View source for Cyanobacteria

{{Short description|Phylum of photosynthesising prokaryotes}}
{{Redirect|Cyanobacterium|the genus|Cyanobacterium (genus)}}
{{Redirect2|Blue-green algae|Green-blue algae|the eukaryotic algae|Green algae}}
{{Use British English|date=August 2021}}
{{Use dmy dates|date=March 2020}}
{{Automatic taxobox
| fossil_range = {{long fossil range|2100|0|earliest=3600}}(Possible Paleoarchean records)
| image = CSIRO ScienceImage 4203 A bluegreen algae species Cylindrospermum sp under magnification.jpg
| image_caption = Microscope image of ''[[Cylindrospermum]]'', a filamentous genus of cyanobacteria
| image_upright = 1.15
| display_parents = 3
| taxon = Cyanobacteria
| authority = [[Roger Stanier|Stanier]] 1973 ex Cavalier-Smith, 2002 (ICNafp)
| subdivision_ranks = Classes, orders and genera
| subdivision_ref = <ref name="Komárek">{{cite journal |vauthors=Komárek J, Kaštovský J, Mareš J, Johansen JR |year=2014 |title=Taxonomic classification of cyanoprokaryotes (cyanobacterial genera) 2014, using a polyphasic approach |url=https://preslia.cz/P144Komarek.pdf |journal=[[Preslia]] |volume=86 |pages=295–335}}</ref>
| subdivision = Short list reflecting c. 2014 taxonomy. For an up-to-date list which is too long to fit here, see [[#Current taxonomy|§ Current taxonomy]].
* [[Cyanophyceae]]
** [[Aegeococcales]]
** [[Chroococcales]]
** [[Chroococcidiopsidales]]
** [[Gloeobacterales]]
** [[Nostocales]]
** [[Oscillatoriales]]
** [[Pleurocapsales]]
** [[Spirulinales]]
** [[Synechococcales]]
| synonyms_ref = {{NoteTag|See note on [[#Phylogeny]].}}
| synonyms = {{collapsible list|expand=yes|bullets = true|title=<small>List (ICNP phylum)</small>
| '''Cyanobacteriota''' {{small|Oren et al. 2022}}<ref name=Oren22>{{Cite journal |last1=Oren |first1=Aharon |last2=Mareš |first2=Jan |last3=Rippka† |first3=Rosmarie |date=2022 |title=Validation of the names Cyanobacterium and Cyanobacterium stanieri, and proposal of Cyanobacteriota phyl. nov |journal=International Journal of Systematic and Evolutionary Microbiology |volume=72 |issue=10 |pages=005528 |doi=10.1099/ijsem.0.005528 |pmid=36251754 }}</ref> 
| "Cyanobacteria" {{small|Woese et al. 1985}} nom. inval.<ref name="auto">{{cite journal |last1=Woese |first1=C.R. |last2=Stackebrandt |first2=E. |last3=Macke |first3=T.J. |last4=Fox |first4=G.E. |title=A Phylogenetic Definition of the Major Eubacterial Taxa |journal=Systematic and Applied Microbiology |date=September 1985 |volume=6 |issue=2 |pages=143–151 |doi=10.1016/S0723-2020(85)80047-3 |pmid=11542017 |bibcode=1985SyApM...6..143W }}</ref>
}}
{{collapsible list|bullets = true|title=<small>List (ICNafp phylum)</small>
| Chloroxybacteria <small>Margulis & Schwartz 1982</small>
| "Cyanophycota" <small>Parker, Schanen & Renner 1969</small>
| "Cyanophyta" <small>Steinecke 1931</small>
| "Diploschizophyta" <small>Dillon 1963</small>
| "Endoschizophyta" <small>Dillon 1963</small>
| "Exoschizophyta" <small>Dillon 1963</small>
| Gonidiophyta <small>Schaffner 1909</small>
}}<!-- Class-level synonyms moved to: [[Cyanophyceae]] -->
{{collapsible list|bullets = true|title=<small>List (other ranks)</small>
| Subdivision "Phycobacteria" <small>Cavalier-Smith 1998</small>
}}<!-- Prochlorophyta and higher moved to: [[Prochlorophyta]] -->
}}

'''Cyanobacteria''' ({{IPAc-en|s|aɪ|ˌ|æ|n|oʊ|b|æ|k|ˈ|t|ɪər|i|ə}} {{respell|sy|AN|oh|bak|TEER|ee|ə}}) are a group of [[autotrophic]] [[gram-negative bacteria]]<ref>{{cite journal | vauthors = Sinha RP, Häder DP |title=UV-protectants in cyanobacteria |journal=[[Plant Science (journal)|Plant Science]] |year=2008 |volume=174 |issue=3 |pages=278–289 |doi=10.1016/j.plantsci.2007.12.004|bibcode=2008PlnSc.174..278S }}</ref> that can obtain [[biological energy]] via [[oxygenic photosynthesis]]. The name "cyanobacteria" ({{etymology|grc|''{{wikt-lang|grc|κύανος}}'' ({{grc-transl|κύανος}})|blue}}) refers to their bluish green ([[cyan]]) color,<ref>{{OEtymD|cyan |access-date=2018-01-21}}</ref><ref>{{LSJ|ku/anos|κύανος|ref |access-date=2018-01-21}}.</ref> which forms the basis of cyanobacteria's informal [[common name]], '''blue-green algae.'''<ref name="ucmp-cyanobacteria-lh">{{cite web |title=Life History and Ecology of Cyanobacteria |url=https://ucmp.berkeley.edu/bacteria/cyanolh.html |url-status=live |archive-url=https://web.archive.org/web/20120919015239/http://www.ucmp.berkeley.edu/bacteria/cyanolh.html |archive-date=19 September 2012 |access-date=2012-07-17 |publisher=[[University of California Museum of Paleontology]]}}</ref><ref name="ncbi-taxonomy">{{cite web |title=Taxonomy Browser – Cyanobacteria |url=https://ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Info&id=1117&lvl=3 |publisher=[[National Center for Biotechnology Information]] |id=NCBI:txid1117 |access-date=12 April 2018}}</ref><ref name="Allaby 92">{{cite encyclopedia |year=1992 |title=Algae |encyclopedia=The Concise Dictionary of Botany |publisher=[[Oxford University Press]] |location=Oxford | veditors = Allaby M }}</ref>{{NoteTag|Some botanists restrict the name ''[[algae]]'' to [[protist]] [[eukaryote]]s, which does not extend to cyanobacteria, which are [[prokaryote]]s. However, the common name blue-green algae continues to be used synonymously with cyanobacteria outside of the biological sciences.{{citation needed|reason=This contradicts the position of the [[ICNafp]], so is not true for all botanists. |date=September 2024}}}}

Cyanobacteria are probably the most numerous [[taxon]] to have ever existed on Earth and the first organisms known to have produced [[oxygen]],<ref name=pwc>{{cite journal | vauthors = Crockford PW, Bar On YM, Ward LM, Milo R, Halevy I | title = The geologic history of primary productivity | journal = Current Biology | volume = 33 | issue = 21 | pages = 4741–4750.e5 | date = November 2023 | pmid = 37827153 | doi = 10.1016/j.cub.2023.09.040 | bibcode = 2023CBio...33E4741C }}</ref> having appeared in the middle [[Archean|Archean eon]] and apparently originated in a [[freshwater]] or [[terrestrial environment]].<ref name=freshwater>{{cite journal |last1=Sánchez-Baracaldo |first1=Patricia |title=Origin of marine planktonic cyanobacteria |journal=Scientific Reports |date=December 2015 |volume=5 |issue=1 |page=17418 |doi=10.1038/srep17418 |pmid=26621203 |pmc=4665016 |bibcode=2015NatSR...517418S }}</ref><ref>{{cite book |url=https://books.google.com/books?id=MJ1PDAAAQBAJ&pg=PA65 |title=The Marine Microbiome: An Untapped Source of Biodiversity and Biotechnological Potential | vauthors = Stal LJ, Cretoiu MS |date=2016 |publisher=[[Springer Science+Business Media]] |isbn=978-3319330006}}</ref> Their [[photopigment]]s can absorb the red- and blue-spectrum frequencies of [[sunlight]] (thus reflecting a greenish color) to split [[water molecule]]s into [[hydrogen ion]]s and oxygen. The hydrogen ions are used to react with [[carbon dioxide]] to produce complex [[organic compound]]s such as [[carbohydrate]]s (a process known as [[carbon fixation]]), and the oxygen is released as a [[byproduct]]. By continuously producing and releasing oxygen over billions of years, cyanobacteria are thought to have converted the [[early Earth]]'s anoxic, [[reducing atmosphere|weakly reducing]] [[prebiotic atmosphere]], into an [[oxidation|oxidizing]] one with free gaseous oxygen (which previously would have been immediately removed by various [[Earth's surface|surface]] [[reductant]]s), resulting in the [[Great Oxidation Event]] and the "[[banded iron formation|rusting of the Earth]]" during the early [[Proterozoic]],<ref name="Whitton2012">{{cite book | veditors = Whitton BA |title=Ecology of Cyanobacteria II: Their Diversity in Space and Time |chapter-url={{google books |plainurl=y |id=4oJ_vi27s18C |page=17}} |year=2012 |chapter=The fossil record of cyanobacteria |page=17 |publisher=[[Springer Science+Business Media]] |isbn=978-94-007-3855-3}}</ref> dramatically changing the composition of life forms on Earth.<ref>{{cite web |url=https://basicbiology.net/micro/microorganisms/bacteria |publisher=Basic Biology |date=18 March 2016 |title=Bacteria}}</ref> The subsequent [[adaptation]] of early [[single-celled organism]]s to survive in oxygenous environments likely had led to [[symbiogenesis|endosymbiosis]] between [[anaerobe]]s and [[aerobe]]s, and hence the evolution of [[eukaryote]]s during the [[Paleoproterozoic]].

Cyanobacteria use [[photosynthetic pigment]]s such as various forms of [[chlorophyll]], [[carotenoid]]s, [[phycobilin]]s to convert the [[light energy|photonic energy]] in sunlight to [[chemical energy]]. Unlike [[heterotroph]]ic prokaryotes, cyanobacteria have [[endomembrane system|internal membranes]]. These are flattened sacs called [[thylakoids]] where photosynthesis is performed.<ref>{{cite book | vauthors = Liberton M, Pakrasi HB |chapter=Chapter 10. Membrane Systems in Cyanobacteria | veditors = Herrero A, Flore E |title=The Cyanobacteria: Molecular Biology, Genomics, and Evolution |publisher=[[Horizon Scientific Press]] |year=2008 |isbn=978-1-904455-15-8 |location=Norwich, United Kingdom |pages=217–287  }}</ref><ref>{{cite journal | vauthors = Liberton M, Page LE, O'Dell WB, O'Neill H, Mamontov E, Urban VS, Pakrasi HB | title = Organization and flexibility of cyanobacterial thylakoid membranes examined by neutron scattering | journal = The Journal of Biological Chemistry | volume = 288 | issue = 5 | pages = 3632–3640 | date = February 2013 | pmid = 23255600 | pmc = 3561581 | doi = 10.1074/jbc.M112.416933 | doi-access = free }}</ref> [[Photoautotroph]]ic eukaryotes such as [[red algae]], [[green algae]] and [[plant]]s perform photosynthesis in chlorophyllic [[organelle]]s that are thought to have their ancestry in cyanobacteria, acquired long ago via endosymbiosis. These [[endosymbiont]] cyanobacteria in eukaryotes then evolved and differentiated into specialized organelles such as [[chloroplast]]s, [[chromoplast]]s, [[etioplast]]s, and [[leucoplast]]s, collectively known as [[plastid]]s.

Sericytochromatia, the proposed name of the [[paraphyly|paraphyletic]] and most basal group, is the ancestor of both the non-photosynthetic group [[Melainabacteria]] and the photosynthetic cyanobacteria, also called Oxyphotobacteria.<ref>{{cite journal | vauthors = Monchamp ME, Spaak P, Pomati F | title = Long Term Diversity and Distribution of Non-photosynthetic Cyanobacteria in Peri-Alpine Lakes | journal = Frontiers in Microbiology | volume = 9 | pages = 3344 | date = 27 July 2019 | pmid = 30692982 | pmc = 6340189 | doi = 10.3389/fmicb.2018.03344 | doi-access = free }}</ref>

The cyanobacteria ''[[Synechocystis]]'' and ''[[Cyanothece]]'' are important model organisms with potential applications in biotechnology for [[bioethanol]] production, food colorings, as a source of human and animal food, dietary supplements and raw materials.<ref>{{cite journal | vauthors = Pathak J, Rajneesh, Maurya PK, Singh SP, Haeder DP, Sinha RP |date=2018 |title=Cyanobacterial Farming for Environment Friendly Sustainable Agriculture Practices: Innovations and Perspectives |journal=[[Frontiers in Environmental Science]] |volume=6 |page=7 |doi=10.3389/fenvs.2018.00007 |doi-access=free |bibcode=2018FrEnS...6....7P }}</ref> Cyanobacteria produce a range of toxins known as [[cyanotoxins]] that can cause harmful health effects in humans and animals.

== Overview ==
[[File:Ocean mist and spray 2.jpg|thumb|upright=1.35|right|Cyanobacteria are found almost everywhere. [[Sea spray]] containing [[marine microorganisms]], including cyanobacteria, can be swept high into the atmosphere where they become [[aeroplankton]], and can travel the globe before falling back to earth.<ref>{{Cite web | vauthors = Morrison J |title=Living Bacteria Are Riding Earth's Air Currents |url=https://www.smithsonianmag.com/science-nature/living-bacteria-are-riding-earths-air-currents-180957734/ |access-date=2022-08-10 |website=Smithsonian Magazine |language=en|date=11 January 2016}}</ref>]]

Cyanobacteria are a large and diverse phylum of [[photoautotrophic|photosynthetic]] [[prokaryote]]s.<ref>{{cite book |doi=10.1007/978-94-007-3855-3_1 | vauthors = Whitton BA, Potts M |chapter=Introduction to the Cyanobacteria | veditors = Whitton BA |title=Ecology of Cyanobacteria II |year=2012  |pages=1–13 | publisher = Springer | location = Dordrecht |isbn=978-94-007-3854-6}}</ref> They are defined by their unique combination of [[Biological pigment|pigments]] and their ability to perform [[oxygenic photosynthesis]]. They often live in [[Colony (biology)|colonial aggregates]] that can take on a multitude of forms.<ref name="Donkor1991"/> Of particular interest are the [[Filamentous cyanobacteria|filamentous species]], which often dominate the upper layers of [[microbial mat]]s found in extreme environments such as [[hot spring]]s, [[hypersaline|hypersaline water]], deserts and the polar regions,<ref name="Stal2000book">{{Cite book | vauthors = Stay LJ | chapter = Cyanobacterial Mats and Stromatolites | chapter-url = https://books.google.com/books?id=4oJ_vi27s18C&q=Stal+LJ+%282000%29+%22Cyanobacterial+Mats+and+Stromatolites%22 |title=Ecology of Cyanobacteria II: Their Diversity in Space and Time | veditors = Whitton BA |date=5 July 2012 |publisher=Springer Science & Business Media |isbn=9789400738553 |access-date=15 February 2022|via=Google Books}}</ref> but are also widely distributed in more mundane environments as well.<ref name=Tamulonis2011>{{cite journal | vauthors = Tamulonis C, Postma M, Kaandorp J | title = Modeling filamentous cyanobacteria reveals the advantages of long and fast trichomes for optimizing light exposure | journal = PLOS ONE | volume = 6 | issue = 7 | pages = e22084 | year = 2011 | pmid = 21789215 | pmc = 3138769 | doi = 10.1371/journal.pone.0022084 | doi-access = free | bibcode = 2011PLoSO...622084T }} {{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref> They are evolutionarily optimized for environmental conditions of low oxygen.<ref>{{Cite news |url=http://www.latimes.com/news/local/oceans/la-me-ocean30jul30,0,6670018,full.story |archive-url=https://web.archive.org/web/20060814091723/http://www.latimes.com/news/local/oceans/la-me-ocean30jul30,0,6670018,full.story |url-status=dead |archive-date=2006-08-14 |work=[[Los Angeles Times]] |title=A Primeval Tide of Toxins | vauthors = Weiss KR |date=2006-07-30}}</ref> Some species are [[nitrogen-fixing]] and live in a wide variety of moist soils and water, either freely or in a symbiotic relationship with plants or [[lichen]]-forming [[Fungus|fungi]] (as in the lichen genus ''[[Peltigera]]'').<ref>{{cite journal |vauthors=Dodds WK, Gudder DA, Mollenhauer D |year=1995 |title=The ecology of 'Nostoc' |journal=Journal of Phycology |volume=31 |issue=1 |pages=2–18 |doi=10.1111/j.0022-3646.1995.00002.x |bibcode=1995JPcgy..31....2D }}</ref>

[[File:Prochlorococcus marinus.jpg|thumb|upright=1.35|right| {{center|''[[Prochlorococcus]]'', an influential marine cyanobacterium which produces much of the world's oxygen}}]]

Cyanobacteria are globally widespread photo{{shy}}synthetic prokaryotes and are major contributors to global [[biogeochemical cycle]]s.<ref name=Aguilera2021 /> They are the only oxygenic photosynthetic prokaryotes, and prosper in diverse and extreme habitats.<ref>{{cite book |doi=10.1007/978-94-007-3855-3_17 |chapter=Carbon |title=Ecology of Cyanobacteria II |date=2012 |last1=Raven |first1=John A. |pages=443–460 |publisher=Springer |location=Dordrecht |isbn=978-94-007-3854-6 }}</ref> They are among the oldest organisms on Earth with fossil records dating back at least 2.1 billion years.<ref name="Schirrmeister-2013" /> Since then, cyanobacteria have been essential players in the Earth's ecosystems. Planktonic cyanobacteria are a fundamental component of [[marine food web]]s and are major contributors to global [[carbon cycle|carbon]] and [[nitrogen cycle|nitrogen fluxes]].<ref>{{cite journal | vauthors = Bullerjahn GS, Post AF | title = Physiology and molecular biology of aquatic cyanobacteria | journal = Frontiers in Microbiology | volume = 5 | pages = 359 | year = 2014 | pmid = 25076944 | pmc = 4099938 | doi = 10.3389/fmicb.2014.00359 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Tang W, Wang S, Fonseca-Batista D, Dehairs F, Gifford S, Gonzalez AG, Gallinari M, Planquette H, Sarthou G, Cassar N | display-authors = 6 | title = Revisiting the distribution of oceanic N<sub>2</sub> fixation and estimating diazotrophic contribution to marine production | journal = Nature Communications | volume = 10 | issue = 1 | pages = 831 | date = February 2019 | pmid = 30783106 | pmc = 6381160 | doi = 10.1038/s41467-019-08640-0 }}</ref> Some cyanobacteria form [[harmful algal bloom]]s causing the disruption of aquatic ecosystem services and intoxication of wildlife and humans by the production of powerful toxins ([[cyanotoxin]]s) such as [[microcystin]]s, [[saxitoxin]], and [[cylindrospermopsin]].<ref>{{cite journal | vauthors = Bláha L, Babica P, Maršálek B | title = Toxins produced in cyanobacterial water blooms - toxicity and risks | journal = Interdisciplinary Toxicology | volume = 2 | issue = 2 | pages = 36–41 | date = June 2009 | pmid = 21217843 | pmc = 2984099 | doi = 10.2478/v10102-009-0006-2 }}</ref><ref name=Paerl2013>{{cite journal | vauthors = Paerl HW, Otten TG | title = Harmful cyanobacterial blooms: causes, consequences, and controls | journal = Microbial Ecology | volume = 65 | issue = 4 | pages = 995–1010 | date = May 2013 | pmid = 23314096 | doi = 10.1007/s00248-012-0159-y | bibcode = 2013MicEc..65..995P }}</ref> Nowadays, cyanobacterial blooms pose a serious threat to aquatic environments and public health, and are increasing in frequency and magnitude globally.<ref name=Huisman2018 /><ref name=Aguilera2021 />

Cyanobacteria are ubiquitous in marine environments and play important roles as [[primary producer]]s. They are part of the marine [[phytoplankton]], which currently contributes almost half of the Earth's total primary production.<ref>{{cite journal | vauthors = Field CB, Behrenfeld MJ, Randerson JT, Falkowski P | title = Primary production of the biosphere: integrating terrestrial and oceanic components | journal = Science | volume = 281 | issue = 5374 | pages = 237–240 | date = July 1998 | pmid = 9657713 | doi = 10.1126/science.281.5374.237 | bibcode = 1998Sci...281..237F | url = https://www.escholarship.org/uc/item/9gm7074q }}</ref> About 25% of the global marine primary production is contributed by cyanobacteria.<ref>{{cite journal | vauthors = Cabello-Yeves PJ, Scanlan DJ, Callieri C, Picazo A, Schallenberg L, Huber P, Roda-Garcia JJ, Bartosiewicz M, Belykh OI, Tikhonova IV, Torcello-Requena A, De Prado PM, Millard AD, Camacho A, Rodriguez-Valera F, Puxty RJ | display-authors = 6 | title = α-cyanobacteria possessing form IA RuBisCO globally dominate aquatic habitats | journal = The ISME Journal | volume = 16 | issue = 10 | pages = 2421–2432 | date = October 2022 | pmid = 35851323 | pmc = 9477826 | doi = 10.1038/s41396-022-01282-z | publisher = Springer Science and Business Media LLC | bibcode = 2022ISMEJ..16.2421C }} {{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref>

Within the cyanobacteria, only a few lineages colonized the open ocean: ''[[Crocosphaera]]'' and relatives, [[cyanobacterium UCYN-A]], ''[[Trichodesmium]]'', as well as ''[[Prochlorococcus]]'' and ''[[Synechococcus]]''.<ref name=Zehr2011>{{cite journal | vauthors = Zehr JP | title = Nitrogen fixation by marine cyanobacteria | journal = Trends in Microbiology | volume = 19 | issue = 4 | pages = 162–173 | date = April 2011 | pmid = 21227699 | doi = 10.1016/j.tim.2010.12.004 }}</ref><ref name=Thompson2012>{{cite journal | vauthors = Thompson AW, Foster RA, Krupke A, Carter BJ, Musat N, Vaulot D, Kuypers MM, Zehr JP | display-authors = 6 | title = Unicellular cyanobacterium symbiotic with a single-celled eukaryotic alga | journal = Science | volume = 337 | issue = 6101 | pages = 1546–1550 | date = September 2012 | pmid = 22997339 | doi = 10.1126/science.1222700 | bibcode = 2012Sci...337.1546T }}</ref><ref name=Johnson2006>{{cite journal | vauthors = Johnson ZI, Zinser ER, Coe A, McNulty NP, Woodward EM, Chisholm SW | title = Niche partitioning among Prochlorococcus ecotypes along ocean-scale environmental gradients | journal = Science | volume = 311 | issue = 5768 | pages = 1737–1740 | date = March 2006 | pmid = 16556835 | doi = 10.1126/science.1118052 | bibcode = 2006Sci...311.1737J }}</ref><ref name=Scanlan2009>{{cite journal | vauthors = Scanlan DJ, Ostrowski M, Mazard S, Dufresne A, Garczarek L, Hess WR, Post AF, Hagemann M, Paulsen I, Partensky F | display-authors = 6 | title = Ecological genomics of marine picocyanobacteria | journal = Microbiology and Molecular Biology Reviews | volume = 73 | issue = 2 | pages = 249–299 | date = June 2009 | pmid = 19487728 | pmc = 2698417 | doi = 10.1128/MMBR.00035-08 }}</ref> From these lineages, nitrogen-fixing cyanobacteria are particularly important because they exert a control on [[Marine primary production|primary productivity]] and the [[Biological pump|export of organic carbon]] to the deep ocean,<ref name=Zehr2011 /> by converting nitrogen gas into ammonium, which is later used to make amino acids and proteins. Marine [[picocyanobacteria|pico{{shy}}cyanobacteria]] (''[[Prochlorococcus]]'' and ''[[Synechococcus]]'') numerically dominate most phytoplankton assemblages in modern oceans, contributing importantly to primary productivity.<ref name=Johnson2006 /><ref name=Scanlan2009 /><ref name="Present and future global distribut">{{cite journal | vauthors = Flombaum P, Gallegos JL, Gordillo RA, Rincón J, Zabala LL, Jiao N, Karl DM, Li WK, Lomas MW, Veneziano D, Vera CS, Vrugt JA, Martiny AC | display-authors = 6 | title = Present and future global distributions of the marine Cyanobacteria Prochlorococcus and Synechococcus | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 110 | issue = 24 | pages = 9824–9829 | date = June 2013 | pmid = 23703908 | pmc = 3683724 | doi = 10.1073/pnas.1307701110 | doi-access = free | bibcode = 2013PNAS..110.9824F }}</ref> While some planktonic cyanobacteria are unicellular and free living cells (e.g., ''Crocosphaera'', ''Prochlorococcus'', ''Synechococcus''); others have established symbiotic relationships with [[Haptophyte|haptophyte algae]], such as [[coccolithophore]]s.<ref name=Thompson2012 /> Amongst the filamentous forms, ''Trichodesmium'' are free-living and form aggregates. However, filamentous heterocyst-forming cyanobacteria (e.g., ''[[Richelia]]'', ''[[Calothrix]]'') are found in association with [[diatom]]s such as ''Hemiaulus'', ''Rhizosolenia'' and ''[[Chaetoceros]]''.<ref>{{cite journal | vauthors = Foster RA, Kuypers MM, Vagner T, Paerl RW, Musat N, Zehr JP | title = Nitrogen fixation and transfer in open ocean diatom-cyanobacterial symbioses | journal = The ISME Journal | volume = 5 | issue = 9 | pages = 1484–1493 | date = September 2011 | pmid = 21451586 | pmc = 3160684 | doi = 10.1038/ismej.2011.26 | bibcode = 2011ISMEJ...5.1484F }}</ref><ref>{{cite journal |doi=10.1111/j.1439-0485.1990.tb00233.x |title=Laboratory Culture and Preliminary Characterization of the Nitrogen-Fixing Rhizosolenia-Richelia Symbiosis |year=1990 | vauthors = Villareal TA |journal=Marine Ecology |volume=11 |issue=2 |pages=117–132 |bibcode=1990MarEc..11..117V}}</ref><ref>{{cite journal | vauthors = Janson S, Wouters J, Bergman B, Carpenter EJ | title = Host specificity in the Richelia-diatom symbiosis revealed by hetR gene sequence analysis | journal = Environmental Microbiology | volume = 1 | issue = 5 | pages = 431–438 | date = October 1999 | pmid = 11207763 | doi = 10.1046/j.1462-2920.1999.00053.x | bibcode = 1999EnvMi...1..431J }}</ref><ref name="Sánchez-Baracaldo2016" />

Marine cyanobacteria include the smallest known photosynthetic organisms. The smallest of all, ''[[Prochlorococcus]]'', is just 0.5 to 0.8 micrometres across.<ref>{{cite journal | vauthors = Kettler GC, Martiny AC, Huang K, Zucker J, Coleman ML, Rodrigue S, Chen F, Lapidus A, Ferriera S, Johnson J, Steglich C, Church GM, Richardson P, Chisholm SW | display-authors = 6 | title = Patterns and implications of gene gain and loss in the evolution of Prochlorococcus | journal = PLOS Genetics | volume = 3 | issue = 12 | pages = e231 | date = December 2007 | pmid = 18159947 | pmc = 2151091 | doi = 10.1371/journal.pgen.0030231 | doi-access = free }}</ref> In terms of numbers of individuals, ''Prochlorococcus'' is possibly the most plentiful genus on Earth: a single millilitre of surface seawater can contain 100,000 cells of this genus or more. Worldwide there are estimated to be several [[octillion]] (10<sup>27</sup>, a billion billion billion) individuals.<ref>{{Cite journal |last1=Flombaum |first1=Pedro |last2=Gallegos |first2=José L. |last3=Gordillo |first3=Rodolfo A. |last4=Rincón |first4=José |last5=Zabala |first5=Lina L. |last6=Jiao |first6=Nianzhi |last7=Karl |first7=David M. |last8=Li |first8=William K. W. |last9=Lomas |first9=Michael W. |last10=Veneziano |first10=Daniele |last11=Vera |first11=Carolina S. |last12=Vrugt |first12=Jasper A. |last13=Martiny |first13=Adam C. |date=2013-06-11 |title=Present and future global distributions of the marine Cyanobacteria Prochlorococcus and Synechococcus |journal=Proceedings of the National Academy of Sciences |volume=110 |issue=24 |pages=9824–9829 |doi=10.1073/pnas.1307701110 |doi-access=free |pmc=3683724 |pmid=23703908|bibcode=2013PNAS..110.9824F }}</ref> ''Prochlorococcus'' is ubiquitous between latitudes 40°N and 40°S, and dominates in the [[oligotroph]]ic (nutrient-poor) regions of the oceans.<ref name="Partensky-1999">{{cite journal | vauthors = Partensky F, Hess WR, Vaulot D | title = Prochlorococcus, a marine photosynthetic prokaryote of global significance | journal = Microbiology and Molecular Biology Reviews | volume = 63 | issue = 1 | pages = 106–127 | date = March 1999 | pmid = 10066832 | pmc = 98958 | doi = 10.1128/MMBR.63.1.106-127.1999 }}</ref> The bacterium accounts for about 20% of the oxygen in the Earth's atmosphere.<ref name="npr">{{cite news |last1=Palca |first1=Joe |title=The Most Important Microbe You've Never Heard Of |url=https://www.npr.org/2008/06/12/91448837/the-most-important-microbe-youve-never-heard-of |work=NPR |date=12 June 2008 }}</ref>

== Morphology ==
{{main|Cyanobacterial morphology}}
{{multiple image
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 | header    = Diversity in cyanobacteria morphology
 | image1    = Cyanobacteriaunicellularandcolonial020.jpg
 | caption1  = Unicellular and colonial cyanobacteria.<br />(scale bars about 10 μm)
 | image2    = Simplefilaments022.jpg
 | caption2  = Simple cyanobacterial filaments ''[[Nostocales]]'', ''[[Oscillatoriales]]'' and ''[[Spirulinales]]''
}}

Cyanobacteria are variable in morphology, ranging from [[unicellular]] and [[Filamentous cyanobacteria|filamentous]] to [[Colony (biology)|colonial forms]]. Filamentous forms exhibit functional cell differentiation such as [[heterocyst]]s (for nitrogen fixation), [[akinetes]] (resting stage cells), and [[hormogonia]] (reproductive, motile filaments). These, together with the intercellular connections they possess, are considered the first signs of multicellularity.<ref name=Claessen2014>{{cite journal | vauthors = Claessen D, Rozen DE, Kuipers OP, Søgaard-Andersen L, van Wezel GP | title = Bacterial solutions to multicellularity: a tale of biofilms, filaments and fruiting bodies | journal = Nature Reviews. Microbiology | volume = 12 | issue = 2 | pages = 115–124 | date = February 2014 | pmid = 24384602 | doi = 10.1038/nrmicro3178 | hdl-access = free | hdl = 11370/0db66a9c-72ef-4e11-a75d-9d1e5827573d | url = https://pure.rug.nl/ws/files/2328477/2014NatRevMicrobiolClaessen.pdf }}</ref><ref>{{cite journal | vauthors = Nürnberg DJ, Mariscal V, Parker J, Mastroianni G, Flores E, Mullineaux CW | title = Branching and intercellular communication in the Section V cyanobacterium Mastigocladus laminosus, a complex multicellular prokaryote | journal = Molecular Microbiology | volume = 91 | issue = 5 | pages = 935–949 | date = March 2014 | pmid = 24383541 | doi = 10.1111/mmi.12506 | hdl-access = free | hdl = 10261/99110 }}</ref><ref name=Herrero2016>{{cite journal | vauthors = Herrero A, Stavans J, Flores E | title = The multicellular nature of filamentous heterocyst-forming cyanobacteria | journal = FEMS Microbiology Reviews | volume = 40 | issue = 6 | pages = 831–854 | date = November 2016 | pmid = 28204529 | doi = 10.1093/femsre/fuw029 | hdl-access = free | hdl = 10261/140753 }}</ref><ref name=Aguilera2021 />

Many cyanobacteria form motile filaments of cells, called [[hormogonium|hormogonia]], that travel away from the main biomass to bud and form new colonies elsewhere.<ref>{{cite journal | vauthors = Risser DD, Chew WG, Meeks JC | title = Genetic characterization of the hmp locus, a chemotaxis-like gene cluster that regulates hormogonium development and motility in Nostoc punctiforme | journal = Molecular Microbiology | volume = 92 | issue = 2 | pages = 222–233 | date = April 2014 | pmid = 24533832 | doi = 10.1111/mmi.12552 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Khayatan B, Bains DK, Cheng MH, Cho YW, Huynh J, Kim R, Omoruyi OH, Pantoja AP, Park JS, Peng JK, Splitt SD, Tian MY, Risser DD | display-authors = 6 | title = A Putative O-Linked β-''N''-Acetylglucosamine Transferase Is Essential for Hormogonium Development and Motility in the Filamentous Cyanobacterium Nostoc punctiforme | journal = Journal of Bacteriology | volume = 199 | issue = 9 | pages = e00075–17 | date = May 2017 | pmid = 28242721 | pmc = 5388816 | doi = 10.1128/JB.00075-17 }}</ref> The cells in a hormogonium are often thinner than in the vegetative state, and the cells on either end of the motile chain may be tapered. To break away from the parent colony, a hormogonium often must tear apart a weaker cell in a filament, called a necridium.{{cn|date=March 2025}}

[[File:Morphological variation within cyanobacterial genera.jpg|thumb|upright=1.8| '''Morphological variations''':<ref>{{cite journal | vauthors = Esteves-Ferreira AA, Cavalcanti JH, Vaz MG, Alvarenga LV, Nunes-Nesi A, Araújo WL | title = Cyanobacterial nitrogenases: phylogenetic diversity, regulation and functional predictions | journal = Genetics and Molecular Biology | volume = 40 | issue = 1 suppl 1 | pages = 261–275 | year = 2017 | pmid = 28323299 | pmc = 5452144 | doi = 10.1590/1678-4685-GMB-2016-0050 }}</ref> {{unordered list|item_style=margin-bottom: 0|Unicellular: (a) ''[[Synechocystis]]'' and (b) ''[[Synechococcus elongatus]]'' |Non-[[Heterocyst|heterocytous]]: (c) ''[[Arthrospira|Arthrospira maxima]]'',(d) ''[[Trichodesmium]]'' and (e) ''Phormidium''| False- or non-branching heterocytous: (f) ''[[Nostoc]]'' and (g) ''Brasilonema octagenarum'' | True-branching heterocytous: (h) ''Stigonema'' (ak) akinetes (fb) false branching (tb) true branching}}]]

Some filamentous species can differentiate into several different [[cell (biology)|cell]] types:
* Vegetative cells – the normal, photosynthetic cells that are formed under favorable growing conditions
* [[Akinete]]s – climate-resistant spores that may form when environmental conditions become harsh
* Thick-walled [[heterocysts]] – which contain the enzyme [[nitrogenase]] vital for [[nitrogen fixation]]<ref>{{cite journal | vauthors = Meeks JC, Elhai J, Thiel T, Potts M, Larimer F, Lamerdin J, Predki P, Atlas R | display-authors = 6 | title = An overview of the genome of Nostoc punctiforme, a multicellular, symbiotic cyanobacterium | journal = Photosynthesis Research | volume = 70 | issue = 1 | pages = 85–106 | date = 2001 | pmid = 16228364 | doi = 10.1023/A:1013840025518 | bibcode = 2001PhoRe..70...85M }}</ref><ref name="Golden_1998">{{cite journal | vauthors = Golden JW, Yoon HS | title = Heterocyst formation in Anabaena | journal = Current Opinion in Microbiology | volume = 1 | issue = 6 | pages = 623–629 | date = December 1998 | pmid = 10066546 | doi = 10.1016/s1369-5274(98)80106-9 }}</ref><ref name="Fay_1992">{{cite journal | vauthors = Fay P | title = Oxygen relations of nitrogen fixation in cyanobacteria | journal = Microbiological Reviews | volume = 56 | issue = 2 | pages = 340–373 | date = June 1992 | pmid = 1620069 | pmc = 372871 | doi = 10.1128/MMBR.56.2.340-373.1992 }}</ref> in an anaerobic environment due to its sensitivity to oxygen.<ref name="Fay_1992" />

Each individual cell (each single cyanobacterium) typically has a thick, gelatinous [[cell wall]].<ref>{{Cite book | veditors = Singh V, Pande PC, Jain DK | chapter = Cyanobacteria, Actinomycetes, Mycoplasma, and Rickettsias |page=72 |title=Text Book of Botany Diversity of Microbes And Cryptogams |publisher=Rastogi Publications |isbn=978-81-7133-889-4 }}</ref> They lack [[flagellum|flagella]], but hormogonia of some species can move about by [[bacterial gliding|gliding]] along surfaces.<ref>{{Cite news |url=http://www.microbiologynotes.com/differences-between-bacteria-and-cyanobacteria/ |title=Differences between Bacteria and Cyanobacteria |date=2015-10-29 |work=Microbiology Notes |access-date=2018-01-21}}</ref> Many of the multicellular filamentous forms of ''[[Oscillatoria]]'' are capable of a waving motion; the filament oscillates back and forth. In water columns, some cyanobacteria float by forming [[gas vesicle]]s, as in [[archaea]].<ref>{{cite journal | vauthors = Walsby AE | title = Gas vesicles | journal = Microbiological Reviews | volume = 58 | issue = 1 | pages = 94–144 | date = March 1994 | pmid = 8177173 | pmc = 372955 | doi = 10.1128/MMBR.58.1.94-144.1994 }}</ref> These vesicles are not [[organelle]]s as such. They are not bounded by [[lipid membranes]], but by a protein sheath.

{{multiple image 
|total_width = 410
|align = left
|image1=Gloeotrichia in Sytox.jpg |caption1=Ball-shaped colony of [[Gloeotrichia|Gloeotrichia echinulata]] stained with [[SYTOX]]
|image2=CyanobacteriaColl1.jpg |caption2=Colonies of ''[[Nostoc pruniforme]]''
}}
{{clear}}

== Nitrogen fixation ==
[[File:Nitrogen-fixing cyanobacteria.png|thumb|{{center|Nitrogen-fixing cyanobacteria}}]]

Some cyanobacteria can fix atmospheric [[nitrogen]] in anaerobic conditions by means of specialized cells called [[heterocyst]]s.<ref name="Golden_1998" /><ref name="Fay_1992" /> Heterocysts may also form under the appropriate environmental conditions (anoxic) when fixed nitrogen is scarce. Heterocyst-forming species are specialized for nitrogen fixation and are able to fix nitrogen gas into [[ammonia]] ({{chem2|NH3}}), [[nitrites]] ({{chem2|NO2-}}) or [[nitrates]] ({{chem2|NO3-}}), which can be absorbed by plants and converted to protein and nucleic acids (atmospheric nitrogen is not [[bioavailable]] to plants, except for those having endosymbiotic [[nitrogen-fixing bacteria]], especially the family [[Fabaceae]], among
others). Nitrogen fixation commonly occurs on a cycle of nitrogen fixation during the night because photosynthesis can inhibit nitrogen fixation.<ref>{{Cite journal |last1=Berman-Frank |first1=Ilana |last2=Quigg |first2=Antonietta |last3=Finkel |first3=Zoe V. |last4=Irwin |first4=Andrew J. |last5=Haramaty |first5=Liti |date=2007 |title=Nitrogen-fixation strategies and Fe requirements in cyanobacteria |journal=Limnology and Oceanography |language=en |volume=52 |issue=5 |pages=2260–2269 |doi=10.4319/lo.2007.52.5.2260 |bibcode=2007LimOc..52.2260B }}</ref>

Free-living cyanobacteria are present in the water of [[paddy field|rice paddies]], and cyanobacteria can be found growing as [[epiphyte]]s on the surfaces of the green alga, ''[[Chara (alga)|Chara]]'', where they may fix nitrogen.<ref>
{{cite journal |vauthors=Sims GK, Dunigan EP |year=1984 |title=Diurnal and seasonal variations in nitrogenase activity {{chem|C|2|H|2}} reduction) of rice roots |journal=Soil Biology and Biochemistry |volume=16 |issue=1 |pages=15–18 |doi=10.1016/0038-0717(84)90118-4}}</ref> Cyanobacteria such as ''[[Anabaena]]'' (a symbiont of the aquatic fern ''[[Azolla]]'') can provide rice plantations with [[biofertilizer]].<ref name="Azolla-Anabaena as a Biofertilizer for Rice Paddy Fields in the Po Valley, a Temperate Rice Area in Northern Italy">{{cite journal |title=Azolla-Anabaena as a Biofertilizer for Rice Paddy Fields in the Po Valley, a Temperate Rice Area in Northern Italy |journal=[[International Journal of Agronomy]] |doi=10.1155/2010/152158 |year=2010 | vauthors = Bocchi S, Malgioglio A |volume=2010 |pages=1–5 |doi-access=free|hdl=2434/149583 |hdl-access=free }}</ref>

{{clear}}

== Photosynthesis ==
[[File:Cyanobacterium-inline.svg|thumb|upright=1.5| {{center|'''Diagram of a typical cyanobacterial cell'''}}]]
[[File:Cyanobacterial thylakoid membrane.png|thumb|upright=1.5| {{center|'''Cyanobacterial thylakoid membrane'''{{hsp}}<ref>{{cite journal | vauthors = Huokko T, Ni T, Dykes GF, Simpson DM, Brownridge P, Conradi FD, Beynon RJ, Nixon PJ, Mullineaux CW, Zhang P, Liu LN | display-authors = 6 | title = Probing the biogenesis pathway and dynamics of thylakoid membranes | journal = Nature Communications | volume = 12 | issue = 1 | pages = 3475 | date = June 2021 | pmid = 34108457 | pmc = 8190092 | doi = 10.1038/s41467-021-23680-1 | bibcode = 2021NatCo..12.3475H }}</ref>}} Outer and plasma membranes are in blue, thylakoid membranes in gold, glycogen granules in cyan, [[carboxysome]]s (C) in green, and a large dense polyphosphate granule (G) in pink]]

=== Carbon fixation ===
The [[thylakoids]] of cyanobacteria use the energy of [[sunlight]] to drive [[photosynthesis]], a process where the energy of light is used to synthesize [[organic compound]]s from carbon dioxide. Because they are aquatic organisms, they typically employ several strategies which are collectively known as a "{{CO2}} concentrating mechanism" to aid in the acquisition of inorganic carbon ({{CO2}} or [[bicarbonate]]). Among the more specific strategies is the widespread prevalence of the bacterial microcompartments known as [[carboxysome]]s,<ref name="KerfeldHeinhorst2010">{{cite journal | vauthors = Kerfeld CA, Heinhorst S, Cannon GC | title = Bacterial microcompartments | journal = Annual Review of Microbiology | volume = 64 | issue = 1 | pages = 391–408 | year = 2010 | pmid = 20825353 | pmc = 6022854 | doi = 10.1146/annurev.micro.112408.134211 }}</ref> which co-operate with active transporters of CO<sub>2</sub> and bicarbonate, in order to accumulate bicarbonate into the cytoplasm of the cell.<ref>{{cite journal | vauthors = Rae BD, Long BM, Badger MR, Price GD | title = Functions, compositions, and evolution of the two types of carboxysomes: polyhedral microcompartments that facilitate CO2 fixation in cyanobacteria and some proteobacteria | journal = Microbiology and Molecular Biology Reviews | volume = 77 | issue = 3 | pages = 357–379 | date = September 2013 | pmid = 24006469 | pmc = 3811607 | doi = 10.1128/MMBR.00061-12 }}</ref> Carboxysomes are [[Icosahedron|icosahedral]] structures composed of hexameric shell proteins that assemble into cage-like structures that can be several hundreds of nanometres in diameter. It is believed that these structures tether the {{CO2}}-fixing enzyme, [[RuBisCO]], to the interior of the shell, as well as the enzyme [[carbonic anhydrase]], using [[metabolic channeling]] to enhance the local {{CO2}} concentrations and thus increase the efficiency of the RuBisCO enzyme.<ref name="LongBadger2007">{{cite journal | vauthors = Long BM, Badger MR, Whitney SM, Price GD | title = Analysis of carboxysomes from Synechococcus PCC7942 reveals multiple Rubisco complexes with carboxysomal proteins CcmM and CcaA | journal = The Journal of Biological Chemistry | volume = 282 | issue = 40 | pages = 29323–29335 | date = October 2007 | pmid = 17675289 | doi = 10.1074/jbc.M703896200 | doi-access = free }}</ref>

=== Electron transport ===
In contrast to [[purple bacteria]] and other bacteria performing [[anoxygenic photosynthesis]], thylakoid membranes of cyanobacteria are not continuous with the plasma membrane but are separate compartments.<ref name="Vothknecht&Westhoff20012">{{cite journal | vauthors = Vothknecht UC, Westhoff P | title = Biogenesis and origin of thylakoid membranes | journal = Biochimica et Biophysica Acta (BBA) - Molecular Cell Research | volume = 1541 | issue = 1–2 | pages = 91–101 | date = December 2001 | pmid = 11750665 | doi = 10.1016/S0167-4889(01)00153-7 | doi-access = free }}</ref> The photosynthetic machinery is embedded in the [[thylakoid]] membranes, with [[phycobilisome]]s acting as [[Light-harvesting complex|light-harvesting antennae]] attached to the membrane, giving the green pigmentation observed (with wavelengths from 450&nbsp;nm to 660&nbsp;nm) in most cyanobacteria.<ref name="Sobiechowska-Sasim_2014">{{cite journal | vauthors = Sobiechowska-Sasim M, Stoń-Egiert J, Kosakowska A | title = Quantitative analysis of extracted phycobilin pigments in cyanobacteria-an assessment of spectrophotometric and spectrofluorometric methods | journal = Journal of Applied Phycology | volume = 26 | issue = 5 | pages = 2065–2074 | date = February 2014 | pmid = 25346572 | pmc = 4200375 | doi = 10.1007/s10811-014-0244-3 | bibcode = 2014JAPco..26.2065S }}</ref>

While most of the high-energy [[electron]]s derived from water are used by the cyanobacterial cells for their own needs, a fraction of these electrons may be donated to the external environment via [[electrogenic]] activity.<ref name="Pisciotta JM, Zou Y, Baskakov IV 2010 e108212">{{cite journal | vauthors = Pisciotta JM, Zou Y, Baskakov IV | title = Light-dependent electrogenic activity of cyanobacteria | journal = PLOS ONE | volume = 5 | issue = 5 | pages = e10821 | date = May 2010 | pmid = 20520829 | pmc = 2876029 | doi = 10.1371/journal.pone.0010821 | veditors = Yang CH | doi-access = free | bibcode = 2010PLoSO...510821P }} {{Creative Commons text attribution notice|cc=by4|from this source=yes}}
</ref>

==== Respiration ====
[[Cellular respiration|Respiration]] in cyanobacteria can occur in the thylakoid membrane alongside photosynthesis,<ref name="Vermaas_2001">{{cite book |title=Photosynthesis and Respiration in Cyanobacteria. eLS. | vauthors = Vermaas WF |date=2001 |publisher=[[John Wiley & Sons]], Ltd |isbn=978-0-470-01590-2 |doi=10.1038/npg.els.0001670 |chapter=Photosynthesis and Respiration in Cyanobacteria}}</ref> with their photosynthetic [[Electron transport chain|electron transport]] sharing the same compartment as the components of respiratory electron transport. While the goal of photosynthesis is to store energy by building carbohydrates from CO<sub>2</sub>, respiration is the reverse of this, with carbohydrates turned back into CO<sub>2</sub> accompanying energy release.

Cyanobacteria appear to separate these two processes with their plasma membrane containing only components of the respiratory chain, while the thylakoid membrane hosts an interlinked respiratory and photosynthetic electron transport chain.<ref name="Vermaas_2001" /> Cyanobacteria use electrons from [[succinate dehydrogenase]] rather than from [[Nicotinamide adenine dinucleotide phosphate|NADPH]] for respiration.<ref name="Vermaas_2001" />

Cyanobacteria only respire during the night (or in the dark) because the facilities used for electron transport are used in reverse for photosynthesis while in the light.<ref name="Armstrong_2014">{{cite book |url=https://www.press.uchicago.edu/ucp/books/book/chicago/H/bo16465693.html |title=How the Earth Turned Green: A Brief 3.8-Billion-Year History of Plants | vauthors = Armstronf JE |date=2015 |publisher=The [[University of Chicago Press]] |isbn=978-0-226-06977-7}}</ref>

==== Electron transport chain ====
Many cyanobacteria are able to reduce nitrogen and carbon dioxide under [[Aerobic cellular respiration|aerobic]] conditions (using different methods to circumvent the deleterious effect of dioxygen on [[nitrogenase]]s), a fact that may be responsible for their evolutionary and ecological success. The water-oxidizing photosynthesis is accomplished by coupling the activity of [[photosystem]] (PS) II and I ([[Z-scheme]]). In contrast to [[green sulfur bacteria]] which only use one photosystem, the use of water as an electron donor is energetically demanding, requiring two photosystems.<ref name="Klatt_2016">{{cite journal | vauthors = Klatt JM, de Beer D, Häusler S, Polerecky L | title = Cyanobacteria in Sulfidic Spring Microbial Mats Can Perform Oxygenic and Anoxygenic Photosynthesis Simultaneously during an Entire Diurnal Period | journal = Frontiers in Microbiology | volume = 7 | pages = 1973 | date = 2016 | pmid = 28018309 | pmc = 5156726 | doi = 10.3389/fmicb.2016.01973 | doi-access = free }}</ref>

Attached to the thylakoid membrane, [[phycobilisome]]s act as [[Light-harvesting complex|light-harvesting antennae]] for the photosystems.<ref>{{cite journal | vauthors = Grossman AR, Schaefer MR, Chiang GG, Collier JL | title = The phycobilisome, a light-harvesting complex responsive to environmental conditions | journal = Microbiological Reviews | volume = 57 | issue = 3 | pages = 725–749 | date = September 1993 | pmid = 8246846 | pmc = 372933 | doi = 10.1128/MMBR.57.3.725-749.1993 }}</ref> The phycobilisome components ([[phycobiliprotein]]s) are responsible for the blue-green pigmentation of most cyanobacteria.<ref>{{Cite web |url=http://www.webexhibits.org/causesofcolor/5D.html |title=Colors from bacteria {{!}} Causes of Color |website=www.webexhibits.org |access-date=2018-01-22}}</ref> The variations on this theme are due mainly to [[carotenoid]]s and [[phycoerythrin]]s that give the cells their red-brownish coloration. In some cyanobacteria, the color of light influences the composition of the phycobilisomes.<ref>{{cite book |doi=10.1016/B978-012373944-5.00250-9 | vauthors = Garcia-Pichel F |chapter=Cyanobacteria |title=Encyclopedia of Microbiology |edition=third |pages=107–24 | veditors = Schaechter M |isbn=978-0-12-373944-5 |year=2009| publisher = Elsevier }}</ref><ref>{{cite journal | vauthors = Kehoe DM | title = Chromatic adaptation and the evolution of light color sensing in cyanobacteria | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 107 | issue = 20 | pages = 9029–9030 | date = May 2010 | pmid = 20457899 | pmc = 2889117 | doi = 10.1073/pnas.1004510107 | doi-access = free | bibcode = 2010PNAS..107.9029K }}</ref> In green light, the cells accumulate more phycoerythrin, which absorbs green light, whereas in red light they produce more [[phycocyanin]] which absorbs red. Thus, these bacteria can change from brick-red to bright blue-green depending on whether they are exposed to green light or to red light.<ref>{{cite journal | vauthors = Kehoe DM, Gutu A | title = Responding to color: the regulation of complementary chromatic adaptation | journal = Annual Review of Plant Biology | volume = 57 | pages = 127–150 | date = 2006 | issue = 1 | pmid = 16669758 | doi = 10.1146/annurev.arplant.57.032905.105215 | bibcode = 2006AnRPB..57..127K }}</ref> This process of "complementary chromatic adaptation" is a way for the cells to maximize the use of available light for photosynthesis.

A few genera lack phycobilisomes and have [[chlorophyll b]] instead (''[[Prochloron]]'', ''[[Prochlorococcus]]'', ''Prochlorothrix''). These were originally grouped together as the [[Prochlorophyta|prochlorophytes]] or chloroxybacteria, but appear to have developed in several different lines of cyanobacteria. For this reason, they are now considered as part of the cyanobacterial group.<ref>{{cite journal | vauthors = Palenik B, Haselkorn R | title = Multiple evolutionary origins of prochlorophytes, the chlorophyll b-containing prokaryotes | journal = Nature | volume = 355 | issue = 6357 | pages = 265–267 | date = January 1992 | pmid = 1731224 | doi = 10.1038/355265a0 | bibcode = 1992Natur.355..265P }}</ref><ref>{{cite journal | vauthors = Urbach E, Robertson DL, Chisholm SW | title = Multiple evolutionary origins of prochlorophytes within the cyanobacterial radiation | journal = Nature | volume = 355 | issue = 6357 | pages = 267–270 | date = January 1992 | pmid = 1731225 | doi = 10.1038/355267a0 | bibcode = 1992Natur.355..267U }}</ref>

==== Metabolism ====
In general, photosynthesis in cyanobacteria uses water as an [[Redox|electron donor]] and produces [[oxygen]] as a byproduct, though some may also use [[hydrogen sulfide]]<ref name="Cohen Y, Jørgensen BB, Revsbech NP, Poplawski R 19862">{{cite journal | vauthors = Cohen Y, Jørgensen BB, Revsbech NP, Poplawski R | title = Adaptation to Hydrogen Sulfide of Oxygenic and Anoxygenic Photosynthesis among Cyanobacteria | journal = Applied and Environmental Microbiology | volume = 51 | issue = 2 | pages = 398–407 | date = February 1986 | pmid = 16346996 | pmc = 238881 | doi = 10.1128/AEM.51.2.398-407.1986 | bibcode = 1986ApEnM..51..398C }}</ref> a process which occurs among other photosynthetic bacteria such as the [[purple sulfur bacteria]].

[[Carbon dioxide]] is reduced to form [[carbohydrate]]s via the [[Calvin cycle]].<ref>{{cite book |title=Molecular Mechanisms of Photosynthesis | vauthors = Blankenship RE |author1-link=Robert E. Blankenship |publisher=[[Wiley-Blackwell]] |year=2014 |isbn=978-1-4051-8975-0 |pages=147–73}}</ref> The large amounts of oxygen in the atmosphere are considered to have been first created by the activities of ancient cyanobacteria.<ref name="Och_2012">{{cite journal | vauthors = Och LM, Shields-Zhou GA |title=The Neoproterozoic oxygenation event: Environmental perturbations and biogeochemical cycling |journal=[[Earth-Science Reviews]] |volume=110 |issue=1–4 |pages=26–57 |doi=10.1016/j.earscirev.2011.09.004 |date=January 2012 |bibcode=2012ESRv..110...26O}}</ref> They are often found as [[symbiont]]s with a number of other groups of organisms such as fungi (lichens), [[coral]]s, [[pteridophyte]]s (''[[Azolla]]''), [[angiosperm]]s (''[[Gunnera]]''), etc.<ref>{{cite book | vauthors = Adams DG, Bergman B, Nierzwicki-Bauer SA, Duggan PS, Rai AN, Schüßler A | chapter = Cyanobacterial-Plant Symbioses | veditors = Rosenberg E, DeLong EF, Lory S, Stackebrandt E, Thompson F |title=The Prokaryotes  |date=2013 |publisher=Springer, Berlin, Heidelberg |isbn=978-3-642-30193-3 |pages=359–400 |doi=10.1007/978-3-642-30194-0_17}}</ref> The carbon metabolism of cyanobacteria include the incomplete [[Citric acid cycle|Krebs cycle]],<ref>{{cite journal | vauthors = Zhang S, Bryant DA | title = The tricarboxylic acid cycle in cyanobacteria | journal = Science | volume = 334 | issue = 6062 | pages = 1551–1553 | date = December 2011 | pmid = 22174252 | doi = 10.1126/science.1210858 | bibcode = 2011Sci...334.1551Z }}</ref> the [[pentose phosphate pathway]], and [[glycolysis]].<ref>{{cite journal | vauthors = Xiong W, Lee TC, Rommelfanger S, Gjersing E, Cano M, Maness PC, Ghirardi M, Yu J | display-authors = 6 | title = Phosphoketolase pathway contributes to carbon metabolism in cyanobacteria | journal = Nature Plants | volume = 2 | issue = 1 | pages = 15187 | date = December 2015 | pmid = 27250745 | doi = 10.1038/nplants.2015.187 }}</ref>

There are some groups capable of [[heterotrophic]] growth,<ref name="CarrWhitton1973">{{cite book | vauthors = Smith A |chapter=Synthesis of metabolic intermediates | veditors =  Carr NG, Whitton BA |title=The Biology of Blue-green Algae|chapter-url={{google books |plainurl=y |id=fSRPg-D0Jk0C |page=30}} |year=1973 |publisher=University of California Press |isbn=978-0-520-02344-4 |pages=[https://archive.org/details/biologyofbluegre0000carr/page/30 30–] |url=https://archive.org/details/biologyofbluegre0000carr}}</ref> while others are [[parasitic]], causing diseases in invertebrates or algae (e.g., the [[black band disease]]).<ref>{{cite journal | vauthors = Jangoux M |year=1987 |title=Diseases of Echinodermata. I. Agents microorganisms and protistans |journal=Diseases of Aquatic Organisms |volume=2 |pages=147–62 |doi=10.3354/dao002147 |doi-access=free}}</ref><ref>{{cite book |veditors=Kinne O |year=1980 |title=Diseases of Marine Animals |volume=1 |url=https://www.int-res.com/archive/doma_books/DOMA_Vol_I_(general_aspects,_protozoa_to%20gastropoda).pdf |publisher=John Wiley & Sons |location=Chichester, UK |isbn=978-0-471-99584-5}}</ref><ref>{{cite journal | vauthors = Kristiansen A |doi=10.2216/i0031-8884-4-1-19.1 |title=''Sarcinastrum urosporae'', a Colourless Parasitic Blue-green Alga |journal=Phycologia |year=1964 |volume=4 |issue=1 |pages=19–22 |bibcode=1964Phyco...4...19K }}</ref>
{{clear}}

== Ecology ==
[[File:Environmental impact of aquatic photosynthetic microorganisms.png|thumb|upright=2|Environmental impact of cyanobacteria and other photosynthetic microorganisms in aquatic systems. Different classes of photosynthetic microorganisms are found in aquatic and marine environments where they form the base of healthy food webs and participate in symbioses with other organisms. However, shifting environmental conditions can result in community dysbiosis, where the growth of opportunistic species can lead to harmful blooms and toxin production with negative consequences to human health, livestock and fish stocks. Positive interactions are indicated by arrows; negative interactions are indicated by closed circles on the ecological model.<ref>{{cite journal | vauthors = Mazard S, Penesyan A, Ostrowski M, Paulsen IT, Egan S | title = Tiny Microbes with a Big Impact: The Role of Cyanobacteria and Their Metabolites in Shaping Our Future | journal = Marine Drugs | volume = 14 | issue = 5 | page = 97 | date = May 2016 | pmid = 27196915 | pmc = 4882571 | doi = 10.3390/md14050097 | doi-access = free }}</ref>]]

Cyanobacteria can be found in almost every terrestrial and [[aquatic habitat]]&nbsp;– [[ocean]]s, [[fresh water]], damp soil, temporarily moistened rocks in [[desert]]s, bare rock and soil, and even [[Antarctic]] rocks. They can occur as [[planktonic]] cells or form [[phototrophic biofilms]]. They are found inside stones and shells (in [[endolithic ecosystem]]s).<ref>{{cite journal | vauthors = de los Ríos A, Grube M, Sancho LG, Ascaso C | title = Ultrastructural and genetic characteristics of endolithic cyanobacterial biofilms colonizing Antarctic granite rocks | journal = FEMS Microbiology Ecology | volume = 59 | issue = 2 | pages = 386–395 | date = February 2007 | pmid = 17328119 | doi = 10.1111/j.1574-6941.2006.00256.x | bibcode = 2007FEMME..59..386D | doi-access = free }}</ref> A few are [[endosymbiont]]s in [[lichen]]s, plants, various [[protist]]s, or [[Sea sponge|sponges]] and provide energy for the [[Host (biology)|host]]. Some live in the fur of [[sloth]]s, providing a form of [[camouflage]].<ref>{{cite book | vauthors = Vaughan T |title=Mammalogy |year=2011 |publisher=Jones and Barlett |page=21 |url={{google books|plainurl=y |id=LD1nDlzXYicC |page=21}} |isbn=978-0763762995}}</ref>

Aquatic cyanobacteria are known for their extensive and highly visible [[Algal bloom|blooms]] that can form in both [[freshwater]] and marine environments. The blooms can have the appearance of blue-green paint or scum. These blooms can be [[toxic]], and frequently lead to the closure of recreational waters when spotted. [[Marine bacteriophage]]s are significant [[parasites]] of unicellular marine cyanobacteria.<ref>{{cite magazine | vauthors = Schultz N |date=30 August 2009 |url=https://www.newscientist.com/article/mg20327235.000-photosynthetic-viruses-keep-worlds-oxygen-levels-up.html |title=Photosynthetic viruses keep world's oxygen levels up |magazine=[[New Scientist]]}}</ref>

Cyanobacterial growth is favoured in ponds and lakes where waters are calm and have little turbulent mixing.<ref name="Jöhnk_2008">{{cite journal | vauthors = Jöhnk KD, Huisman J, Sharples J, Sommeijer B, Visser PM, Stroom JM |title=Summer heatwaves promote blooms of harmful cyanobacteria |journal=[[Global Change Biology]] |date=1 March 2008 |volume=14 |issue=3 |pages=495–512 |doi=10.1111/j.1365-2486.2007.01510.x |bibcode=2008GCBio..14..495J |url=https://ir.cwi.nl/pub/12731 }}</ref> Their lifecycles are disrupted when the water naturally or artificially mixes from churning currents caused by the flowing water of streams or the churning water of fountains. For this reason blooms of cyanobacteria seldom occur in rivers unless the water is flowing slowly. Growth is also favoured at higher temperatures which enable ''[[Microcystis]]'' species to outcompete [[diatoms]] and [[green algae]], and potentially allow development of toxins.<ref name="Jöhnk_2008"/>

Based on environmental trends, models and observations suggest cyanobacteria will likely increase their dominance in aquatic environments. This can lead to serious consequences, particularly the contamination of sources of [[drinking water]]. Researchers including [[Linda Lawton]] at [[Robert Gordon University]], have developed techniques to study these.<ref>{{Cite web |title=Linda Lawton – 11th International Conference on Toxic Cyanobacteria |url=http://ictc11.org/speakers/linda-lawton/ |access-date=2021-06-25|language=en-US}}</ref> Cyanobacteria can interfere with [[water treatment]] in various ways, primarily by plugging filters (often large beds of sand and similar media) and by producing [[cyanotoxin]]s, which have the potential to cause serious illness if consumed. Consequences may also lie within fisheries and waste management practices. Anthropogenic [[eutrophication]], rising temperatures, vertical stratification and increased [[atmospheric carbon dioxide]] are contributors to cyanobacteria increasing dominance of aquatic ecosystems.<ref>{{cite journal | vauthors = Paerl HW, Paul VJ | title = Climate change: links to global expansion of harmful cyanobacteria | journal = Water Research | volume = 46 | issue = 5 | pages = 1349–1363 | date = April 2012 | pmid = 21893330 | doi = 10.1016/j.watres.2011.08.002 | bibcode = 2012WatRe..46.1349P }}</ref>

[[File:Cyanobacteriaassociatedwithtufa014 Microcoleus v.jpg|thumb|upright|right|Diagnostic Drawing: Cyanobacteria associated with tufa: ''Microcoleus vaginatus'']]

Cyanobacteria have been found to play an important role in terrestrial habitats and organism communities. It has been widely reported that cyanobacteria [[soil crust]]s help to stabilize soil to prevent [[erosion]] and retain water.<ref>{{cite journal |vauthors=Thomas AD, Dougill AJ |date=15 March 2007 |title=Spatial and temporal distribution of cyanobacterial soil crusts in the Kalahari: Implications for soil surface properties |journal=Geomorphology |volume=85 |issue=1 |pages=17–29 |bibcode=2007Geomo..85...17T |doi=10.1016/j.geomorph.2006.03.029}}</ref> An example of a cyanobacterial species that does so is ''Microcoleus vaginatus''. ''M. vaginatus'' stabilizes soil using a [[polysaccharide]] sheath that binds to sand particles and absorbs water.<ref>{{cite journal |vauthors=Belnap J, Gardner JS |date=1993 |title=Soil Microstructure in Soils of the Colorado Plateau: The Role of the Cyanobacterium Microcoleus Vaginatus |journal=The Great Basin Naturalist |volume=53 |issue=1 |pages=40–47 |jstor=41712756}}</ref> ''M. vaginatus'' also makes a significant contribution to the cohesion of [[biological soil crust]].<ref name="SpatialSelfSegregation">{{cite journal |last1=Nelson |first1=Corey |last2=Giraldo-Silva |first2=Ana |last3=Warsop Thomas |first3=Finlay |last4=Garcia-Pichel |first4=Ferran |date=16 November 2022 |title=Spatial self-segregation of pioneer cyanobacterial species drives microbiome organization in biocrusts |journal=ISME Communications |volume=2 |issue=1 |page=114 |doi=10.1038/s43705-022-00199-0 |pmid=37938289 |pmc=9723579 |bibcode=2022ISMEC...2..114N }}</ref>

Some of these organisms contribute significantly to global ecology and the [[oxygen cycle]]. The tiny marine cyanobacterium ''[[Prochlorococcus]]'' was discovered in 1986 and accounts for more than half of the photosynthesis of the open ocean.<ref>{{cite journal | vauthors = Nadis S | title = The cells that rule the seas | journal = Scientific American | volume = 289 | issue = 6 | pages = 52–53 | date = December 2003 | pmid = 14631732 | doi = 10.1038/scientificamerican1203-52 | bibcode = 2003SciAm.289f..52N }}</ref> [[Circadian rhythm]]s were once thought to only exist in eukaryotic cells but many cyanobacteria display a [[bacterial circadian rhythm]].

<blockquote>"Cyanobacteria are arguably the most successful group of [[microorganisms]] on earth. They are the most genetically diverse; they occupy a broad range of habitats across all latitudes, widespread in freshwater, marine, and terrestrial ecosystems, and they are found in the most extreme niches such as hot springs, salt works, and hypersaline bays. [[Photoautotrophic]], oxygen-producing cyanobacteria created the conditions in the planet's early atmosphere that directed the evolution of aerobic metabolism and eukaryotic photosynthesis. Cyanobacteria fulfill vital ecological functions in the world's oceans, being important contributors to global carbon and nitrogen budgets." – Stewart and Falconer<ref name="WalshSmith2011">{{cite book | veditors = Walsh PJ, Smith S, Fleming L, Solo-Gabriele H, Gerwick WH
 |title=Oceans and Human Health: Risks and Remedies from the Seas |chapter-url={{google books |plainurl=y |id=LMZPqW-PmFYC |page=271}} |date=2 September 2011 | vauthors = Stewart I, Falconer IR |chapter=Cyanobacteria and cyanobacterial toxins |pages=271–296 |publisher=[[Academic Press]] |isbn=978-0-08-087782-2}}</ref>
</blockquote>

=== Cyanobionts ===
[[File:Leaf and root colonization by cyanobacteria.jpg|thumb|upright=2|right| {{center|'''Symbiosis with land plants'''{{hsp}}<ref name=Lee2021>{{cite journal | vauthors = Lee SM, Ryu CM | title = Algae as New Kids in the Beneficial Plant Microbiome | journal = Frontiers in Plant Science | volume = 12 | pages = 599742 | date = 4 Feb 2021 | pmid = 33613596 | pmc = 7889962 | doi = 10.3389/fpls.2021.599742 | publisher = Frontiers Media SA | doi-access = free | bibcode = 2021FrPS...1299742L }} {{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref><br />Leaf and root colonization by cyanobacteria}} (1) Cyanobacteria enter the leaf tissue through the [[stomata]] and colonize the intercellular space, forming a cyanobacterial loop.<br /> (2) On the root surface, cyanobacteria exhibit two types of colonization pattern; in the [[root hair]], filaments of ''[[Anabaena]]'' and ''[[Nostoc]]'' species form loose colonies, and in the restricted zone on the root surface, specific ''Nostoc'' species form cyanobacterial colonies.<br /> (3) Co-inoculation with [[2,4-Dichlorophenoxyacetic acid|2,4-D]] and ''Nostoc'' spp. increases para-nodule formation and nitrogen fixation. A large number of ''Nostoc'' spp. isolates colonize the root [[endosphere]] and form para-nodules.<ref name=Lee2021 />]]
{{main|Cyanobiont}}

Some cyanobacteria, the so-called [[cyanobiont]]s (cyanobacterial symbionts), have a [[symbiotic]] relationship with other organisms, both unicellular and multicellular.<ref name=Kim2020 /> As illustrated on the right, there are many examples of cyanobacteria interacting [[symbiotically]] with [[land plant]]s.<ref>{{cite journal |doi=10.1046/j.1469-8137.1999.00352.x |title=Colonization of wheatpara-nodules by the N2-fixing cyanobacterium ''Nostocsp''. Strain 2S9B |year=1999 | vauthors = Gantar M, Elhai J |journal=[[New Phytologist]] |volume=141 |issue=3 |pages=373–379 |doi-access=free}}</ref><ref>{{cite journal |doi=10.1007/s003740000243 |title=Mechanical damage of roots provides enhanced colonization of the wheat endorhizosphere by the dinitrogen-fixing cyanobacterium Nostoc sp. Strain 2S9B |year=2000 | vauthors = Gantar M |journal=Biology and Fertility of Soils |volume=32 |issue=3 |pages=250–255 |bibcode=2000BioFS..32..250G }}</ref><ref name=Treves2016>{{cite journal | vauthors = Treves H, Raanan H, Kedem I, Murik O, Keren N, Zer H, Berkowicz SM, Giordano M, Norici A, Shotland Y, Ohad I, Kaplan A | display-authors = 6 | title = The mechanisms whereby the green alga Chlorella ohadii, isolated from desert soil crust, exhibits unparalleled photodamage resistance | journal = The New Phytologist | volume = 210 | issue = 4 | pages = 1229–1243 | date = June 2016 | pmid = 26853530 | doi = 10.1111/nph.13870 | doi-access = free | bibcode = 2016NewPh.210.1229T }}</ref><ref>{{cite journal | vauthors = Zhu H, Li S, Hu Z, Liu G | title = Molecular characterization of eukaryotic algal communities in the tropical phyllosphere based on real-time sequencing of the 18S rDNA gene | journal = BMC Plant Biology | volume = 18 | issue = 1 | pages = 365 | date = December 2018 | pmid = 30563464 | pmc = 6299628 | doi = 10.1186/s12870-018-1588-7 | doi-access = free | bibcode = 2018BMCPB..18..365Z }}</ref> Cyanobacteria can enter the plant through the [[stomata]] and colonize the intercellular space, forming loops and intracellular coils.<ref>{{cite journal |doi=10.1016/j.revpalbo.2008.06.006 |title=Endophytic cyanobacteria in a 400-million-yr-old land plant: A scenario for the origin of a symbiosis? |year=2009 | vauthors = Krings M, Hass H, Kerp H, Taylor TN, Agerer R, Dotzler N |journal=[[Review of Palaeobotany and Palynology]] |volume=153 |issue=1–2 |pages=62–69|bibcode=2009RPaPa.153...62K }}</ref> ''[[Anabaena]]'' spp. colonize the roots of wheat and cotton plants.<ref>{{cite journal | vauthors = Karthikeyan N, Prasanna R, Sood A, Jaiswal P, Nayak S, Kaushik BD | title = Physiological characterization and electron microscopic investigation of cyanobacteria associated with wheat rhizosphere | journal = Folia Microbiologica | volume = 54 | issue = 1 | pages = 43–51 | year = 2009 | pmid = 19330544 | doi = 10.1007/s12223-009-0007-8 }}</ref><ref name=Babu2015>{{cite journal |doi=10.1007/s10811-014-0322-6 |title=Analysing the colonisation of inoculated cyanobacteria in wheat plants using biochemical and molecular tools |year=2015 | vauthors = Babu S, Prasanna R, Bidyarani N, Singh R |journal=Journal of Applied Phycology |volume=27 |issue=1 |pages=327–338 |bibcode=2015JAPco..27..327B }}</ref><ref name=Bidyarani2015>{{cite journal | vauthors = Bidyarani N, Prasanna R, Chawla G, Babu S, Singh R | title = Deciphering the factors associated with the colonization of rice plants by cyanobacteria | journal = Journal of Basic Microbiology | volume = 55 | issue = 4 | pages = 407–419 | date = April 2015 | pmid = 25515189 | doi = 10.1002/jobm.201400591 }}</ref> ''[[Calothrix]]'' sp. has also been found on the root system of wheat.<ref name=Babu2015 /><ref name=Bidyarani2015 /> [[Monocot]]s, such as wheat and rice, have been colonised by ''[[Nostoc]]'' spp.,<ref name=Gantar1991>{{cite journal |doi=10.1111/j.1469-8137.1991.tb00031.x |title=Colonization of wheat (Triticum vulgare L.) by N2-fixing cyanobacteria: II. An ultrastructural study |year=1991 | vauthors = Gantar M, Kerby NW, Rowell P |journal=[[New Phytologist]] |volume=118 |issue=3 |pages=485–492 |doi-access=free|bibcode=1991NewPh.118..485G }}</ref><ref>{{cite journal |doi=10.1007/s11104-010-0488-x |title=Association of non-heterocystous cyanobacteria with crop plants |year=2010 | vauthors = Ahmed M, Stal LJ, Hasnain S |journal=[[Plant and Soil]] |volume=336 |issue=1–2 |pages=363–375 |bibcode=2010PlSoi.336..363A |url=http://dare.uva.nl/personal/pure/en/publications/association-of-nonheterocystous-cyanobacteria-with-crop-plants(a90f9b12-6bd1-47cc-87c4-389ab649d969).html }}</ref><ref>{{cite journal | vauthors = Hussain A, Hamayun M, Shah ST | title = Root colonization and phytostimulation by phytohormones producing entophytic Nostoc sp. AH-12 | journal = Current Microbiology | volume = 67 | issue = 5 | pages = 624–630 | date = November 2013 | pmid = 23794014 | doi = 10.1007/s00284-013-0408-4 }}</ref><ref>{{cite journal | vauthors = Hussain A, Shah ST, Rahman H, Irshad M, Iqbal A | title = Effect of IAA on in vitro growth and colonization of Nostoc in plant roots | journal = Frontiers in Plant Science | volume = 6 | pages = 46 | year = 2015 | pmid = 25699072 | pmc = 4318279 | doi = 10.3389/fpls.2015.00046 | doi-access = free | bibcode = 2015FrPS....6...46H }}</ref> In 1991, Ganther and others isolated diverse [[heterocystous]] nitrogen-fixing cyanobacteria, including ''Nostoc'', ''Anabaena'' and ''[[Cylindrospermum]]'', from plant root and soil. Assessment of wheat seedling roots revealed two types of association patterns: loose colonization of root hair by ''Anabaena'' and tight colonization of the root surface within a restricted zone by ''Nostoc''.<ref name=Gantar1991 /><ref name=Lee2021 />

[[File:Cyanobacterial symbionts of Ornithocercus dinoflagellate 2.png|thumb|upright=2|left| {{center|'''Cyanobionts of ''Ornithocercus'' dinoflagellates'''{{hsp}}<ref name=Kim2020>{{cite journal | vauthors = Kim M, Choi DH, Park MG | title = Cyanobiont genetic diversity and host specificity of cyanobiont-bearing dinoflagellate Ornithocercus in temperate coastal waters | journal = Scientific Reports | volume = 11 | issue = 1 | pages = 9458 | date = May 2021 | pmid = 33947914 | pmc = 8097063 | doi = 10.1038/s41598-021-89072-z | bibcode = 2021NatSR..11.9458K }} {{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref>}} Live cyanobionts (cyanobacterial symbionts) belonging to ''[[Ornithocercus]]'' [[dinoflagellate]] host consortium<br />(a) ''O. magnificus'' with numerous cyanobionts present in the upper and lower girdle lists (black arrowheads) of the cingulum termed the symbiotic chamber.<br />(b) ''O. steinii'' with numerous cyanobionts inhabiting the symbiotic chamber.<br />(c) Enlargement of the area in (b) showing two cyanobionts that are being divided by binary transverse fission (white arrows).]]

[[File:Cyanobacteria in symbiosis with a diatom.png|thumb|upright=1|right| {{center|[[Epiphytic]] ''[[Calothrix]] ''cyanobacteria (arrows) in symbiosis with a ''[[Chaetoceros]]'' diatom. Scale bar 50 μm.}}]]

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The relationships between [[cyanobiont]]s (cyanobacterial symbionts) and protistan hosts are particularly noteworthy, as some nitrogen-fixing cyanobacteria ([[diazotroph]]s) play an important role in [[Marine primary production|primary production]], especially in nitrogen-limited [[oligotrophic]] oceans.<ref>{{cite journal |doi=10.1126/science.276.5316.1221 |title=Trichodesmium, a Globally Significant Marine Cyanobacterium |year=1997 | vauthors = Capone DG |journal=Science |volume=276 |issue=5316 |pages=1221–1229}}</ref><ref>{{cite journal | vauthors = Falkowski PG, Barber RT, Smetacek V | title = Biogeochemical Controls and Feedbacks on Ocean Primary Production | journal = Science | volume = 281 | issue = 5374 | pages = 200–207 | date = July 1998 | pmid = 9660741 | doi = 10.1126/science.281.5374.200 }}</ref><ref>{{cite journal |doi=10.4319/lo.2007.52.4.1293 |title=CO2 control of Trichodesmium N2 fixation, photosynthesis, growth rates, and elemental ratios: Implications for past, present, and future ocean biogeochemistry |year=2007 | vauthors = Hutchins DA, Fu FX, Zhang Y, Warner ME, Feng Y, Portune K, Bernhardt PW, Mulholland MR |author-link8=Margaret Mulholland |journal=[[Limnology and Oceanography]] |volume=52 |issue=4 |pages=1293–1304 |bibcode=2007LimOc..52.1293H |doi-access=free }}</ref> Cyanobacteria, mostly [[pico-]]sized ''[[Synechococcus]]'' and ''[[Prochlorococcus]]'', are ubiquitously distributed and are the most abundant photosynthetic organisms on Earth, accounting for a quarter of all carbon fixed in marine ecosystems.<ref name="Present and future global distribut"/><ref>{{cite journal | vauthors = Huang S, Wilhelm SW, Harvey HR, Taylor K, Jiao N, Chen F | title = Novel lineages of Prochlorococcus and Synechococcus in the global oceans | journal = The ISME Journal | volume = 6 | issue = 2 | pages = 285–297 | date = February 2012 | pmid = 21955990 | pmc = 3260499 | doi = 10.1038/ismej.2011.106 | bibcode = 2012ISMEJ...6..285H }}</ref><ref name="Partensky-1999"/> In contrast to free-living marine cyanobacteria, some cyanobionts are known to be responsible for nitrogen fixation rather than carbon fixation in the host.<ref>{{cite book |doi=10.1201/b13853 |title=Stress Biology of Cyanobacteria |date=2013 |publisher=CRC Press |isbn=978-0-429-10135-9 |editor-last1=Srivastava |editor-last2=Rai |editor-last3=Neilan |editor-first1=Ashish Kumar |editor-first2=Amar Nath |editor-first3=Brett A. }}{{pn|date=February 2025}}</ref><ref>{{cite journal | vauthors = Zehr JP, Bench SR, Carter BJ, Hewson I, Niazi F, Shi T, Tripp HJ, Affourtit JP | display-authors = 6 | title = Globally distributed uncultivated oceanic N2-fixing cyanobacteria lack oxygenic photosystem II | journal = Science | volume = 322 | issue = 5904 | pages = 1110–1112 | date = November 2008 | pmid = 19008448 | doi = 10.1126/science.1165340 | bibcode = 2008Sci...322.1110Z }}</ref> However, the physiological functions of most cyanobionts remain unknown. Cyanobionts have been found in numerous protist groups, including [[dinoflagellate]]s, [[tintinnid]]s, [[radiolarian]]s, [[amoebae]], [[diatom]]s, and [[haptophyte]]s.<ref>{{cite book |doi=10.1007/978-4-431-55130-0_19 |chapter=Photosymbiosis in Marine Planktonic Protists |title=Marine Protists |year=2015 | vauthors = Decelle J, Colin S, Foster RA |pages=465–500 |publisher=Springer |location=Tokyo |isbn=978-4-431-55129-4}}</ref><ref>{{cite journal | vauthors = Foster RA, Zehr JP | title = Diversity, Genomics, and Distribution of Phytoplankton-Cyanobacterium Single-Cell Symbiotic Associations | journal = Annual Review of Microbiology | volume = 73 | pages = 435–456 | date = September 2019 | issue = 1 | pmid = 31500535 | doi = 10.1146/annurev-micro-090817-062650 }}</ref> Among these cyanobionts, little is known regarding the nature (e.g., genetic diversity, host or cyanobiont specificity, and cyanobiont seasonality) of the symbiosis involved, particularly in relation to dinoflagellate host.<ref name=Kim2020 />

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=== Collective behaviour ===
{{further|Algal bloom}}

[[File:Collective behaviour and lifestyle choices in single-celled cyanobacteria.webp|thumb|upright=2| {{center|Collective behaviour and  buoyancy strategies in single-celled cyanobacteria{{hsp}}<ref name=Mullineaux2021>{{cite journal | vauthors = Mullineaux CW, Wilde A | title = The social life of cyanobacteria | journal = eLife | volume = 10 | date = June 2021 | pmid = 34132636 | pmc = 8208810 | doi = 10.7554/eLife.70327 | doi-access = free }} {{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref>}}]]

Some cyanobacteria – even single-celled ones – show striking collective behaviours and form colonies (or [[algal bloom|blooms]]) that can float on water and have important ecological roles. For instance, billions of years ago, communities of marine [[Paleoproterozoic]] cyanobacteria could have helped create the [[biosphere]] as we know it by burying carbon compounds and allowing the initial build-up of oxygen in the atmosphere.<ref>{{cite journal | vauthors = Kamennaya NA, Zemla M, Mahoney L, Chen L, Holman E, Holman HY, Auer M, Ajo-Franklin CM, Jansson C | display-authors = 6 | title = High pCO<sub>2</sub>-induced exopolysaccharide-rich ballasted aggregates of planktonic cyanobacteria could explain Paleoproterozoic carbon burial | journal = Nature Communications | volume = 9 | issue = 1 | pages = 2116 | date = May 2018 | pmid = 29844378 | pmc = 5974010 | doi = 10.1038/s41467-018-04588-9 | bibcode = 2018NatCo...9.2116K }}</ref> On the other hand, [[Harmful algal bloom|toxic cyanobacterial bloom]]s are an increasing issue for society, as their toxins can be harmful to animals.<ref name=Huisman2018>{{cite journal | vauthors = Huisman J, Codd GA, Paerl HW, Ibelings BW, Verspagen JM, Visser PM | title = Cyanobacterial blooms | journal = Nature Reviews. Microbiology | volume = 16 | issue = 8 | pages = 471–483 | date = August 2018 | pmid = 29946124 | doi = 10.1038/s41579-018-0040-1 }}</ref> Extreme blooms can also deplete water of oxygen and reduce the penetration of sunlight and visibility, thereby compromising the feeding and mating behaviour of light-reliant species.<ref name=Mullineaux2021 />

As shown in the diagram on the right, bacteria can stay in suspension as individual cells, adhere collectively to surfaces to form biofilms, passively sediment, or flocculate to form suspended aggregates. Cyanobacteria are able to produce sulphated [[polysaccharide]]s (yellow haze surrounding clumps of cells) that enable them to form floating aggregates. In 2021, Maeda et al. discovered that oxygen produced by cyanobacteria becomes trapped in the network of polysaccharides and cells, enabling the microorganisms to form buoyant blooms.<ref name=Maeda2021>{{cite journal | vauthors = Maeda K, Okuda Y, Enomoto G, Watanabe S, Ikeuchi M | title = Biosynthesis of a sulfated exopolysaccharide, synechan, and bloom formation in the model cyanobacterium ''Synechocystis'' sp. strain PCC 6803 | journal = eLife | volume = 10 | date = June 2021 | pmid = 34127188 | pmc = 8205485 | doi = 10.7554/eLife.66538 | doi-access = free }}</ref> It is thought that specific protein fibres known as [[Pilus|pili]] (represented as lines radiating from the cells) may act as an additional way to link cells to each other or onto surfaces. Some cyanobacteria also use sophisticated intracellular [[gas vesicle]]s as floatation aids.<ref name=Mullineaux2021 />

[[File:Model of a clumped cyanobacterial mat.webp|thumb|upright=1.35|left| {{center|Model of a clumped cyanobacterial mat{{hsp}}<ref name=Sim2012>{{cite journal |doi=10.3390/geosciences2040235 |doi-access=free |title=Oxygen-Dependent Morphogenesis of Modern Clumped Photosynthetic Mats and Implications for the Archean Stromatolite Record |year=2012 | vauthors = Sim MS, Liang B, Petroff AP, Evans A, Klepac-Ceraj V, Flannery DT, Walter MR, Bosak T | display-authors = 6 |journal=Geosciences |volume=2 |issue=4 |pages=235–259 |bibcode=2012Geosc...2..235S|hdl=1721.1/85544 |hdl-access=free }} {{Creative Commons text attribution notice|cc=by3|from this source=yes}}</ref>}}]]

[[File:Cyanobacteria guerrero negro.jpg|thumb|upright=1.15| Light microscope view of cyanobacteria from a [[microbial mat]]]]

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The diagram on the left above shows a proposed model of microbial distribution, spatial organization, carbon and O<sub>2</sub> cycling in clumps and adjacent areas. (a) Clumps contain denser cyanobacterial filaments and heterotrophic microbes. The initial differences in density depend on cyanobacterial motility and can be established over short timescales. Darker blue color outside of the clump indicates higher oxygen concentrations in areas adjacent to clumps. Oxic media increase the reversal frequencies of any filaments that begin to leave the clumps, thereby reducing the net migration away from the clump. This enables the persistence of the initial clumps over short timescales; (b) Spatial coupling between photosynthesis and respiration in clumps. Oxygen produced by cyanobacteria diffuses into the overlying medium or is used for aerobic respiration. [[Dissolved inorganic carbon]] (DIC) diffuses into the clump from the overlying medium and is also produced within the clump by respiration. In oxic solutions, high O<sub>2</sub> concentrations reduce the efficiency of CO<sub>2</sub> fixation and result in the excretion of glycolate. Under these conditions, clumping can be beneficial to cyanobacteria if it stimulates the retention of carbon and the assimilation of inorganic carbon by cyanobacteria within clumps. This effect appears to promote the accumulation of [[particulate organic carbon]] (cells, sheaths and heterotrophic organisms) in clumps.<ref name=Sim2012 />

It has been unclear why and how cyanobacteria form communities. Aggregation must divert resources away from the core business of making more cyanobacteria, as it generally involves the production of copious quantities of extracellular material. In addition, cells in the centre of dense aggregates can also suffer from both shading and shortage of nutrients.<ref name=Conradi2019>{{cite journal | vauthors = Conradi FD, Zhou RQ, Oeser S, Schuergers N, Wilde A, Mullineaux CW | title = Factors Controlling Floc Formation and Structure in the Cyanobacterium ''Synechocystis'' sp. Strain PCC 6803 | journal = Journal of Bacteriology | volume = 201 | issue = 19 | date = October 2019 | pmid = 31262837 | pmc = 6755745 | doi = 10.1128/JB.00344-19 }}</ref><ref>{{cite journal | vauthors = Enomoto G, Ikeuchi M | title = Blue-/Green-Light-Responsive Cyanobacteriochromes Are Cell Shade Sensors in Red-Light Replete Niches | journal = iScience | volume = 23 | issue = 3 | pages = 100936 | date = March 2020 | pmid = 32146329 | pmc = 7063230 | doi = 10.1016/j.isci.2020.100936 | bibcode = 2020iSci...23j0936E }}</ref> So, what advantage does this communal life bring for cyanobacteria?<ref name=Mullineaux2021 />

[[File:Cell death in eukaryotes and cyanobacteria.jpg|thumb|upright=2| {{center|'''Cell death in eukaryotes and cyanobacteria'''{{hsp}}<ref name=Aguilera2021>{{cite journal | vauthors = Aguilera A, Klemenčič M, Sueldo DJ, Rzymski P, Giannuzzi L, Martin MV | title = Cell Death in Cyanobacteria: Current Understanding and Recommendations for a Consensus on Its Nomenclature | journal = Frontiers in Microbiology | volume = 12 | pages = 631654 | year = 2021 | pmid = 33746925 | pmc = 7965980 | doi = 10.3389/fmicb.2021.631654 | doi-access = free }} {{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref>}} Types of cell death according to the [[Nomenclature Committee on Cell Death]] (upper panel;<ref>{{cite journal | vauthors = Galluzzi L, Bravo-San Pedro JM, Vitale I, Aaronson SA, Abrams JM, Adam D, Alnemri ES, Altucci L, Andrews D, Annicchiarico-Petruzzelli M, Baehrecke EH, Bazan NG, Bertrand MJ, Bianchi K, Blagosklonny MV, Blomgren K, Borner C, Bredesen DE, Brenner C, Campanella M, Candi E, Cecconi F, Chan FK, Chandel NS, Cheng EH, Chipuk JE, Cidlowski JA, Ciechanover A, Dawson TM, Dawson VL, De Laurenzi V, De Maria R, Debatin KM, Di Daniele N, Dixit VM, Dynlacht BD, El-Deiry WS, Fimia GM, Flavell RA, Fulda S, Garrido C, Gougeon ML, Green DR, Gronemeyer H, Hajnoczky G, Hardwick JM, Hengartner MO, Ichijo H, Joseph B, Jost PJ, Kaufmann T, Kepp O, Klionsky DJ, Knight RA, Kumar S, Lemasters JJ, Levine B, Linkermann A, Lipton SA, Lockshin RA, López-Otín C, Lugli E, Madeo F, Malorni W, Marine JC, Martin SJ, Martinou JC, Medema JP, Meier P, Melino S, Mizushima N, Moll U, Muñoz-Pinedo C, Nuñez G, Oberst A, Panaretakis T, Penninger JM, Peter ME, Piacentini M, Pinton P, Prehn JH, Puthalakath H, Rabinovich GA, Ravichandran KS, Rizzuto R, Rodrigues CM, Rubinsztein DC, Rudel T, Shi Y, Simon HU, Stockwell BR, Szabadkai G, Tait SW, Tang HL, Tavernarakis N, Tsujimoto Y, Vanden Berghe T, Vandenabeele P, Villunger A, Wagner EF, Walczak H, White E, Wood WG, Yuan J, Zakeri Z, Zhivotovsky B, Melino G, Kroemer G | display-authors = 6 | title = Essential versus accessory aspects of cell death: recommendations of the NCCD 2015 | journal = Cell Death and Differentiation | volume = 22 | issue = 1 | pages = 58–73 | date = January 2015 | pmid = 25236395 | pmc = 4262782 | doi = 10.1038/cdd.2014.137 }}</ref> and proposed for cyanobacteria (lower panel). Cells exposed to extreme injury die in an uncontrollable manner, reflecting the loss of structural integrity. This type of cell death is called "accidental cell death" (ACD). "Regulated cell death (RCD)" is encoded by a genetic pathway that can be modulated by genetic or pharmacologic interventions. [[Programmed cell death]] (PCD) is a type of RCD that occurs as a developmental program, and has not been addressed in cyanobacteria yet. RN, regulated necrosis.]]

New insights into how cyanobacteria form blooms have come from a 2021 study on the cyanobacterium ''[[Synechocystis]]''. These use a set of genes that regulate the production and export of sulphated [[polysaccharide]]s, chains of sugar molecules modified with [[sulphate]] groups that can often be found in marine algae and animal tissue. Many bacteria generate extracellular polysaccharides, but sulphated ones have only been seen in cyanobacteria. In ''Synechocystis'' these sulphated polysaccharide help the cyanobacterium form buoyant aggregates by trapping oxygen bubbles in the slimy web of cells and polysaccharides.<ref name=Maeda2021 /><ref name=Mullineaux2021 />

Previous studies on ''Synechocystis'' have shown [[type IV pili]], which decorate the surface of cyanobacteria, also play a role in forming blooms.<ref>{{cite journal | vauthors = Allen R, Rittmann BE, Curtiss R | title = Axenic Biofilm Formation and Aggregation by ''Synechocystis'' sp. Strain PCC 6803 Are Induced by Changes in Nutrient Concentration and Require Cell Surface Structures | journal = Applied and Environmental Microbiology | volume = 85 | issue = 7 | date = April 2019 | pmid = 30709828 | pmc = 6585507 | doi = 10.1128/AEM.02192-18 | bibcode = 2019ApEnM..85E2192A }}</ref><ref name=Conradi2019 /> These retractable and adhesive protein fibres are important for motility, adhesion to substrates and DNA uptake.<ref>{{cite journal | vauthors = Schuergers N, Wilde A | title = Appendages of the cyanobacterial cell | journal = Life | volume = 5 | issue = 1 | pages = 700–715 | date = March 2015 | pmid = 25749611 | pmc = 4390875 | doi = 10.3390/life5010700 | bibcode = 2015Life....5..700S | doi-access = free }}</ref> The formation of blooms may require both type IV pili and Synechan – for example, the pili may help to export the polysaccharide outside the cell. Indeed, the activity of these protein fibres may be connected to the production of extracellular polysaccharides in filamentous cyanobacteria.<ref name=Khayatan2015>{{cite journal | vauthors = Khayatan B, Meeks JC, Risser DD | title = Evidence that a modified type IV pilus-like system powers gliding motility and polysaccharide secretion in filamentous cyanobacteria | journal = Molecular Microbiology | volume = 98 | issue = 6 | pages = 1021–1036 | date = December 2015 | pmid = 26331359 | doi = 10.1111/mmi.13205 | doi-access = free }}</ref> A more obvious answer would be that pili help to build the aggregates by binding the cells with each other or with the extracellular polysaccharide. As with other kinds of bacteria,<ref>{{cite journal | vauthors = Adams DW, Stutzmann S, Stoudmann C, Blokesch M | title = DNA-uptake pili of Vibrio cholerae are required for chitin colonization and capable of kin recognition via sequence-specific self-interaction | journal = Nature Microbiology | volume = 4 | issue = 9 | pages = 1545–1557 | date = September 2019 | pmid = 31182799 | pmc = 6708440 | doi = 10.1038/s41564-019-0479-5 }}</ref> certain components of the pili may allow cyanobacteria from the same species to recognise each other and make initial contacts, which are then stabilised by building a mass of extracellular polysaccharide.<ref name=Mullineaux2021 />

The bubble flotation mechanism identified by Maeda et al. joins a range of known strategies that enable cyanobacteria to control their buoyancy, such as using gas vesicles or accumulating carbohydrate ballasts.<ref>{{cite journal |doi=10.1093/plankt/12.1.161 |title=A computer model of buoyancy and vertical migration in cyanobacteria |year=1990 | vauthors = Kromkamp J, Walsby AE |journal=[[Journal of Plankton Research]] |volume=12 |issue=1 |pages=161–183}}</ref> Type IV pili on their own could also control the position of marine cyanobacteria in the water column by regulating viscous drag.<ref>{{cite journal | vauthors = Aguilo-Ferretjans MD, Bosch R, Puxty RJ, Latva M, Zadjelovic V, Chhun A, Sousoni D, Polin M, Scanlan DJ, Christie-Oleza JA | display-authors = 6 | title = Pili allow dominant marine cyanobacteria to avoid sinking and evade predation | journal = Nature Communications | volume = 12 | issue = 1 | pages = 1857 | date = March 2021 | pmid = 33767153 | pmc = 7994388 | doi = 10.1038/s41467-021-22152-w | bibcode = 2021NatCo..12.1857A }}</ref> Extracellular polysaccharide appears to be a multipurpose asset for cyanobacteria, from floatation device to food storage, defence mechanism and mobility aid.<ref name=Khayatan2015 /><ref name=Mullineaux2021 />

=== Cellular death ===
One of the most critical processes determining cyanobacterial eco-physiology is [[cellular death]]. Evidence supports the existence of controlled cellular demise in cyanobacteria, and various forms of cell death have been described as a response to biotic and abiotic stresses. However, cell death research in cyanobacteria is a relatively young field and understanding of the underlying mechanisms and molecular machinery underpinning this fundamental process remains largely elusive.<ref name=Aguilera2021 /> However, reports on cell death of marine and freshwater cyanobacteria indicate this process has major implications for the ecology of microbial communities/<ref name="Agustí2004">{{cite journal | vauthors = Agustí S | title = Viability and niche segregation of Prochlorococcus and Synechococcus cells across the Central Atlantic Ocean. | journal = Aquatic Microbial Ecology | date = June 2004 | volume = 36 | issue = 1 | pages = 53–59 | doi = 10.3354/ame036053 | doi-access = free | hdl = 10261/86957 | hdl-access = free }}</ref><ref name="Agustí2006">{{cite journal |doi=10.1111/j.1365-2427.2006.01584.x |title=Cell death in lake phytoplankton communities |year=2006 | vauthors = Agusti S, Alou EV, Hoyer MV, Frazer TK, Canfield DE |author4-link=Thomas K. Frazer |journal=[[Freshwater Biology]] |volume=51 |issue=8 |pages=1496–1506|bibcode=2006FrBio..51.1496A }}</ref><ref name=Franklin2006>{{cite journal |doi=10.1080/09670260500505433 |title=What is the role and nature of programmed cell death in phytoplankton ecology? |year=2006 | vauthors = Franklin DJ, Brussaard CP, Berges JA |journal=European Journal of Phycology |volume=41 |issue=1 |pages=1–14 |bibcode=2006EJPhy..41....1F |doi-access=free}}</ref><ref name=Sigee2007>{{cite journal |doi=10.2216/06-69.1 |title=Patterns of cell death in freshwater colonial cyanobacteria during the late summer bloom |year=2007 | vauthors = Sigee DC, Selwyn A, Gallois P, Dean AP |journal=[[Phycologia]] |volume=46 |issue=3 |pages=284–292 |bibcode=2007Phyco..46..284S }}</ref> Different forms of cell demise have been observed in cyanobacteria under several stressful conditions,<ref name=BermanFrank2004>{{cite journal |doi=10.4319/lo.2004.49.4.0997 |title=The demise of the marine cyanobacterium, Trichodesmium SPP., via an autocatalyzed cell death pathway |year=2004 | vauthors = Berman-Frank I, Bidle KD, Haramaty L, Falkowski PG |journal=[[Limnology and Oceanography]] |volume=49 |issue=4 |pages=997–1005 |bibcode=2004LimOc..49..997B |doi-access=free}}</ref><ref name=Hu2019>{{cite journal | vauthors = Hu C, Rzymski P | title = Programmed Cell Death-Like and Accompanying Release of Microcystin in Freshwater Bloom-Forming Cyanobacterium ''Microcystis'': From Identification to Ecological Relevance | journal = Toxins | volume = 11 | issue = 12 | page = 706 | date = December 2019 | pmid = 31817272 | pmc = 6950475 | doi = 10.3390/toxins11120706 | doi-access = free }}</ref> and cell death has been suggested to play a key role in developmental processes, such as [[akinete]] and [[heterocyst]] differentiation, as well as strategy for population survival.<ref name="Programmed Cell Death-Like and Acco"/><ref>{{cite journal | vauthors = Rzymski P, Klimaszyk P, Jurczak T, Poniedziałek B | title = Oxidative Stress, Programmed Cell Death and Microcystin Release in ''Microcystis aeruginosa'' in Response to ''Daphnia'' Grazers | journal = Frontiers in Microbiology | volume = 11 | pages = 1201 | date = 2020 | pmid = 32625177 | pmc = 7311652 | doi = 10.3389/fmicb.2020.01201 | doi-access = free }}</ref><ref name="Meeks-2001">{{cite journal | vauthors = Meeks JC, Elhai J, Thiel T, Potts M, Larimer F, Lamerdin J, Predki P, Atlas R | display-authors = 6 | title = An overview of the genome of Nostoc punctiforme, a multicellular, symbiotic cyanobacterium | journal = Photosynthesis Research | volume = 70 | issue = 1 | pages = 85–106 | year = 2001 | pmid = 16228364 | doi = 10.1023/A:1013840025518 | bibcode = 2001PhoRe..70...85M }}</ref><ref name=Claessen2014 /><ref name=Aguilera2021 />

{| class="wikitable"
|-
! Programmed cell death and microcystins
|-
|[[File:Toxins-11-00706-g001.png|right|thumb|upright=1.5]]
<small>A hypothetical conceptual model couples [[programmed cell death]] and the role of [[microcystin]]s in ''[[Microcystis]]''. (1) The extracellular stressor (e.g., ultraviolet radiation) acts on the cell. (2) Intracellular oxidative stress increases; the intracellular [[reactive oxygen species]] content exceeds the antioxidative capacity of the cell (mediated mostly by an enzymatic system involving a [[superoxide dismutase]], [[catalase]], and [[glutathione peroxidase]] and causes molecular damage. (3) The damage further activates the [[caspase]]-like activity, and [[apoptosis]]-like death is initiated. Simultaneously, intracellular microcystins begin to be released into the extracellular environment. (4) The extracellular microcystins have been significantly released from dead ''Microcystis'' cells. (5) They act on the remaining ''Microcystis'' cells, and exert extracellular roles, for example, extracellular microcystins can increase the production of extracellular [[polysaccharide]]s that are involved in colony formation. Eventually, the colonial form improves the survival of the remaining cells under stressful conditions.<ref name="Programmed Cell Death-Like and Acco">{{cite journal | vauthors = Hu C, Rzymski P | title = Programmed Cell Death-Like and Accompanying Release of Microcystin in Freshwater Bloom-Forming Cyanobacterium ''Microcystis'': From Identification to Ecological Relevance | journal = Toxins | volume = 11 | issue = 12 | pages = 706 | date = December 2019 | pmid = 31817272 | pmc = 6950475 | doi = 10.3390/toxins11120706 | doi-access = free }}</ref></small>
|}

=== Cyanophages ===
{{main|Cyanophage}}
{{further|Marine viruses}}

{{multiple image |total_width=450
|caption_align = center
|image1=Cyanophages.png |caption1=[[Electron micrograph]] of [[negative-stained]] ''[[Prochlorococcus]]'' [[myovirus]]es
|image2=Structure of a Myoviridae bacteriophage 2.jpg |caption2=Typical structure of a myovirus
}}

[[Cyanophage]]s are viruses that infect cyanobacteria. Cyanophages can be found in both freshwater and marine environments.<ref>{{Cite book |title=The Ecology of Cyanobacteria | vauthors = Suttle CA |date=2000-01-01 |publisher=Springer Netherlands |isbn=9780792347354 | veditors = Whitton BA, Potts M |pages=563–589|language=en |doi=10.1007/0-306-46855-7_20 |chapter=Cyanophages and Their Role in the Ecology of Cyanobacteria}}</ref> Marine and freshwater cyanophages have [[Regular icosahedron|icosahedral]] heads, which contain double-stranded DNA, attached to a tail by connector proteins.<ref name=Suttle1993>{{Cite journal | vauthors = Suttle CA, Chan AM |year=1993 |title=Marine cyanophages infecting oceanic and coastal strains of Synechococcus: abundance, . morphology, cross-infectivity and growth characteristics |journal=[[Marine Ecology Progress Series]] |volume=92 |pages=99–109 |bibcode=1993MEPS...92...99S |doi=10.3354/meps092099 |doi-access=free}}</ref> The size of the head and tail vary among species of cyanophages.
Cyanophages, like other [[bacteriophage]]s, rely on [[Brownian motion]] to collide with bacteria, and then use receptor binding proteins to recognize cell surface proteins, which leads to adherence. Viruses with contractile tails then rely on receptors found on their tails to recognize highly conserved proteins on the surface of the host cell.<ref name="Fokine-2014">{{cite journal | vauthors = Fokine A, Rossmann MG | title = Molecular architecture of tailed double-stranded DNA phages | journal = Bacteriophage | volume = 4 | issue = 1 | pages = e28281 | date = January 2014 | pmid = 24616838 | pmc = 3940491 | doi = 10.4161/bact.28281 }}</ref>

Cyanophages infect a wide range of cyanobacteria and are key regulators of the cyanobacterial populations in aquatic environments, and may aid in the prevention of cyanobacterial blooms in freshwater and marine ecosystems. These blooms can pose a danger to humans and other animals, particularly in [[eutrophic]] freshwater lakes. Infection by these viruses is highly prevalent in cells belonging to ''[[Synechococcus]]'' spp. in marine environments, where up to 5% of cells belonging to marine cyanobacterial cells have been reported to contain mature phage particles.<ref name="Proctor">{{Cite journal | vauthors = Proctor LM, Fuhrman JA |year=1990 |title=Viral mortality of marine bacteria and cyanobacteria |journal=[[Nature (journal)|Nature]] |volume=343 |issue=6253 |pages=60–62 |doi=10.1038/343060a0 |bibcode=1990Natur.343...60P }}</ref>

The first cyanophage, [[Cyanophage LPP-1|LPP-1]], was discovered in 1963.<ref name=Sarma /> Cyanophages are classified within the [[bacteriophage]] families ''[[Myoviridae]]'' (e.g. [[cyanophage AS-1|AS-1]], [[cyanophage N-1|N-1]]), ''[[Podoviridae]]'' (e.g. LPP-1) and ''[[Siphoviridae]]'' (e.g. [[cyanophage S-1|S-1]]).<ref name=Sarma>{{cite book |doi=10.1201/b14316 |title=Handbook of Cyanobacteria |date=2012 |last1=Sarma |first1=T. A. |isbn=978-1-4665-5941-7 |chapter=Cyanophages |pages=417–486 |publisher=CRC Press }}</ref>

{{clear}}

== Movement ==
{{further|Cyanobacterial movement|Bacterial motility}}
[[File:Cyanobacteriaunicellularandcolonial020 Synechococcus.jpg|thumb|upright=1| ''[[Synechococcus]]'' uses a gliding technique to move at 25 μm/s. Scale bar is about 10 μm.]]

It has long been known that [[filamentous cyanobacteria]] perform surface motions, and that these movements result from [[type IV pili]].<ref>{{cite journal | vauthors = Duggan PS, Gottardello P, Adams DG | title = Molecular analysis of genes in Nostoc punctiforme involved in pilus biogenesis and plant infection | journal = Journal of Bacteriology | volume = 189 | issue = 12 | pages = 4547–4551 | date = June 2007 | pmid = 17416648 | pmc = 1913353 | doi = 10.1128/JB.01927-06 }}</ref><ref name="Khayatan2015"/><ref>{{cite journal | vauthors = Wilde A, Mullineaux CW | title = Motility in cyanobacteria: polysaccharide tracks and Type IV pilus motors | journal = Molecular Microbiology | volume = 98 | issue = 6 | pages = 998–1001 | date = December 2015 | pmid = 26447922 | doi = 10.1111/mmi.13242 | doi-access = free }}</ref> Additionally, ''[[Synechococcus]]'', a marine cyanobacteria, is known to swim at a speed of 25 μm/s by a mechanism different to that of bacterial flagella.<ref>{{cite journal | vauthors = Waterbury JB, Willey JM, Franks DG, Valois FW, Watson SW | title = A cyanobacterium capable of swimming motility | journal = Science | volume = 230 | issue = 4721 | pages = 74–76 | date = October 1985 | pmid = 17817167 | doi = 10.1126/science.230.4721.74 | bibcode = 1985Sci...230...74W }}</ref> Formation of waves on the cyanobacteria surface is thought to push surrounding water backwards.<ref>{{cite journal | vauthors = Ehlers K, Oster G | title = On the mysterious propulsion of Synechococcus | journal = PLOS ONE | volume = 7 | issue = 5 | pages = e36081 | year = 2012 | pmid = 22567124 | pmc = 3342319 | doi = 10.1371/journal.pone.0036081 | doi-access = free | bibcode = 2012PLoSO...736081E }}</ref><ref name=Miyata2020>{{cite journal | vauthors = Miyata M, Robinson RC, Uyeda TQ, Fukumori Y, Fukushima SI, Haruta S, Homma M, Inaba K, Ito M, Kaito C, Kato K, Kenri T, Kinosita Y, Kojima S, Minamino T, Mori H, Nakamura S, Nakane D, Nakayama K, Nishiyama M, Shibata S, Shimabukuro K, Tamakoshi M, Taoka A, Tashiro Y, Tulum I, Wada H, Wakabayashi KI | display-authors = 6 | title = Tree of motility - A proposed history of motility systems in the tree of life | journal = Genes to Cells | volume = 25 | issue = 1 | pages = 6–21 | date = January 2020 | pmid = 31957229 | pmc = 7004002 | doi = 10.1111/gtc.12737 }} {{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref> Cells are known to be [[motile]] by a gliding method<ref>{{cite book | vauthors = Castenholz RW |year=1982 |chapter=Motility and taxes | veditors = Carr NG, Whitton BA |title=The biology of cyanobacteria |publisher=University of California Press, Berkeley and Los Angeles |pages=413–439 |isbn=978-0-520-04717-4}}</ref> and a novel uncharacterized, non-phototactic swimming method<ref>{{cite journal | vauthors = Waterbury JB, Willey JM, Franks DG, Valois FW, Watson SW | title = A cyanobacterium capable of swimming motility | journal = Science | volume = 230 | issue = 4721 | pages = 74–76 | date = October 1985 | pmid = 17817167 | doi = 10.1126/science.230.4721.74 | name-list-style = amp | bibcode = 1985Sci...230...74W }}</ref> that does not involve flagellar motion.

Many species of cyanobacteria are capable of gliding. [[Gliding motility|Gliding]] is a form of cell movement that differs from crawling or swimming in that it does not rely on any obvious external organ or change in cell shape and it occurs only in the presence of a [[substrate (biology)|substrate]].<ref>{{cite journal | vauthors = McBride MJ | title = Bacterial gliding motility: multiple mechanisms for cell movement over surfaces | journal = Annual Review of Microbiology | volume = 55 | pages = 49–75 | year = 2001 | issue = 1 | pmid = 11544349 | doi = 10.1146/annurev.micro.55.1.49 }}</ref><ref>{{cite journal | vauthors = Reichenbach H | title = Taxonomy of the gliding bacteria | journal = Annual Review of Microbiology | volume = 35 | pages = 339–364 | year = 1981 | issue = 1 | pmid = 6794424 | doi = 10.1146/annurev.mi.35.100181.002011 }}</ref> Gliding in filamentous cyanobacteria appears to be powered by a "slime jet" mechanism, in which the cells extrude a gel that expands quickly as it hydrates providing a propulsion force,<ref>{{cite journal | vauthors = Hoiczyk E, Baumeister W | title = The junctional pore complex, a prokaryotic secretion organelle, is the molecular motor underlying gliding motility in cyanobacteria | journal = Current Biology | volume = 8 | issue = 21 | pages = 1161–1168 | date = October 1998 | pmid = 9799733 | doi = 10.1016/S0960-9822(07)00487-3 | doi-access = free | bibcode = 1998CBio....8.1161H }}</ref><ref>{{cite journal | vauthors = Hoiczyk E | title = Gliding motility in cyanobacterial: observations and possible explanations | journal = Archives of Microbiology | volume = 174 | issue = 1–2 | pages = 11–17 | year = 2000 | pmid = 10985737 | doi = 10.1007/s002030000187 | bibcode = 2000ArMic.174...11H }}</ref> although some [[unicellular]] cyanobacteria use [[type IV pili]] for gliding.<ref>{{cite journal | vauthors = Bhaya D, Watanabe N, Ogawa T, Grossman AR | title = The role of an alternative sigma factor in motility and pilus formation in the cyanobacterium Synechocystis sp. strain PCC6803 | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 96 | issue = 6 | pages = 3188–3193 | date = March 1999 | pmid = 10077659 | pmc = 15917 | doi = 10.1073/pnas.96.6.3188 | doi-access = free | bibcode = 1999PNAS...96.3188B }}</ref><ref name="Tamulonis2011" />

Cyanobacteria have strict light requirements. Too little light can result in insufficient energy production, and in some species may cause the cells to resort to heterotrophic respiration.<ref name="Stal2000book" /> Too much light can inhibit the cells, decrease photosynthesis efficiency and cause damage by bleaching. UV radiation is especially deadly for cyanobacteria, with normal solar levels being significantly detrimental for these microorganisms in some cases.<ref name=Donkor1991>{{cite journal | vauthors = Tamulonis C, Postma M, Kaandorp J | title = Modeling filamentous cyanobacteria reveals the advantages of long and fast trichomes for optimizing light exposure | journal = PLOS ONE | volume = 6 | issue = 7 | pages = e22084 | year = 2011 | pmid = 21789215 | pmc = 3138769 | doi = 10.1371/journal.pone.0022084 | doi-access = free | bibcode = 2011PLoSO...622084T }}</ref><ref name=Donkor1993>{{cite journal |doi=10.1111/j.1574-6941.1993.tb00026.x |title=Effects of tropical solar radiation on the motility of filamentous cyanobacteria |year=1993 | vauthors = Donkor VA, Amewowor DH, Häder DP |journal=FEMS Microbiology Ecology |volume=12 |issue=2 |pages=143–147 |bibcode=1993FEMME..12..143D |doi-access=free}}</ref><ref name=Tamulonis2011 />

Filamentous cyanobacteria that live in microbial mats often migrate vertically and horizontally within the mat in order to find an optimal niche that balances their light requirements for photosynthesis against their sensitivity to photodamage. For example, the filamentous cyanobacteria ''[[Oscillatoria]]'' sp. and ''[[Spirulina (genus)|Spirulina subsalsa]]'' found in the hypersaline benthic mats of [[Guerrero Negro]], Mexico migrate downwards into the lower layers during the day in order to escape the intense sunlight and then rise to the surface at dusk.<ref>{{cite journal | vauthors = Garcia-Pichel F, Mechling M, Castenholz RW | title = Diel Migrations of Microorganisms within a Benthic, Hypersaline Mat Community | journal = Applied and Environmental Microbiology | volume = 60 | issue = 5 | pages = 1500–1511 | date = May 1994 | pmid = 16349251 | pmc = 201509 | doi = 10.1128/aem.60.5.1500-1511.1994 | bibcode = 1994ApEnM..60.1500G }}</ref> In contrast, the population of ''Microcoleus chthonoplastes'' found in hypersaline mats in [[Camargue]], France migrate to the upper layer of the mat during the day and are spread homogeneously through the mat at night.<ref>{{cite journal | vauthors = Fourçans A, Solé A, Diestra E, Ranchou-Peyruse A, Esteve I, Caumette P, Duran R | title = Vertical migration of phototrophic bacterial populations in a hypersaline microbial mat from Salins-de-Giraud (Camargue, France) | journal = FEMS Microbiology Ecology | volume = 57 | issue = 3 | pages = 367–377 | date = September 2006 | pmid = 16907751 | doi = 10.1111/j.1574-6941.2006.00124.x | bibcode = 2006FEMME..57..367F | doi-access = free }}</ref> An in vitro experiment using ''[[Phormidium|Phormidium uncinatum]]'' also demonstrated this species' tendency to migrate in order to avoid damaging radiation.<ref name=Donkor1991 /><ref name=Donkor1993 /> These migrations are usually the result of some sort of photomovement, although other forms of taxis can also play a role.<ref>{{cite journal | vauthors = Richardson LL, Castenholz RW | title = Diel Vertical Movements of the Cyanobacterium Oscillatoria terebriformis in a Sulfide-Rich Hot Spring Microbial Mat | journal = Applied and Environmental Microbiology | volume = 53 | issue = 9 | pages = 2142–2150 | date = September 1987 | pmid = 16347435 | pmc = 204072 | doi = 10.1128/aem.53.9.2142-2150.1987 | bibcode = 1987ApEnM..53.2142R }}</ref><ref name=Tamulonis2011 />

Photomovement – the modulation of cell movement as a function of the incident light – is employed by the cyanobacteria as a means to find optimal light conditions in their environment. There are three types of photomovement: photokinesis, phototaxis and photophobic responses.<ref name="Häder1987">{{cite journal |doi=10.1111/j.1751-1097.1987.tb04745.x |title=EFFECTS OF UV-B IRRADIATION ON PHOTOMOVEMENT IN THE DESMID, Cosmarium cucumis |year=1987 | vauthors = Häder DP |journal=Photochemistry and Photobiology |volume=46 |issue=1 |pages=121–126 }}</ref><ref>{{cite journal | vauthors = Nultsch W, Häder DP | title = Photomovement in motile microorganisms--II | journal = Photochemistry and Photobiology | volume = 47 | issue = 6 | pages = 837–869 | date = June 1988 | pmid = 3064112 | doi = 10.1111/j.1751-1097.1988.tb01668.x | doi-access = free }}</ref><ref>{{cite book |doi=10.1201/9780203495902-126 |chapter=Photomovements of Microorganisms: An Introduction |title=CRC Handbook of Organic Photochemistry and Photobiology |date=2003 |pages=2393–2402 |publisher=CRC Press |isbn=978-0-429-20964-2 |editor1-first=William M. |editor1-last=Horspool |editor2-first=Francesco |editor2-last=Lenci }}</ref><ref name=Tamulonis2011 />

Photokinetic microorganisms modulate their gliding speed according to the incident light intensity. For example, the speed with which ''Phormidium autumnale'' glides increases linearly with the incident light intensity.<ref>{{cite journal |last1=Nultsch |first1=Wilhelm |title=DER EINFLUSS DES LICHTES AUF DIE BEWEGUNG DER CYANOPHYCEEN: III. Mitteilung: PHOTOPHOBOTAXIS VON PHORMIDIUM UNCINATUM |journal=Planta |date=1962 |volume=58 |issue=6 |pages=647–663 |doi=10.1007/BF01914754 |jstor=23364646 |bibcode=1962Plant..58..647N }}</ref><ref name=Tamulonis2011 />

Phototactic microorganisms move according to the direction of the light within the environment, such that positively phototactic species will tend to move roughly parallel to the light and towards the light source. Species such as ''Phormidium uncinatum'' cannot steer directly towards the light, but rely on random collisions to orient themselves in the right direction, after which they tend to move more towards the light source. Others, such as ''[[Anabaena variabilis]]'', can steer by bending the [[trichome]].<ref>{{cite journal |doi=10.1007/BF00408050 |title=Investigations on the phototactic orientation of Anabaena variabilis |year=1979 | vauthors = Nultsch W, Schuchart H, Höhl M |journal=Archives of Microbiology |volume=122 |issue=1 |pages=85–91 |bibcode=1979ArMic.122...85N }}</ref><ref name=Tamulonis2011 />

Finally, photophobic microorganisms respond to spatial and temporal light gradients. A step-up photophobic reaction occurs when an organism enters a brighter area field from a darker one and then reverses direction, thus avoiding the bright light. The opposite reaction, called a step-down reaction, occurs when an organism enters a dark area from a bright area and then reverses direction, thus remaining in the light.<ref name=Tamulonis2011 />

{{clear}}

== Evolution ==

=== Earth history ===
{{life timeline}}
[[Stromatolites]] are layered biochemical [[accretion (geology)|accretion]]ary [[structure]]s formed in shallow water by the trapping, binding, and cementation of sedimentary grains by [[biofilm]]s ([[microbial mat]]s) of [[microorganism]]s, especially cyanobacteria.<ref>{{cite journal | vauthors = Riding R |year=2007 |title=The term stromatolite: towards an essential definition |journal=[[Lethaia]] |volume=32 |issue=4 |pages=321–30 |doi=10.1111/j.1502-3931.1999.tb00550.x}}</ref>

Cyanobacteria likely first evolved in a freshwater environment.<ref name=freshwater/> During the [[Precambrian]], stromatolite communities of microorganisms grew in most marine and non-marine environments in the [[photic zone]]. After the Cambrian explosion of marine animals, grazing on the stromatolite mats by herbivores greatly reduced the occurrence of the stromatolites in marine environments. Since then, they are found mostly in hypersaline conditions where grazing invertebrates cannot live (e.g. [[Shark Bay]], Western Australia). Stromatolites provide ancient records of life on Earth by fossil remains which date from 3.5 [[annus#SI prefix multipliers|Ga]] ago.<ref>{{cite journal |last1=Baumgartner |first1=Raphael J. |last2=Van Kranendonk |first2=Martin J. |last3=Wacey |first3=David |last4=Fiorentini |first4=Marco L. |last5=Saunders |first5=Martin |last6=Caruso |first6=Stefano |last7=Pages |first7=Anais |last8=Homann |first8=Martin |last9=Guagliardo |first9=Paul |title=Nano−porous pyrite and organic matter in 3.5-billion-year-old stromatolites record primordial life |journal=Geology |date=November 2019 |volume=47 |issue=11 |pages=1039–1043 |doi=10.1130/G46365.1 |bibcode=2019Geo....47.1039B |url=https://archimer.ifremer.fr/doc/00637/74900/ }}</ref> The oldest undisputed evidence of cyanobacteria is dated to be 2.1 Ga ago, but there is some evidence for them as far back as 2.7 Ga ago.<ref name="Schirrmeister-2013">{{cite journal | vauthors = Schirrmeister BE, de Vos JM, Antonelli A, Bagheri HC | title = Evolution of multicellularity coincided with increased diversification of cyanobacteria and the Great Oxidation Event | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 110 | issue = 5 | pages = 1791–1796 | date = January 2013 | pmid = 23319632 | pmc = 3562814 | doi = 10.1073/pnas.1209927110 | doi-access = free | bibcode = 2013PNAS..110.1791S }}</ref> Cyanobacteria might have also emerged 3.5 Ga ago.<ref>{{cite journal | vauthors = Schopf JW, Packer BM | title = Early Archean (3.3-billion to 3.5-billion-year-old) microfossils from Warrawoona Group, Australia | journal = Science | volume = 237 | issue = 4810 | pages = 70–73 | date = July 1987 | pmid = 11539686 | doi = 10.1126/science.11539686 | bibcode = 1987Sci...237...70S }}</ref> Oxygen concentrations in the atmosphere remained around or below 0.001% of today's level until 2.4 Ga ago (the [[Great Oxygenation Event]]).<ref>{{cite journal | vauthors = Farquhar J, Bao H, Thiemens M | title = Atmospheric influence of Earth's earliest sulfur cycle | journal = Science | volume = 289 | issue = 5480 | pages = 756–759 | date = August 2000 | pmid = 10926533 | doi = 10.1126/science.289.5480.756 | bibcode = 2000Sci...289..756F }}</ref> The rise in oxygen may have caused a fall in the concentration of [[atmospheric methane]], and triggered the [[Huronian glaciation]] from around 2.4 to 2.1 Ga ago. In this way, cyanobacteria may have killed off most of the other bacteria of the time.<ref name=Lane>{{cite news |author-link=Nick Lane | vauthors = Lane N |date=6 February 2010 |url=https://www.newscientist.com/article/mg20527461-100-first-breath-earths-billion-year-struggle-for-oxygen/ |title=First breath: Earth's billion-year struggle for oxygen |work=[[New Scientist]] |pages=36–39 }} See accompanying graph as well.</ref>

[[Oncolite]]s are [[sedimentary structures]] composed of oncoids, which are layered structures formed by cyanobacterial growth. Oncolites are similar to stromatolites, but instead of forming columns, they form approximately spherical structures that were not attached to the underlying substrate as they formed.<ref name=Corsetti2003>{{cite journal | vauthors = Corsetti FA, Awramik SM, Pierce D | title = A complex microbiota from snowball Earth times: microfossils from the Neoproterozoic Kingston Peak Formation, Death Valley, USA | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 8 | pages = 4399–4404 | date = April 2003 | pmid = 12682298 | pmc = 153566 | doi = 10.1073/pnas.0730560100 | doi-access = free | bibcode = 2003PNAS..100.4399C }}</ref> The oncoids often form around a central nucleus, such as a shell fragment,<ref>{{cite journal |last1=Gutschick |first1=Raymond Charles |last2=Perry |first2=Thomas Gregory |title=Sappington (Kinderhookian) sponges and their environment [Montana] |journal=Journal of Paleontology |date=November 1959 |volume=33 |issue=6 |pages=977–985 |url=https://pubs.geoscienceworld.org/paleosoc/jpaleontol/article/33/6/977/79261/Sappington-Kinderhookian-sponges-and-their }}
</ref> and a [[calcium carbonate]] structure is deposited by encrusting [[microbes]]. Oncolites are indicators of warm waters in the [[photic zone]], but are also known in contemporary freshwater environments.<ref>{{cite book |doi=10.1007/978-3-642-52335-9 |title=Calcareous Algae and Stromatolites |date=1991 |isbn=978-3-642-52337-3 |editor-last1=Riding |editor-first1=Robert |page=32 |publisher=Springer |location=Berlin, Heidelberg }}</ref> These structures rarely exceed 10&nbsp;cm in diameter.

One former classification scheme of cyanobacterial fossils divided them into the [[porostromata]] and the [[spongiostromata]]. These are now recognized as [[form taxa]] and considered taxonomically obsolete; however, some authors have advocated for the terms remaining informally to describe form and structure of bacterial fossils.<ref>{{Cite book | vauthors = Monty CL |chapter=Spongiostromate vs. Porostromate Stromatolites and Oncolites | veditors = Monty CL |title=Phanerozoic Stromatolites  |date=1981 |location=Berlin, Heidelberg |publisher=Springer |pages=1–4 |doi=10.1007/978-3-642-67913-1_1 |isbn=978-3-642-67913-1 }}</ref>

<gallery mode="packed" style="float:left" heights="145px">
File:Stromatolites.jpg| [[Stromatolites]] left behind by cyanobacteria are the oldest known fossils of life on Earth. This fossil is one billion years old.
File:Oncolitic limestone (central Utah, USA) 3.jpg| Oncolitic limestone formed from successive layers of calcium carbonate precipitated by cyanobacteria
File:OncolitesAlamoBreccia.jpg| [[Oncolite]]s from the [[Late Devonian]] [[Alamo bolide impact]] in Nevada
File:Oscillatoriopsis longa fossil.jpg| {{center|Cyanobacterial remains of an annulated tubular [[microfossil]] ''Oscillatoriopsis longa''{{hsp}}<ref>{{cite journal |doi=10.1111/pala.12374 |title=First record of Cyanobacteria in Cambrian Orsten deposits of Sweden |year=2018 | vauthors = Castellani C, Maas A, Eriksson ME, Haug JT, Haug C, Waloszek D |journal=[[Palaeontology (journal)|Palaeontology]] |volume=61 |issue=6 |pages=855–880 |bibcode=2018Palgy..61..855C }}</ref><br /><small>Scale bar: 100 μm</small>}}
</gallery>

{{clear left}}

=== Origin of photosynthesis ===
{{further|Evolution of photosynthesis}}

[[Oxygenic photosynthesis]] only evolved once (in prokaryotic cyanobacteria), and all photosynthetic [[eukaryote]]s (including all [[plant]]s and [[algae]]) have acquired this ability from [[symbiogenesis|endosymbiosis]] with cyanobacteria or their [[endosymbiont]] hosts. In other words, all the oxygen that makes the atmosphere breathable for [[aerobic organism]]s originally comes from cyanobacteria or their [[plastid]] descendants.<ref>{{Cite web |url=https://phys.org/news/2017-03-oxygen-cyanobacteria.html |title=How do plants make oxygen? Ask cyanobacteria |date=30 March 2017 |website=[[Phys.org]] |publisher=Science X |access-date=2017-10-26}}</ref>

Cyanobacteria remained the principal [[primary producers]] throughout the latter half of the [[Archean]] [[eon (geology)|eon]] and most of the [[Proterozoic eon]], in part because the redox structure of the oceans favored photoautotrophs capable of [[nitrogen fixation]]. However, their population is argued to have varied considerably across this eon.<ref name=pwc/><ref>{{Cite journal | vauthors = Crockford PW, Kunzmann M, Bekker A, Hayles J, Bao H, Halverson GP, Peng Y, Bui TH, Cox GM, Gibson TM, Wörndle S | display-authors = 6 |date=2019-05-20 |title=Claypool continued: Extending the isotopic record of sedimentary sulfate |journal=Chemical Geology |volume=513 |pages=200–225 |doi=10.1016/j.chemgeo.2019.02.030 | bibcode = 2019ChGeo.513..200C }}</ref><ref>{{cite journal | vauthors = Hodgskiss MS, Crockford PW, Peng Y, Wing BA, Horner TJ | title = A productivity collapse to end Earth's Great Oxidation | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 116 | issue = 35 | pages = 17207–17212 | date = August 2019 | pmid = 31405980 | pmc = 6717284 | doi = 10.1073/pnas.1900325116 | bibcode = 2019PNAS..11617207H | doi-access = free }}</ref> [[Archaeplastid]]s such as [[green algae|green]] and [[red algae]] eventually surpassed cyanobacteria as major primary producers on [[continental shelf|continental shelves]] near the end of the [[Neoproterozoic]], but only with the [[Mesozoic]] (251–65 Ma) radiations of secondary photoautotrophs such as [[dinoflagellate]]s, [[Coccolithophore|coccolithophorids]] and [[diatom]]s did [[primary production]] in marine shelf waters take modern form. Cyanobacteria remain critical to [[marine ecosystem]]s as primary producers in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the plastids of [[marine algae]].<ref name=Herrero>{{cite book | vauthors = Herrero A | veditors = Flores E |title=The Cyanobacteria: Molecular Biology, Genomics and Evolution |edition=1st |publisher=Caister Academic Press |year=2008 |isbn=978-1-904455-15-8}}</ref>

=== Origin of chloroplasts ===
{{See also|Chloroplast#Chloroplast lineages and evolution}}

Primary chloroplasts are cell organelles found in some [[eukaryote|eukaryotic]] lineages, where they are specialized in performing photosynthesis. They are considered to have evolved from [[endosymbiotic]] cyanobacteria.<ref name="ReferenceA">{{cite journal | vauthors = Keeling PJ | title = The number, speed, and impact of plastid endosymbioses in eukaryotic evolution | journal = Annual Review of Plant Biology | volume = 64 | pages = 583–607 | year = 2013 | issue = 1 | pmid = 23451781 | doi = 10.1146/annurev-arplant-050312-120144 | bibcode = 2013AnRPB..64..583K }}</ref><ref>{{cite journal | vauthors = Moore KR, Magnabosco C, Momper L, Gold DA, Bosak T, Fournier GP | title = An Expanded Ribosomal Phylogeny of Cyanobacteria Supports a Deep Placement of Plastids | journal = Frontiers in Microbiology | volume = 10 | pages = 1612 | date = 2019 | pmid = 31354692 | pmc = 6640209 | doi = 10.3389/fmicb.2019.01612 | doi-access = free }}</ref> After some years of debate,<ref>{{cite journal | vauthors = Howe CJ, Barbrook AC, Nisbet RE, Lockhart PJ, Larkum AW | title = The origin of plastids | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 363 | issue = 1504 | pages = 2675–2685 | date = August 2008 | pmid = 18468982 | pmc = 2606771 | doi = 10.1098/rstb.2008.0050 }}</ref> it is now generally accepted that the three major groups of primary endosymbiotic eukaryotes (i.e. [[Viridiplantae|green plants]], [[Rhodophytes|red algae]] and [[glaucophyte]]s) form one large [[Monophyly|monophyletic group]] called [[Archaeplastida]], which evolved after one unique endosymbiotic event.<ref name="Rodríguez-Ezpeleta N 2005">{{cite journal | vauthors = Rodríguez-Ezpeleta N, Brinkmann H, Burey SC, Roure B, Burger G, Löffelhardt W, Bohnert HJ, Philippe H, Lang BF | display-authors = 6 | title = Monophyly of primary photosynthetic eukaryotes: green plants, red algae, and glaucophytes | journal = Current Biology | volume = 15 | issue = 14 | pages = 1325–1330 | date = July 2005 | pmid = 16051178 | doi = 10.1016/j.cub.2005.06.040 | doi-access = free | bibcode = 2005CBio...15.1325R }}</ref><ref>{{cite journal | vauthors = Adl SM, Simpson AG, Lane CE, Lukeš J, Bass D, Bowser SS, Brown MW, Burki F, Dunthorn M, Hampl V, Heiss A, Hoppenrath M, Lara E, Le Gall L, Lynn DH, McManus H, Mitchell EA, Mozley-Stanridge SE, Parfrey LW, Pawlowski J, Rueckert S, Shadwick L, Schoch CL, Smirnov A, Spiegel FW | display-authors = 6 | title = The revised classification of eukaryotes | journal = The Journal of Eukaryotic Microbiology | volume = 59 | issue = 5 | pages = 429–493 | date = September 2012 | pmid = 23020233 | pmc = 3483872 | doi = 10.1111/j.1550-7408.2012.00644.x }}</ref><ref>{{cite journal | vauthors = Price DC, Chan CX, Yoon HS, Yang EC, Qiu H, Weber AP, Schwacke R, Gross J, Blouin NA, Lane C, Reyes-Prieto A, Durnford DG, Neilson JA, Lang BF, Burger G, Steiner JM, Löffelhardt W, Meuser JE, Posewitz MC, Ball S, Arias MC, Henrissat B, Coutinho PM, Rensing SA, Symeonidi A, Doddapaneni H, Green BR, Rajah VD, Boore J, Bhattacharya D | display-authors = 6 | title = Cyanophora paradoxa genome elucidates origin of photosynthesis in algae and plants | journal = Science | volume = 335 | issue = 6070 | pages = 843–847 | date = February 2012 | pmid = 22344442 | doi = 10.1126/science.1213561 | bibcode = 2012Sci...335..843P | url = https://digitalcommons.uri.edu/bio_facpubs/480 }}</ref><ref name="Ponce-Toledo RI 2016">{{cite journal | vauthors = Ponce-Toledo RI, Deschamps P, López-García P, Zivanovic Y, Benzerara K, Moreira D | title = An Early-Branching Freshwater Cyanobacterium at the Origin of Plastids | journal = Current Biology | volume = 27 | issue = 3 | pages = 386–391 | date = February 2017 | pmid = 28132810 | pmc = 5650054 | doi = 10.1016/j.cub.2016.11.056 | bibcode = 2017CBio...27..386P }}</ref>

The [[Morphology (biology)|morphological]] similarity between chloroplasts and cyanobacteria was first reported by German botanist [[Andreas Franz Wilhelm Schimper]] in the 19th century<ref name="Schimper">{{cite journal | vauthors = Schimper AF |author-link=Andreas Franz Wilhelm Schimper |title=Über die Entwicklung der Chlorophyllkörner und Farbkörper |trans-title=About the development of the chlorophyll grains and stains |language=de |journal=Bot. Zeitung |year=1883 |volume=41 |pages=105–14, 121–31, 137–46, 153–62 |url=http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/19551 |url-status=dead |archive-url=https://web.archive.org/web/20131019121025/http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/19551 |archive-date=19 October 2013|df=dmy-all}}</ref> Chloroplasts are only found in [[plant]]s and [[algae]],<ref name="Molecular biology of the cell—chloroplasts and photosynthesis">{{cite book |last1=Alberts |first1=Bruce |last2=Johnson |first2=Alexander |last3=Lewis |first3=Julian |last4=Raff |first4=Martin |last5=Roberts |first5=Keith |last6=Walter |first6=Peter |title=Molecular Biology of the Cell |edition=4th |date=2002 |publisher=Garland Science |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK26819/ |chapter=Chloroplasts and Photosynthesis }}</ref> thus paving the way for Russian biologist [[Konstantin Mereschkowski]] to suggest in 1905 the symbiogenic origin of the plastid.<ref>{{cite journal |vauthors=Mereschkowsky C |title=Über Natur und Ursprung der Chromatophoren im Pflanzenreiche |trans-title=About the nature and origin of chromatophores in the vegetable kingdom |language=de |journal=Biol Centralbl |year=1905 |volume=25 |pages=593–604 |url=https://archive.org/details/cbarchive_51353_bernaturundursprungderchromato1881}}</ref> [[Lynn Margulis]] brought this hypothesis back to attention more than 60 years later<ref>{{cite journal | vauthors = Sagan L | title = On the origin of mitosing cells | journal = Journal of Theoretical Biology | volume = 14 | issue = 3 | pages = 255–274 | date = March 1967 | pmid = 11541392 | doi = 10.1016/0022-5193(67)90079-3 | bibcode = 1967JThBi..14..225S }}</ref> but the idea did not become fully accepted until supplementary data started to accumulate. The cyanobacterial origin of plastids is now supported by various pieces of [[Phylogenetics|phylogenetic]],<ref>{{cite journal | vauthors = Schwartz RM, Dayhoff MO | title = Origins of prokaryotes, eukaryotes, mitochondria, and chloroplasts | journal = Science | volume = 199 | issue = 4327 | pages = 395–403 | date = January 1978 | pmid = 202030 | doi = 10.1126/science.202030 | bibcode = 1978Sci...199..395S }}</ref><ref name="Rodríguez-Ezpeleta N 2005"/><ref name="Ponce-Toledo RI 2016"/> [[Genomics|genomic]],<ref>{{cite journal | vauthors = Archibald JM | title = Genomic perspectives on the birth and spread of plastids | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 112 | issue = 33 | pages = 10147–10153 | date = August 2015 | pmid = 25902528 | pmc = 4547232 | doi = 10.1073/pnas.1421374112 | doi-access = free | bibcode = 2015PNAS..11210147A }}</ref> biochemical<ref>{{cite journal | vauthors = Blankenship RE | title = Early evolution of photosynthesis | journal = Plant Physiology | volume = 154 | issue = 2 | pages = 434–438 | date = October 2010 | pmid = 20921158 | pmc = 2949000 | doi = 10.1104/pp.110.161687 }}</ref><ref>{{cite journal | vauthors = Rockwell NC, Lagarias JC, Bhattacharya D | title = Primary endosymbiosis and the evolution of light and oxygen sensing in photosynthetic eukaryotes | journal = Frontiers in Ecology and Evolution | volume = 2 | issue = 66 | year = 2014 | pmid = 25729749 | pmc = 4343542 | doi = 10.3389/fevo.2014.00066 | doi-access = free | bibcode = 2014FrEEv...2...66R }}</ref> and structural evidence.<ref>Summarised in {{cite journal | vauthors = Cavalier-Smith T | title = Membrane heredity and early chloroplast evolution | journal = Trends in Plant Science | volume = 5 | issue = 4 | pages = 174–182 | date = April 2000 | pmid = 10740299 | doi = 10.1016/S1360-1385(00)01598-3 }}</ref> The description of another independent and more recent primary endosymbiosis event between a cyanobacterium and a separate eukaryote lineage (the [[rhizaria]]n ''[[Paulinella]] chromatophora'') also gives credibility to the endosymbiotic origin of the plastids.<ref>{{cite journal | vauthors = Nowack EC, Melkonian M, Glöckner G | title = Chromatophore genome sequence of Paulinella sheds light on acquisition of photosynthesis by eukaryotes | journal = Current Biology | volume = 18 | issue = 6 | pages = 410–418 | date = March 2008 | pmid = 18356055 | doi = 10.1016/j.cub.2008.02.051 | doi-access = free | bibcode = 2008CBio...18..410N }}</ref>

{{multiple image |total_width=450|caption_align = center
|image1=Glaucocystis sp.jpg |caption1=The chloroplasts of [[glaucophyte]]s have a [[peptidoglycan]] layer, evidence suggesting their endosymbiotic origin from cyanobacteria.<ref name="keeling">{{cite journal | vauthors = Keeling PJ | title = Diversity and evolutionary history of plastids and their hosts | journal = American Journal of Botany | volume = 91 | issue = 10 | pages = 1481–1493 | date = October 2004 | pmid = 21652304 | doi = 10.3732/ajb.91.10.1481 | doi-access = free | bibcode = 2004AmJB...91.1481K }}</ref>
|image2=Plagiomnium affine laminazellen.jpeg |caption2=Plant cells with visible chloroplasts (from a moss, ''[[Plagiomnium affine]]'')
}}
In addition to this primary endosymbiosis, many eukaryotic lineages have been subject to [[Secondary endosymbiosis|secondary]] or even [[tertiary endosymbiotic events]], that is the "[[Matryoshka doll|Matryoshka]]-like" engulfment by a eukaryote of another plastid-bearing eukaryote.<ref>{{cite journal | vauthors = Archibald JM | title = The puzzle of plastid evolution | journal = Current Biology | volume = 19 | issue = 2 | pages = R81–R88 | date = January 2009 | pmid = 19174147 | doi = 10.1016/j.cub.2008.11.067 | doi-access = free | bibcode = 2009CBio...19..R81A }}</ref><ref name="ReferenceA"/>

[[Chloroplast]]s have many similarities with cyanobacteria, including a circular [[chromosome]], prokaryotic-type [[ribosome]]s, and similar proteins in the photosynthetic reaction center.<ref>{{cite journal | vauthors = Douglas SE | title = Plastid evolution: origins, diversity, trends | journal = Current Opinion in Genetics & Development | volume = 8 | issue = 6 | pages = 655–661 | date = December 1998 | pmid = 9914199 | doi = 10.1016/S0959-437X(98)80033-6 }}</ref><ref>{{cite journal | vauthors = Reyes-Prieto A, Weber AP, Bhattacharya D | title = The origin and establishment of the plastid in algae and plants | journal = Annual Review of Genetics | volume = 41 | pages = 147–168 | year = 2007 | issue = 1 | pmid = 17600460 | doi = 10.1146/annurev.genet.41.110306.130134 }}</ref> The [[endosymbiotic theory]] suggests that photosynthetic bacteria were acquired (by [[endocytosis]]) by early [[Eukaryote|eukaryotic]] cells to form the first [[plant]] cells. Therefore, chloroplasts may be photosynthetic bacteria that adapted to life inside plant cells. Like [[mitochondrion|mitochondria]], chloroplasts still possess their own DNA, separate from the [[nuclear DNA]] of their plant host cells and the genes in this chloroplast DNA resemble those in cyanobacteria.<ref>{{cite journal | vauthors = Raven JA, Allen JF | title = Genomics and chloroplast evolution: what did cyanobacteria do for plants? | journal = Genome Biology | volume = 4 | issue = 3 | pages = 209 | year = 2003 | pmid = 12620099 | pmc = 153454 | doi = 10.1186/gb-2003-4-3-209 | doi-access = free }}</ref> DNA in chloroplasts codes for [[redox]] proteins such as photosynthetic reaction centers. The [[CoRR hypothesis]] proposes this co-location is required for redox regulation.

=== Origin of marine planktonic cyanobacteria ===
[[File:Timing and trends in cell diameter, loss of filamentous forms and habitat preference within cyanobacteria.webp|thumb|upright=1.5|left|{{center|Timing and trends in cell diameter, loss of filamentous forms and habitat preference within cyanobacteria}} Based on data: nodes (1–10) and stars representing common ancestors from Sánchez-Baracaldo et al., 2015,<ref name="Sánchez-Baracaldo2016">{{cite journal | vauthors = Sánchez-Baracaldo P | title = Origin of marine planktonic cyanobacteria | journal = Scientific Reports | volume = 5 | pages = 17418 | date = December 2015 | issue = 1 | pmid = 26621203 | pmc = 4665016 | doi = 10.1038/srep17418 | bibcode = 2015NatSR...517418S }} {{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref> timing of the [[Great Oxidation Event]] (GOE),<ref name="Bekker2004">{{cite journal | vauthors = Bekker A, Holland HD, Wang PL, Rumble D, Stein HJ, Hannah JL, Coetzee LL, Beukes NJ | display-authors = 6 | title = Dating the rise of atmospheric oxygen | journal = Nature | volume = 427 | issue = 6970 | pages = 117–120 | date = January 2004 | pmid = 14712267 | doi = 10.1038/nature02260 | bibcode = 2004Natur.427..117B }}</ref> the [[Lomagundi-Jatuli Excursion]],<ref>{{cite journal | vauthors = Kump LR, Junium C, Arthur MA, Brasier A, Fallick A, Melezhik V, Lepland A, Crne AE, Luo G | display-authors = 6 | title = Isotopic evidence for massive oxidation of organic matter following the great oxidation event | journal = Science | volume = 334 | issue = 6063 | pages = 1694–1696 | date = December 2011 | pmid = 22144465 | doi = 10.1126/science.1213999 | doi-access = free | bibcode = 2011Sci...334.1694K }}</ref> and [[Gunflint formation]].<ref>{{cite journal |doi=10.1139/e02-028 |title=The age of the Gunflint Formation, Ontario, Canada: Single zircon U–Pb age determinations from reworked volcanic ash |year=2002 | vauthors = Fralick P, Davis DW, Kissin SA |journal=Canadian Journal of Earth Sciences |volume=39 |issue=7 |pages=1085–1091 |bibcode=2002CaJES..39.1085F}}</ref> Green lines represent freshwater lineages and blue lines represent marine lineages are based on Bayesian inference of character evolution (stochastic character mapping analyses).<ref name="Sánchez-Baracaldo2016" /> {{center|<small>Taxa are not drawn to scale – those with smaller cell diameters are at the bottom and larger at the top</small>}}]]

{{plankton sidebar|taxonomy}}

Cyanobacteria have fundamentally transformed the geochemistry of the planet.<ref name=Holland2006>{{cite journal | vauthors = Holland HD | title = The oxygenation of the atmosphere and oceans | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 361 | issue = 1470 | pages = 903–915 | date = June 2006 | pmid = 16754606 | pmc = 1578726 | doi = 10.1098/rstb.2006.1838 }}</ref><ref name="Bekker2004"/> Multiple lines of geochemical evidence support the occurrence of intervals of profound global environmental change at the beginning and end of the [[Proterozoic]] (2,500–542 Mya).<ref name=Planavsky2014>{{cite journal | vauthors = Planavsky NJ, Reinhard CT, Wang X, Thomson D, McGoldrick P, Rainbird RH, Johnson T, Fischer WW, Lyons TW | display-authors = 6 | title = Earth history. Low mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals | journal = Science | volume = 346 | issue = 6209 | pages = 635–638 | date = October 2014 | pmid = 25359975 | doi = 10.1126/science.1258410 | bibcode = 2014Sci...346..635P | url = https://resolver.caltech.edu/CaltechAUTHORS:20141009-185334240 }}</ref>
<ref name=Lyons2014>{{cite journal | vauthors = Lyons TW, Reinhard CT, Planavsky NJ | title = The rise of oxygen in Earth's early ocean and atmosphere | journal = Nature | volume = 506 | issue = 7488 | pages = 307–315 | date = February 2014 | pmid = 24553238 | doi = 10.1038/nature13068 | bibcode = 2014Natur.506..307L }}</ref><ref name=Sahoo2012>{{cite journal | vauthors = Sahoo SK, Planavsky NJ, Kendall B, Wang X, Shi X, Scott C, Anbar AD, Lyons TW, Jiang G | display-authors = 6 | title = Ocean oxygenation in the wake of the Marinoan glaciation | journal = Nature | volume = 489 | issue = 7417 | pages = 546–549 | date = September 2012 | pmid = 23018964 | doi = 10.1038/nature11445 | bibcode = 2012Natur.489..546S }}</ref> While it is widely accepted that the presence of molecular oxygen in the early fossil record was the result of cyanobacteria activity, little is known about how cyanobacteria evolution (e.g., habitat preference) may have contributed to changes in [[biogeochemical cycle]]s through Earth history. Geochemical evidence has indicated that there was a first step-increase in the oxygenation of the Earth's surface, which is known as the [[Great Oxidation Event]] (GOE), in the early [[Paleoproterozoic]] (2,500–1,600 Mya).<ref name=Holland2006 /><ref name=Bekker2004 /> A second but much steeper increase in oxygen levels, known as the Neoproterozoic Oxygenation Event (NOE),<ref name=Lyons2014 /><ref name="Och_2012" /><ref name=Canfield2007>{{cite journal | vauthors = Canfield DE, Poulton SW, Narbonne GM | title = Late-Neoproterozoic deep-ocean oxygenation and the rise of animal life | journal = Science | volume = 315 | issue = 5808 | pages = 92–95 | date = January 2007 | pmid = 17158290 | doi = 10.1126/science.1135013 | bibcode = 2007Sci...315...92C | doi-access = free }}</ref> occurred at around 800 to 500 Mya.<ref name=Sahoo2012 /><ref name=Fike2006>{{cite journal | vauthors = Fike DA, Grotzinger JP, Pratt LM, Summons RE | title = Oxidation of the Ediacaran ocean | journal = Nature | volume = 444 | issue = 7120 | pages = 744–747 | date = December 2006 | pmid = 17151665 | doi = 10.1038/nature05345 | bibcode = 2006Natur.444..744F | url = https://resolver.caltech.edu/CaltechAUTHORS:20130212-145114208 }}</ref> Recent [[chromium]] isotope data point to low levels of atmospheric oxygen in the Earth's surface during the mid-Proterozoic,<ref name=Planavsky2014 /> which is consistent with the late evolution of marine planktonic cyanobacteria during the [[Cryogenian]];<ref name="Sánchez-Baracaldo2014">{{cite journal | vauthors = Sánchez-Baracaldo P, Ridgwell A, Raven JA | title = A neoproterozoic transition in the marine nitrogen cycle | journal = Current Biology | volume = 24 | issue = 6 | pages = 652–657 | date = March 2014 | pmid = 24583016 | doi = 10.1016/j.cub.2014.01.041 | doi-access = free | bibcode = 2014CBio...24..652S }}</ref> both types of evidence help explain the late emergence and diversification of animals.<ref name=Erwin2011>{{cite journal | vauthors = Erwin DH, Laflamme M, Tweedt SM, Sperling EA, Pisani D, Peterson KJ | title = The Cambrian conundrum: early divergence and later ecological success in the early history of animals | journal = Science | volume = 334 | issue = 6059 | pages = 1091–1097 | date = November 2011 | pmid = 22116879 | doi = 10.1126/science.1206375 | bibcode = 2011Sci...334.1091E }}</ref><ref name="Sánchez-Baracaldo2016" />

Understanding the evolution of planktonic cyanobacteria is important because their origin fundamentally transformed the [[nitrogen cycle|nitrogen]] and [[carbon cycle]]s towards the end of the [[Pre-Cambrian]].<ref name=Fike2006 /> It remains unclear, however, what evolutionary events led to the emergence of open-ocean planktonic forms within cyanobacteria and how these events relate to geochemical evidence during the Pre-Cambrian.<ref name=Lyons2014 /> So far, it seems that ocean geochemistry (e.g., [[euxinic]] conditions during the early- to mid-Proterozoic)<ref name=Lyons2014 /><ref name=Canfield2007 /><ref>{{cite journal |doi=10.2113/gselements.7.2.107 |title=Ferruginous Conditions: A Dominant Feature of the Ocean through Earth's History |year=2011 | vauthors = Poulton SW, Canfield DE |journal=Elements |volume=7 |issue=2 |pages=107–112|bibcode=2011Eleme...7..107P }}</ref> and nutrient availability{{hsp}}<ref>{{cite journal | vauthors = Anbar AD, Knoll AH | title = Proterozoic ocean chemistry and evolution: a bioinorganic bridge? | journal = Science | volume = 297 | issue = 5584 | pages = 1137–1142 | date = August 2002 | pmid = 12183619 | doi = 10.1126/science.1069651 | bibcode = 2002Sci...297.1137A }}</ref> likely contributed to the apparent delay in diversification and widespread colonization of open ocean environments by planktonic cyanobacteria during the [[Neoproterozoic]].<ref name=Fike2006 /><ref name="Sánchez-Baracaldo2016" />
{{clear}}

== Genetics ==
Cyanobacteria are capable of natural genetic [[transformation (genetics)|transformation]].<ref name="pmid4209657">{{cite journal | vauthors = Orkwiszewski KG, Kaney AR | title = Genetic transformation of the blue-green bacterium, Anacystis nidulans | journal = Archiv für Mikrobiologie | volume = 98 | issue = 1 | pages = 31–37 | date = June 1974 | pmid = 4209657 | doi = 10.1007/BF00425265 | bibcode = 1974ArMic..98...31O }}</ref><ref name="pmid16592896">{{cite journal | vauthors = Stevens SE, Porter RD | title = Transformation in Agmenellum quadruplicatum | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 77 | issue = 10 | pages = 6052–6056 | date = October 1980 | pmid = 16592896 | pmc = 350211 | doi = 10.1073/pnas.77.10.6052 | doi-access = free | bibcode = 1980PNAS...77.6052S }}</ref><ref>{{cite journal |vauthors=Grigorieva G, Shestakov S |year=1982 |title=Transformation in the cyanobacterium Synechocystis sp. 6803 |journal=[[FEMS Microbiology Letters]] |volume=13 |issue=4 |pages=367–70 |doi=10.1111/j.1574-6968.1982.tb08289.x |doi-access=free}}</ref> Natural genetic transformation is the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous DNA from its surroundings. For bacterial transformation to take place, the recipient bacteria must be in a state of [[natural competence|competence]], which may occur in nature as a response to conditions such as starvation, high cell density or exposure to DNA damaging agents. In chromosomal transformation, homologous transforming DNA can be integrated into the recipient genome by [[homologous recombination]], and this process appears to be an adaptation for [[DNA repair|repairing DNA damage]].<ref name="pmid29111273">{{cite journal | vauthors = Bernstein H, Bernstein C, Michod RE | title = Sex in microbial pathogens | journal = Infection, Genetics and Evolution | volume = 57 | pages = 8–25 | date = January 2018 | pmid = 29111273 | doi = 10.1016/j.meegid.2017.10.024 | doi-access = free | bibcode = 2018InfGE..57....8B }}</ref>

=== DNA repair ===
Cyanobacteria are challenged by environmental stresses and internally generated [[reactive oxygen species]] that cause [[DNA damage (naturally occurring)|DNA damage]]. Cyanobacteria possess numerous ''[[Escherichia coli|E. coli]]''-like [[DNA repair]] [[gene]]s.<ref name="pmid27881980">{{cite journal | vauthors = Cassier-Chauvat C, Veaudor T, Chauvat F | title = Comparative Genomics of DNA Recombination and Repair in Cyanobacteria: Biotechnological Implications | journal = Frontiers in Microbiology | volume = 7 | pages = 1809 | date = 2016 | pmid = 27881980 | pmc = 5101192 | doi = 10.3389/fmicb.2016.01809 | doi-access = free }}</ref> Several DNA repair genes are highly conserved in cyanobacteria, even in small [[genome]]s, suggesting that core DNA repair processes such as [[homologous recombination|recombinational repair]], [[nucleotide excision repair]] and methyl-directed [[DNA mismatch repair]] are common among cyanobacteria.<ref name="pmid27881980" />

== Classification ==

=== Taxonomy ===
{{See also|Bacterial taxonomy}}
[[File:Haeckel arbol bn.png|thumb|upright|Tree of Life in ''Generelle Morphologie der Organismen'' (1866). Note the location of the genus
''[[Nostoc]]'' with algae and not with bacteria (kingdom "Monera")]]
Historically, bacteria were first classified as plants constituting the class Schizomycetes, which along with the Schizophyceae (blue-green algae/Cyanobacteria) formed the phylum Schizophyta,<ref name=nageli>{{cite journal |vauthors=Von Nägeli C |veditors=Caspary R |year=1857 |title=Bericht über die Verhandlungen der 33. Versammlung deutscher Naturforscher und Ärzte, gehalten in Bonn von 18 bis 24 September 1857 |trans-title=Report on the Proceedings of the 33rd Meeting of German Natural Scientists and Physicians, held in Bonn, 18 to 24 September 1857 |journal=[[Botanische Zeitung]] |volume=15 |pages=749–776}}</ref> then in the phylum [[Monera]] in the kingdom [[Protista]] by [[Haeckel]] in 1866, comprising ''Protogens, Protamaeba, Vampyrella, Protomonae'', and ''Vibrio'', but not ''Nostoc'' and other cyanobacteria, which were classified with algae,<ref>{{cite book | vauthors = Haeckel E |year=1867 |title=Generelle Morphologie der Organismen |url=https://archive.org/details/bub_gb_-5k5AAAAcAAJ |publisher=Reimer, Berlin}}</ref> later reclassified as the ''[[Prokaryotes]]'' by [[Édouard Chatton|Chatton]].<ref>{{cite journal |vauthors=Chatton É |title=''Pansporella perplexa'': amœbien à spores protégées parasite des daphnies réflexions sur la biologie et la phylogénie des protozoaires |trans-title=''Pansporella perplexa'': amoebian with protected spores parasite of daphnia reflections on the biology and phylogeny of protozoa |journal=Annales des sciences naturelles: Zoologie |series=10 |volume=VII |pages=1–84 |year=1925 |language=fr |url=https://gallica.bnf.fr/ark:/12148/bpt6k55415433 }}</ref>

The cyanobacteria were traditionally classified by morphology into five sections, referred to by the numerals I–V. The first three – [[Chroococcales]], [[Pleurocapsales]], and [[Oscillatoriales]] – are not supported by phylogenetic studies. The latter two – [[Nostocales]] and [[Stigonematales]] – are monophyletic as a unit, and make up the heterocystous cyanobacteria.<ref name="Gugger_2004">{{cite journal | vauthors = Gugger MF, Hoffmann L | title = Polyphyly of true branching cyanobacteria (Stigonematales) | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 54 | issue = Pt 2 | pages = 349–357 | date = March 2004 | pmid = 15023942 | doi = 10.1099/ijs.0.02744-0 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Howard-Azzeh M, Shamseer L, Schellhorn HE, Gupta RS | title = Phylogenetic analysis and molecular signatures defining a monophyletic clade of heterocystous cyanobacteria and identifying its closest relatives | journal = Photosynthesis Research | volume = 122 | issue = 2 | pages = 171–185 | date = November 2014 | pmid = 24917519 | doi = 10.1007/s11120-014-0020-x | bibcode = 2014PhoRe.122..171H }}</ref>

The members of Chroococales are unicellular and usually aggregate in colonies. The classic taxonomic criterion has been the cell morphology and the plane of cell division. In Pleurocapsales, the cells have the ability to form internal spores (baeocytes). The rest of the sections include filamentous species. In Oscillatoriales, the cells are uniseriately arranged and do not form specialized cells (akinetes and heterocysts).<ref>{{cite journal |url=http://www.preslia.cz/P144Komarek.pdf |title=Taxonomic classification of cyanoprokaryotes (cyanobacterial genera) 2014, using a polyphasic approach |vauthors=Komárek J, Kaštovský J, Mareš J, Johansen JR |journal=[[Preslia]] |volume=86 |pages=295–335 |year=2014}}</ref> In Nostocales and Stigonematales, the cells have the ability to develop heterocysts in certain conditions. Stigonematales, unlike Nostocales, include species with truly branched trichomes.<ref name="Gugger_2004" />

Most taxa included in the phylum Cyanobacteria have not yet been validly published under ''The International Code of Nomenclature of Prokaryotes'' (ICNP)<ref>https://www.microbiologyresearch.org/content/journal/ijsem/10.1099/ijsem.0.005585a{{dead link|date=February 2025}}{{full|date=February 2025}}</ref> and are instead validly published under the [[International Code of Nomenclature for algae, fungi, and plants]]. These exceptions are validly published under ICNP:
* The phylum Cyanobacteriota
* The families [[Prochloraceae]] and [[Prochlorotrichaceae]]
* The genera ''[[Cyanobacterium (genus)|Cyanobacterium]]'', ''[[Halospirulina]]'', ''[[Planktothricoides]]'', ''[[Prochlorococcus]]'', ''[[Prochloron]]'', and ''[[Prochlorothrix]]''

Formerly, some bacteria, like ''[[Beggiatoa]]'', were thought to be colorless Cyanobacteria.<ref name="Pringsheim1963">{{cite book | vauthors = Pringsheim EG |title=Farblose Algen: Ein Beitrag zur Evolutionsforschung | trans-title = Colorless Algae: A Contribution to Evolutionary Research | language = de |url={{google books |plainurl=y |id=RYnPAAAAMAAJ}} |year=1963 |publisher=Gustav Fischer Verlag}}</ref>

{{as of|2014}} the taxonomy was under revision.<ref name="Cyanophyceae cladistics">{{cite journal |vauthors=Silva PC, Moe RL |date=Dec 2019 |title=Cyanophyceae |publisher=[[McGraw Hill Education]] |journal=AccessScience |doi=10.1036/1097-8542.175300 }}</ref><ref>{{cite journal | vauthors = Oren A | title = A proposal for further integration of the cyanobacteria under the Bacteriological Code | journal = International Journal of Systematic and Evolutionary Microbiology | volume = 54 | issue = Pt 5 | pages = 1895–1902 | date = September 2004 | pmid = 15388760 | doi = 10.1099/ijs.0.03008-0 | doi-access = free }}</ref>

=== Current taxonomy ===
The currently accepted taxonomy as of 2025 is based on [[National Center for Biotechnology Information]] (NCBI).<ref>{{cite web |url=https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Tree&id=1117&lvl=3&lin=f&keep=1&srchmode=1&unlock |title=Cyanobacteria |access-date=2021-03-20 |publisher=[[National Center for Biotechnology Information]] (NCBI) taxonomy database |display-authors=etal}}</ref> More authoritative sources include [[List of Prokaryotic names with Standing in Nomenclature]] (LPSN)<ref>{{cite web | vauthors = Euzéby JP |url=https://lpsn.dsmz.de/phylum/cyanobacteria |title="Cyanobacteria" |access-date=2022-01-22 |publisher=[[List of Prokaryotic names with Standing in Nomenclature]] (LPSN)}}</ref> and [[AlgaeBase]]. The 2023 summary by Strunecký et al. is also useful.<ref name=Strunecky23>{{cite journal |last1=Strunecký |first1=Otakar |last2=Ivanova |first2=Anna Pavlovna |last3=Mareš |first3=Jan |title=An updated classification of cyanobacterial orders and families based on phylogenomic and polyphasic analysis |journal=Journal of Phycology |date=February 2023 |volume=59 |issue=1 |pages=12–51 |doi=10.1111/jpy.13304|pmid=36443823 |bibcode=2023JPcgy..59...12S }}</ref>
* Class [[Cyanophyceae]]
** Order [[Acaryochloridales]] <small>Miyashita et al. 2003 ex Strunecký & Mareš 2022</small> (syn. "Thermosynechococcales")
*** Family Thermosynechococcaceae
** Order [[Aegeococcales]] <small>Strunecký & Mareš 2022</small>
** Order [[Chroococcales]] (synonyms [[Pleurocapsales]])
** Order [[Chroococcidiopsidales]]
** Order [[Coleofasciculales]]
** Order [[Desertifilales]]
** Order [[Geitlerinematales]] <small>Strunecký & Mareš 2022</small>
** Order [[Gloeobacterales]]
** Order [[Gloeomargaritales]] <small>Moreira et al. 2016</small>
** Order [[Gomontiellales]]
** Order [[Graniferales]]
** Order [[Leptolyngbyales]] (synonym Phormidesmiales) <small>Strunecký & Mareš 2022</small>
** Order [[Nodosilineales]] <small>Strunecký & Mareš 2022</small>
** Order [[Nostocales]] (synonym Stigonematales)
** Order [[Oculatellales]] (synonym Elainellales) <small>Strunecký & Mareš 2022</small>
** Order [[Oscillatoriales]]
** Order [[Pelonematales]]
** Order [[Prochlorotrichales]] <small>Strunecký & Mareš 2022</small> (PCC-9006)
*** Family Prochlorococcaceae <small>Komárek & Strunecky 2020</small> {"PCC-6307"}
** Order [[Sarmaellales]]
** Order [[Spirulinales]]
** Order [[Synechococcales]] (synonym Pseudanabaenales) <small>Hoffmann, Komárek & Kastovsky 2005</small>
** Order [[Thermostichales]] <small>Komárek & Strunecký 2020</small>
* Class [[Melainabacteria|Vampirovibrionophyceae]]
** Order [[Vampirovibrionales]]

=== Phylogeny ===
Notes:
# The botanical and bacteriological communities disagree on the name and scope of this phylum or division. Specifically, the bacteriological community prefer the name Cyanobacteriota not necessarily including the non-photosynthetic Vampirovibrionophyceae, while the botanical community prefers the name Cyanobacteria and the inclusion of Vampirovibrionophyceae. Some bacteriologists refer to Vampirovibrionophyceae as a phylum Melainabacteria or Melainobacteriota.<p>In the dedrograms below, botanical (ICNafp) names are put above the line, and bacteriological (ICNP) names below the line if it differs from the botanical. In addition, a popular bacteriological synonym for Cyanobacteriota s.s. is "Cyanobacteriia".</p>
# The discovery and study of non-photosynthetic lineages related to typical photosynthetic cyanobacteria (Cyanophyceae) is still very active. The treatment of these groups may change.
# The GTDB tree contains a lot of links to non-existent pages because GTDB re-assigns the boundaries of taxonomic levels based on genomic divergence. The type genus of these invented taxa can be inferred from the name.
#* For example, Cyanobacteriales is formed from ''Cyanobacterium'' {{au|Rippka & Cohen-Bazire 1983 [validated 2022]}} (ICNP) and includes important genera such as ''Nostoc''.<ref>[https://gtdb.ecogenomic.org/tree?r=s__Nostoc_B%20piscinale GTDB tree at s__Nostoc_B piscinale]</ref>

<div style="overflow-x:scroll">
{| class="wikitable"
|-
! colspan=1 | 16S rRNA based [[The All-Species Living Tree Project|LTP]]_12_2021<ref>{{cite web |title=The LTP |url=https://imedea.uib-csic.es/mmg/ltp/#LTP |access-date=23 February 2021}}</ref><ref>{{cite web |title=LTP_all tree in newick format |url=https://imedea.uib-csic.es/mmg/ltp/wp-content/uploads/ltp/Tree_LTP_all_12_2021.ntree |access-date=23 February 2021}}</ref><ref>{{cite web |title=LTP_12_2021 Release Notes |url=https://imedea.uib-csic.es/mmg/ltp/wp-content/uploads/ltp/LTP_12_2021_release_notes.pdf |access-date=23 February 2021}}</ref>
! colspan=1 | GTDB 08-RS214 by [[Genome Taxonomy Database]]<ref name="about">{{cite web |title=GTDB release 08-RS214 |url=https://gtdb.ecogenomic.org/about#4%7C |website=[[Genome Taxonomy Database]]|access-date=10 May 2023}}</ref><ref name="tree">{{cite web |title=bac120_r214.sp_label |url=https://data.gtdb.ecogenomic.org/releases/release214/214.0/auxillary_files/bac120_r214.sp_labels.tree |website=[[Genome Taxonomy Database]]|access-date=10 May 2023}}</ref><ref name="taxon_history">{{cite web |title=Taxon History |url=https://gtdb.ecogenomic.org/taxon_history/ |website=[[Genome Taxonomy Database]]|access-date=10 May 2023}}</ref>
|-
| style="vertical-align:top|
{{Clade | style=font-size:90%;line-height:80%
|label1=Cyanobacteria
|sublabel1="Cyanobacteria/Melainabacteria clade"
|1={{clade
  |label1=Vampirovibrionophyceae
  |sublabel1="[[Melainobacteriota]]"
  |1="[[Vampirovibrionales]]"
  |label2=Cyanophyceae
  |sublabel2=Cyanobacteriota s.s.
  |2={{clade
    |label1=
    |1=[[Gloeobacterales]]
    |label2="Phycobacteria"
    |2={{clade
      |1="[[Thermosynechococcales]]"
      |2={{clade
        |label1=Synechococcophycidae
        |1=[[Synechococcales]]
        |label2=Nostocophycidae
        |2={{clade
          |1={{clade
            |1={{clade
              |1=[[Pleurocapsales]]
              |2=[[Spirulinales]]
               }}
            |2=[[Chroococcales]]
             }}
          |2={{clade
            |1=[[Oscillatoriales]]
            |2=[[Nostocales]]
             }}
           }}
         }}
       }}
     }}
   }}
}}
|
{{clade|style=font-size:90%;line-height:80%;width:600px
|1={{Clade
  |label1="[[Margulisbacteria]]"
  |1={{Clade
    |1="[[Saganbacteria]]" (WOR1)
    |2={{Clade
      |1="[[Marinamargulisbacteria]]"
      |2={{Clade
        |1="[[Riflemargulisbacteria]]" (GWF2_35_9)
        |2="[[Termititenacia]]"
         }}
       }}
     }}
  |label2=Cyanobacteria
  |sublabel2="Cyanobacteria/Melainabacteria clade"
  |2={{clade
    |1={{clade
      |label1=Vampirovibrionophyceae
      |sublabel1="[[Melainobacteriota]]"
      |1={{clade
        |1={{clade
          |1="[[Caenarcanales]]"
          |2="[[Obscuribacterales]]"
           }}
        |2={{clade
          |1="[[Vampirovibrionales]]"
          |2="[[Gastranaerophilales]]"
           }}
         }}
       }}
    |2={{clade
      |sublabel1="[[Sericytochromatia]]"
      |1={{Clade
        |1=UBA7694 ("Blackallbacteria")
        |2=S15B-MN24 ("Sericytochromatia"; "Tanganyikabacteria")
         }}
      |label2=Cyanophyceae
      |sublabel2=Cyanobacteriota s.s.
      |2={{clade
        |1={{clade
          |sublabel1="[[Gloeobacteria]]"
          |1=[[Gloeobacterales]]
           }}
        |2={"Phycobacteria"}
         }}
       }}
     }}
   }}
|targetA={"Phycobacteria"}
|subcladeA={{Clade
|label1="Phycobacteria"
|1={{Clade hidden |id=1 |mode=left
  |1=[[Thermostichales]]
  |2={{Clade
    |1=[[Pseudanabaenales]] (''[[Pseudanabaena]]'' + ''[[Tumidithrix]]'')
    |2={{Clade
      |1={{Clade
        |1=[[Gloeoemargaritales]]
        |2=[[Prochlorococcaceae]] {PCC-6307}
         }}
      |2={{Clade
        |1="[[Acaryochloridales]]"
        |2={{Clade
          |1={{Clade
            |label1=Synechococcophycidae
            |1={{Clade
              |1="[[Limnotrichales]]"{{failed verification|date=May 2025}}
              |2={{Clade
                |1=[[Prochlorotrichales]] {PCC-9006}
                |2=[[Synechococcales]] (''[[Synechococcus]]'')
                 }}
               }}
             }}
          |2={{Clade
            |1={{Clade
              |1="[[Phormidesmiales]]"
              |2="[[Nodosilineales]]" (non-GTDB split)
               }}
            |2={{Clade
              |1={{Clade
                |1=[[Oculatellales]] (non-GTDB split)
                |2="[[Elainellales]]"
                 }}
              |2={{Clade
                |1="[[Leptolyngbyales]]"
                |2={{Clade
                  |label1=Nostocophycidae
                  |1=[[Cyanobacteriales]]
                   }}
                 }}
               }}
             }}
           }}
         }}
       }}
     }}
   }}
           }}
}}
|}
</div>

Example of different circumscriptions among sources:
* LPSN uses Cyanobacteriota s.l. with two classes, with the botanical -phyceae class suffix.
* GTDB uses Cyanobacteriota s.l. with three classes, the added one being Sericytochromatia. The bacteriological class suffix -ia is used, hence "Cyanobacteriia" and "Vampirovibrionia".
* NCBI uses Cyanobacteriota s.s. In addition, its Cyanobacteriota/Melainabacteria group includes not only Cyanobacteriota s.l., but also "''Ca.'' Margulisiibacteriota" and "''Ca.''  Adamsella". (In GTDB, "''Ca.'' Adamsella" is nested in Gastranaerophilales.)
* AlgaeBase uses Cyanobacteria with only Cyanophyceae.<ref>{{cite web |title=Taxonomy Browser |url=https://www.algaebase.org/browse/taxonomy/#4305 |website=AlgaeBase}}</ref>
* Strunecký et al. (2023) uses Cyanobacteria with two botanical classes.<ref name=Strunecky23/>

== Relation to humans ==
=== Biotechnology ===
[[File:Blue-green algae cultured in specific media.jpg|thumb|right|Cyanobacteria cultured in specific media: Cyanobacteria can be helpful in agriculture as they have the ability to fix atmospheric nitrogen in soil.]]
The unicellular cyanobacterium ''[[Synechocystis]]'' sp. PCC6803 was the third prokaryote and first photosynthetic organism whose [[genome]] was completely [[DNA sequencing|sequenced]].<ref>{{cite journal | vauthors = Kaneko T, Sato S, Kotani H, Tanaka A, Asamizu E, Nakamura Y, Miyajima N, Hirosawa M, Sugiura M, Sasamoto S, Kimura T, Hosouchi T, Matsuno A, Muraki A, Nakazaki N, Naruo K, Okumura S, Shimpo S, Takeuchi C, Wada T, Watanabe A, Yamada M, Yasuda M, Tabata S | display-authors = 6 | title = Sequence analysis of the genome of the unicellular cyanobacterium Synechocystis sp. strain PCC6803. II. Sequence determination of the entire genome and assignment of potential protein-coding regions | journal = DNA Research | volume = 3 | issue = 3 | pages = 109–136 | date = June 1996 | pmid = 8905231 | doi = 10.1093/dnares/3.3.109 | doi-access = free }}</ref> It continues to be an important model organism.<ref>{{cite journal | vauthors = Tabei Y, Okada K, Tsuzuki M | title = Sll1330 controls the expression of glycolytic genes in Synechocystis sp. PCC 6803 | journal = Biochemical and Biophysical Research Communications | volume = 355 | issue = 4 | pages = 1045–1050 | date = April 2007 | pmid = 17331473 | doi = 10.1016/j.bbrc.2007.02.065 }}</ref> ''[[Crocosphaera|Crocosphaera subtropica]]'' ATCC 51142 is an important [[diazotroph]]ic model organism.<ref>{{cite journal |last1=Mareš |first1=Jan |last2=Johansen |first2=Jeffrey R. |last3=Hauer |first3=Tomáš |last4=Zima |first4=Jan |last5=Ventura |first5=Stefano |last6=Cuzman |first6=Oana |last7=Tiribilli |first7=Bruno |last8=Kaštovský |first8=Jan |title=Taxonomic resolution of the genus Cyanothece (Chroococcales, Cyanobacteria), with a treatment on Gloeothece and three new genera, Crocosphaera, Rippkaea , and Zehria |journal=Journal of Phycology |date=June 2019 |volume=55 |issue=3 |pages=578–610 |doi=10.1111/jpy.12853 |pmid=30830691 |bibcode=2019JPcgy..55..578M }}</ref> The smallest genomes of a photosynthetic organism have been found in ''Prochlorococcus'' spp. (1.7 [[Genome size|Mb]])<ref>{{cite journal | vauthors = Rocap G, Larimer FW, Lamerdin J, Malfatti S, Chain P, Ahlgren NA, Arellano A, Coleman M, Hauser L, Hess WR, Johnson ZI, Land M, Lindell D, Post AF, Regala W, Shah M, Shaw SL, Steglich C, Sullivan MB, Ting CS, Tolonen A, Webb EA, Zinser ER, Chisholm SW | display-authors = 6 | title = Genome divergence in two Prochlorococcus ecotypes reflects oceanic niche differentiation | journal = Nature | volume = 424 | issue = 6952 | pages = 1042–1047 | date = August 2003 | pmid = 12917642 | doi = 10.1038/nature01947 | doi-access = free | bibcode = 2003Natur.424.1042R }}</ref><ref>{{cite journal | vauthors = Dufresne A, Salanoubat M, Partensky F, Artiguenave F, Axmann IM, Barbe V, Duprat S, Galperin MY, Koonin EV, Le Gall F, Makarova KS, Ostrowski M, Oztas S, Robert C, Rogozin IB, Scanlan DJ, Tandeau de Marsac N, Weissenbach J, Wincker P, Wolf YI, Hess WR | display-authors = 6 | title = Genome sequence of the cyanobacterium Prochlorococcus marinus SS120, a nearly minimal oxyphototrophic genome | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 17 | pages = 10020–10025 | date = August 2003 | pmid = 12917486 | pmc = 187748 | doi = 10.1073/pnas.1733211100 | doi-access = free | bibcode = 2003PNAS..10010020D }}</ref> and the largest in ''[[Nostoc punctiforme]]'' (9 Mb).<ref name="Meeks-2001"/> Those of ''[[Calothrix]]'' spp. are estimated at 12–15 Mb,<ref>{{Cite journal |doi=10.1099/00221287-111-1-73 |title=Genome Size of Cyanobacteria |journal=Journal of General Microbiology |volume=111 |pages=73–85 |year=1979 |vauthors=Herdman M, Janvier M, Rippka R, Stanier RY |issue=1 |doi-access=free}}</ref> as large as [[yeast]].

Recent research has suggested the potential application of cyanobacteria to the generation of [[renewable energy]] by directly converting sunlight into electricity. Internal photosynthetic pathways can be coupled to chemical mediators that transfer electrons to external [[electrodes]].<ref>{{cite journal | vauthors = Quintana N, Van der Kooy F, Van de Rhee MD, Voshol GP, Verpoorte R | title = Renewable energy from Cyanobacteria: energy production optimization by metabolic pathway engineering | journal = Applied Microbiology and Biotechnology | volume = 91 | issue = 3 | pages = 471–490 | date = August 2011 | pmid = 21691792 | pmc = 3136707 | doi = 10.1007/s00253-011-3394-0 }}</ref><ref>{{cite journal | vauthors = Chen X, Lawrence JM, Wey LT, Schertel L, Jing Q, Vignolini S, Howe CJ, Kar-Narayan S, Zhang JZ | display-authors = 6 | title = 3D-printed hierarchical pillar array electrodes for high-performance semi-artificial photosynthesis | journal = Nature Materials | volume = 21 | issue = 7 | pages = 811–818 | date = July 2022 | pmid = 35256790 | doi = 10.1038/s41563-022-01205-5 | bibcode = 2022NatMa..21..811C | url = https://www.repository.cam.ac.uk/handle/1810/334792 }}</ref> In the shorter term, efforts are underway to commercialize [[algae-based fuels]] such as [[diesel fuel|diesel]], [[gasoline]], and [[jet fuel]].<ref name="Pisciotta JM, Zou Y, Baskakov IV 2010 e108212"/><ref>[http://www.thehindu.com/sci-tech/science/article477049.ece "Blue green bacteria may help generate 'green' electricity"], ''The Hindu'', 21 June 2010</ref><ref>{{Cite web |date=2010-09-14 |title=Joule wins key patent for GMO cyanobacteria that create fuels from sunlight, CO2 and water : Biofuels Digest |url=https://www.biofuelsdigest.com/bdigest/2010/09/14/joule-wins-key-patent-for-gmo-cyanobacteria-that-create-fuels-from-sunlight-co2-and-water/ |access-date=2022-08-10 |language=en-US |archive-date=2 July 2022 |archive-url=https://web.archive.org/web/20220702180847/https://www.biofuelsdigest.com/bdigest/2010/09/14/joule-wins-key-patent-for-gmo-cyanobacteria-that-create-fuels-from-sunlight-co2-and-water/ |url-status=dead }}</ref> Cyanobacteria have been also engineered to produce ethanol<ref>{{cite journal | vauthors = Deng MD, Coleman JR | title = Ethanol synthesis by genetic engineering in cyanobacteria | journal = Applied and Environmental Microbiology | volume = 65 | issue = 2 | pages = 523–528 | date = February 1999 | pmid = 9925577 | pmc = 91056 | doi = 10.1128/AEM.65.2.523-528.1999 | bibcode = 1999ApEnM..65..523D }}</ref> and experiments have shown that when one or two CBB genes are being over expressed, the yield can be even higher.<ref>{{Cite journal |title=Engineering photoautotrophic carbon fixation for enhanced growth and productivity | vauthors = Liang F, Lindberg P, Lindblad P |date=20 November 2018 |journal=Sustainable Energy & Fuels |volume=2 |issue=12 |pages=2583–2600 |doi=10.1039/C8SE00281A|doi-access=free }}</ref><ref>{{cite journal | vauthors = Roussou S, Albergati A, Liang F, Lindblad P | title = Engineered cyanobacteria with additional overexpression of selected Calvin-Benson-Bassham enzymes show further increased ethanol production | journal = Metabolic Engineering Communications | volume = 12 | pages = e00161 | date = June 2021 | pmid = 33520653 | pmc = 7820548 | doi = 10.1016/j.mec.2021.e00161 }}</ref>

Cyanobacteria may possess the ability to produce substances that could one day serve as anti-inflammatory agents and combat bacterial infections in humans.<ref>{{cite journal | vauthors = Choi H, Mascuch SJ, Villa FA, Byrum T, Teasdale ME, Smith JE, Preskitt LB, Rowley DC, Gerwick L, Gerwick WH | display-authors = 6 | title = Honaucins A-C, potent inhibitors of inflammation and bacterial quorum sensing: synthetic derivatives and structure-activity relationships | journal = Chemistry & Biology | volume = 19 | issue = 5 | pages = 589–598 | date = May 2012 | pmid = 22633410 | pmc = 3361693 | doi = 10.1016/j.chembiol.2012.03.014 }}</ref> Cyanobacteria's photosynthetic output of sugar and oxygen has been demonstrated to have therapeutic value in rats with heart attacks.<ref>{{cite news | vauthors = Frischkorn K |title=Need to Fix a Heart Attack? Try Photosynthesis |url=https://www.smithsonianmag.com/science-nature/how-light-activated-bacteria-could-help-heal-heart-attack-180963756/ |access-date=20 May 2021 |work=[[Smithsonian (magazine)|Smithsonian]] |date=19 June 2017}}</ref> While cyanobacteria can naturally produce various secondary metabolites, they can serve as advantageous hosts for plant-derived metabolites production owing to biotechnological advances in systems biology and synthetic biology.<ref>{{cite journal | vauthors = Jeong Y, Cho SH, Lee H, Choi HK, Kim DM, Lee CG, Cho S, Cho BK | display-authors = 6 | title = Current Status and Future Strategies to Increase Secondary Metabolite Production from Cyanobacteria | journal = Microorganisms | volume = 8 | issue = 12 | pages = 1849 | date = November 2020 | pmid = 33255283 | pmc = 7761380 | doi = 10.3390/microorganisms8121849 | doi-access = free }}</ref>

Spirulina's extracted blue color is used as a natural food coloring.<ref>{{cite journal | vauthors = Newsome AG, Culver CA, van Breemen RB | title = Nature's palette: the search for natural blue colorants | journal = Journal of Agricultural and Food Chemistry | volume = 62 | issue = 28 | pages = 6498–6511 | date = July 2014 | pmid = 24930897 | doi = 10.1021/jf501419q | bibcode = 2014JAFC...62.6498N }}</ref>

Researchers from several space agencies argue that cyanobacteria could be used for producing goods for human consumption in future crewed outposts on Mars, by transforming materials available on this planet.<ref name="Verseux2015">{{cite journal | vauthors = Verseux C, Baqué M, Lehto K, de Vera JP, Rothschild LJ, Billi D  |author5-link=Lynn J. Rothschild |title=Sustainable life support on Mars – the potential roles of cyanobacteria |journal=International Journal of Astrobiology |volume=15 |issue=1 |pages=65–92 |date=2016 |doi=10.1017/S147355041500021X |bibcode=2016IJAsB..15...65V |doi-access=free}}</ref>

=== Human nutrition<span class="anchor" id="nutrition"></span> ===
[[File:Spirulina tablets.jpg|thumb|right|[[Spirulina (dietary supplement)|Spirulina]] tablets]]
Some cyanobacteria are sold as food, notably ''[[Arthrospira platensis]] ([[Spirulina (dietary supplement)|Spirulina]]), [[Aphanizomenon flos-aquae]] ([[Klamath Lake AFA]])'', and others.<ref>{{cite journal | vauthors = Spolaore P, Joannis-Cassan C, Duran E, Isambert A | title = Commercial applications of microalgae | journal = Journal of Bioscience and Bioengineering | volume = 101 | issue = 2 | pages = 87–96 | date = February 2006 | pmid = 16569602 | doi = 10.1263/jbb.101.87 | url = https://hal.science/hal-00133263/file/joannis-cassan_2771.pdf }}</ref>

Some [[microalgae]] contain substances of high biological value, such as [[Polyunsaturated fat|polyunsaturated fatty acids]], [[amino acid]]s, [[protein]]s, pigments, [[antioxidant]]s, [[vitamin]]s, and minerals.<ref>{{cite journal | vauthors = Christaki E, Florou-Paneri P, Bonos E | title = Microalgae: a novel ingredient in nutrition | journal = International Journal of Food Sciences and Nutrition | volume = 62 | issue = 8 | pages = 794–799 | date = December 2011 | pmid = 21574818 | doi = 10.3109/09637486.2011.582460 }}</ref> Edible blue-green algae reduce the production of pro-inflammatory cytokines by inhibiting NF-κB pathway in macrophages and splenocytes.<ref>{{cite journal | vauthors = Ku CS, Pham TX, Park Y, Kim B, Shin MS, Kang I, Lee J | title = Edible blue-green algae reduce the production of pro-inflammatory cytokines by inhibiting NF-κB pathway in macrophages and splenocytes | journal = Biochimica et Biophysica Acta (BBA) - General Subjects | volume = 1830 | issue = 4 | pages = 2981–2988 | date = April 2013 | pmid = 23357040 | pmc = 3594481 | doi = 10.1016/j.bbagen.2013.01.018 }}</ref> Sulfate polysaccharides exhibit immunomodulatory, antitumor, antithrombotic, anticoagulant, anti-mutagenic, anti-inflammatory, antimicrobial, and even antiviral activity against [[HIV]], [[herpes]], and [[hepatitis]].<ref>{{cite book |doi=10.1016/B978-0-12-394597-6.00003-3 |chapter=Health Benefits of Algal Polysaccharides in Human Nutrition |title=Advances in Food and Nutrition Research |date=2012 |last1=Mišurcová |first1=Ladislava |last2=Škrovánková |first2=Soňa |last3=Samek |first3=Dušan |last4=Ambrožová |first4=Jarmila |last5=Machů |first5=Ludmila |volume=66 |pages=75–145 |pmid=22909979 |isbn=978-0-12-394597-6 }}</ref>

=== Health risks ===
{{main|Cyanotoxin}}
Some cyanobacteria can produce [[neurotoxins]], [[cytotoxins]], [[endotoxins]], and [[hepatotoxins]] (e.g., the [[microcystin]]-producing bacteria genus [[microcystis]]), which are collectively known as [[cyanotoxin]]s.

Specific toxins include [[anatoxin-a]], [[guanitoxin]], [[aplysiatoxin]], [[cyanopeptolin]], [[cylindrospermopsin]], [[domoic acid]], [[nodularin R]] (from ''[[Nodularia]]''), [[neosaxitoxin]], and [[saxitoxin]]. Cyanobacteria reproduce explosively under certain conditions. This results in [[algal bloom]]s which can become [[Harmful algal blooms|harmful to other species]] and pose a danger to humans and animals if the cyanobacteria involved produce toxins. Several cases of human poisoning have been documented, but a lack of knowledge prevents an accurate assessment of the risks,<ref>{{cite journal | vauthors = Thébault L, Lesne J, Boutin JP | title = [Cyanobacteria, their toxins and health risks] | journal = Médecine Tropicale | volume = 55 | issue = 4 | pages = 375–380 | year = 1995 | pmid = 8830224 }}</ref><ref name="hc-sc1">{{Cite web |date=2008-05-30 |title=Blue-Green Algae (Cyanobacteria) and Their Toxins - Drinking Water |url=http://www.hc-sc.gc.ca/ewh-semt/pubs/water-eau/cyanobacter_e.html |access-date=2022-08-10 |archive-date=30 May 2008 |archive-url=https://web.archive.org/web/20080530024705/http://www.hc-sc.gc.ca/ewh-semt/pubs/water-eau/cyanobacter_e.html }}</ref><ref>{{Cite web |date=2009-12-03 |title=Harmful Bloom in Lake Atitlán, Guatemala |url=https://earthobservatory.nasa.gov/images/41487/harmful-bloom-in-lake-atitlan-guatemala |access-date=2022-08-10 |website=earthobservatory.nasa.gov |language=en}} from [http://earthobservatory.nasa.gov/ NASA Earth Observatory],</ref><ref>{{cite journal | vauthors = Weirich CA, Miller TR | title = Freshwater harmful algal blooms: toxins and children's health | journal = Current Problems in Pediatric and Adolescent Health Care | volume = 44 | issue = 1 | pages = 2–24 | date = January 2014 | pmid = 24439026 | doi = 10.1016/j.cppeds.2013.10.007 }}</ref> and research by [[Linda Lawton]], [[Fellowship of the Royal Society of Edinburgh|FRSE]] at [[Robert Gordon University]], Aberdeen and collaborators has 30 years of examining the phenomenon and methods of improving water safety.<ref>{{Cite web |title=Professor Linda Lawton |url=https://rgu-repository.worktribe.com/person/147344/linda-lawton |access-date=2021-06-25 |website=rgu-repository.worktribe.com}}</ref>

Recent studies suggest that significant exposure to high levels of cyanobacteria producing toxins such as [[beta-Methylamino-L-alanine|BMAA]] can cause [[amyotrophic lateral sclerosis]] (ALS). People living within half a mile of cyanobacterially contaminated lakes have had a 2.3 times greater risk of developing ALS than the rest of the population; people around New Hampshire's [[Lake Mascoma]] had an up to 25 times greater risk of ALS than the expected incidence.<ref>{{cite journal | vauthors = Caller TA, Doolin JW, Haney JF, Murby AJ, West KG, Farrar HE, Ball A, Harris BT, Stommel EW | display-authors = 6 | title = A cluster of amyotrophic lateral sclerosis in New Hampshire: a possible role for toxic cyanobacteria blooms | journal = Amyotrophic Lateral Sclerosis | volume = 10 | issue = Suppl 2 | pages = 101–108 | year = 2009 | pmid = 19929741 | doi = 10.3109/17482960903278485 }}</ref> BMAA from desert crusts found throughout Qatar might have contributed to higher rates of ALS in [[Gulf War]] veterans.<ref name="hc-sc1"/><ref>{{cite journal | vauthors = Cox PA, Richer R, Metcalf JS, Banack SA, Codd GA, Bradley WG | title = Cyanobacteria and BMAA exposure from desert dust: a possible link to sporadic ALS among Gulf War veterans | journal = Amyotrophic Lateral Sclerosis | volume = 10 | issue = Suppl 2 | pages = 109–117 | year = 2009 | pmid = 19929742 | doi = 10.3109/17482960903286066 }}</ref>

=== Chemical control ===
Several chemicals can eliminate cyanobacterial blooms from smaller water-based systems such as swimming pools. They include [[calcium hypochlorite]], [[Copper(II) sulfate|copper sulphate]], Cupricide (chelated copper), and [[simazine]].<ref name="bluegreenalgae2006">{{cite web | vauthors = Main DC |title=Toxic Algae Blooms |work=Veterinary Pathologist, South Perth |publisher=agric.wa.gov.au |date=2006 |url=http://archive.agric.wa.gov.au/objtwr/imported_assets/content/lwe/water/watq/fn052_2004.pdf |access-date=18 November 2014 |archive-date=1 December 2014 |archive-url= https://web.archive.org/web/20141201235311/http://archive.agric.wa.gov.au/objtwr/imported_assets/content/lwe/water/watq/fn052_2004.pdf |url-status=dead}}</ref> The calcium hypochlorite amount needed varies depending on the cyanobacteria bloom, and treatment is needed periodically. According to the Department of Agriculture Australia, a rate of 12&nbsp;g of 70% material in 1000&nbsp;L of water is often effective to treat a bloom.<ref name="bluegreenalgae2006"/> Copper sulfate is also used commonly, but no longer recommended by the Australian Department of Agriculture, as it kills livestock, crustaceans, and fish.<ref name="bluegreenalgae2006"/> Cupricide is a chelated copper product that eliminates blooms with lower toxicity risks than copper sulfate. Dosage recommendations vary from 190&nbsp;mL to 4.8&nbsp;L per 1000&nbsp;m<sup>2</sup>.<ref name="bluegreenalgae2006"/> Ferric alum treatments at the rate of 50&nbsp;mg/L will reduce algae blooms.<ref name="bluegreenalgae2006"/><ref>{{cite journal |vauthors=May V, Baker H |year=1978 |title=Reduction of toxic algae in farm dams by ferric alum |journal=Techn. Bull. |volume=19 |pages=1–16}}</ref> Simazine, which is also a herbicide, will continue to kill blooms for several days after an application. Simazine is marketed at different strengths (25, 50, and 90%), the recommended amount needed for one cubic meter of water per product is 25% product 8&nbsp;mL; 50% product 4&nbsp;mL; or 90% product 2.2&nbsp;mL.<ref name="bluegreenalgae2006"/>

=== Climate change ===
[[Climate change]] is likely to increase the frequency, intensity and duration of cyanobacterial blooms in many [[eutrophic]] lakes, reservoirs and estuaries.<ref>{{cite journal | vauthors = Paerl HW, Huisman J | title = Climate. Blooms like it hot | journal = Science | volume = 320 | issue = 5872 | pages = 57–58 | date = April 2008 | pmid = 18388279 | doi = 10.1126/science.1155398 }}</ref><ref name="Huisman2018"/> Bloom-forming cyanobacteria produce a variety of [[neurotoxin]]s, [[hepatotoxin]]s and [[dermatoxin]]s, which can be fatal to birds and mammals (including waterfowl, cattle and dogs) and threaten the use of waters for recreation, drinking water production, agricultural irrigation and fisheries.<ref name=Huisman2018 /> [[Cyanotoxin|Toxic cyanobacteria]] have caused major water quality problems, for example in [[Lake Taihu]] (China), [[Lake Erie]] (USA), [[Lake Okeechobee]] (USA), [[Lake Victoria]] (Africa) and the [[Baltic Sea]].<ref name=Huisman2018 /><ref>{{cite journal | vauthors = Sitoki L, Kurmayer R, Rott E | title = Spatial variation of phytoplankton composition, biovolume, and resulting microcystin concentrations in the Nyanza Gulf (Lake Victoria, Kenya) | journal = Hydrobiologia | volume = 691 | issue = 1 | pages = 109–122 | date = July 2012 | pmid = 24683268 | pmc = 3968937 | doi = 10.1007/s10750-012-1062-8 | bibcode = 2012HyBio.691..109S }}</ref><ref>{{cite journal |doi=10.2166/wp.2018.012 |title=Public health responses to toxic cyanobacterial blooms: Perspectives from the 2016 Florida event |year=2018 | vauthors = Metcalf JS, Banack SA, Powell JT, Tymm FJ, Murch SJ, Brand LE, Cox PA |journal=Water Policy |volume=20 |issue=5 |pages=919–932 |doi-access=free |bibcode=2018WaPol..20..919M }}</ref><ref name=Cavicchioli2019>{{cite journal | vauthors = Cavicchioli R, Ripple WJ, Timmis KN, Azam F, Bakken LR, Baylis M, Behrenfeld MJ, Boetius A, Boyd PW, Classen AT, Crowther TW, Danovaro R, Foreman CM, Huisman J, Hutchins DA, Jansson JK, Karl DM, Koskella B, Mark Welch DB, Martiny JB, Moran MA, Orphan VJ, Reay DS, Remais JV, Rich VI, Singh BK, Stein LY, Stewart FJ, Sullivan MB, van Oppen MJ, Weaver SC, Webb EA, Webster NS | display-authors = 6 | title = Scientists' warning to humanity: microorganisms and climate change | journal = Nature Reviews. Microbiology | volume = 17 | issue = 9 | pages = 569–586 | date = September 2019 | pmid = 31213707 | pmc = 7136171 | doi = 10.1038/s41579-019-0222-5 }} {{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref>

[[Climate change]] favours cyanobacterial blooms both directly and indirectly.<ref name=Huisman2018 /> Many bloom-forming cyanobacteria can grow at relatively high temperatures.<ref>{{cite journal | vauthors = Visser PM, Verspagen JM, Sandrini G, Stal LJ, Matthijs HC, Davis TW, Paerl HW, Huisman J | display-authors = 6 | title = How rising CO<sub>2</sub> and global warming may stimulate harmful cyanobacterial blooms | journal = Harmful Algae | volume = 54 | pages = 145–159 | date = April 2016 | pmid = 28073473 | doi = 10.1016/j.hal.2015.12.006 | doi-access = free }}</ref> Increased [[thermal stratification]] of lakes and reservoirs enables buoyant cyanobacteria to float upwards and form dense surface blooms, which gives them better access to light and hence a selective advantage over nonbuoyant phytoplankton organisms.<ref>{{cite journal | vauthors = Walsby AE, Hayes PK, Boje R, Stal LJ | title = The selective advantage of buoyancy provided by gas vesicles for planktonic cyanobacteria in the Baltic Sea | journal = The New Phytologist | volume = 136 | issue = 3 | pages = 407–417 | date = July 1997 | pmid = 33863010 | doi = 10.1046/j.1469-8137.1997.00754.x | bibcode = 1997NewPh.136..407W }}</ref><ref name="Jöhnk_2008" /> Protracted droughts during summer increase water residence times in reservoirs, rivers and estuaries, and these stagnant warm waters can provide ideal conditions for cyanobacterial bloom development.<ref>{{cite journal | vauthors = Lehman PW, Kurobe T, Lesmeister S, Baxa D, Tung A, Teh SJ | title = Impacts of the 2014 severe drought on the Microcystis bloom in San Francisco Estuary | journal = Harmful Algae | volume = 63 | pages = 94–108 | date = March 2017 | pmid = 28366405 | doi = 10.1016/j.hal.2017.01.011 | doi-access = free | bibcode = 2017HAlga..63...94L }}</ref><ref name=Cavicchioli2019 />

The capacity of the harmful cyanobacterial genus ''[[Microcystis]]'' to adapt to elevated CO<sub>2</sub> levels was demonstrated in both laboratory and field experiments.<ref name=Sandrini2016>{{cite journal | vauthors = Sandrini G, Ji X, Verspagen JM, Tann RP, Slot PC, Luimstra VM, Schuurmans JM, Matthijs HC, Huisman J | display-authors = 6 | title = Rapid adaptation of harmful cyanobacteria to rising CO2 | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 113 | issue = 33 | pages = 9315–9320 | date = August 2016 | pmid = 27482094 | pmc = 4995953 | doi = 10.1073/pnas.1602435113 | doi-access = free | bibcode = 2016PNAS..113.9315S }}</ref> ''Microcystis'' spp. take up CO<sub>2</sub> and {{chem|HCO|3|-}} and accumulate [[inorganic carbon]] in [[carboxysome]]s, and strain competitiveness was found to depend on the concentration of inorganic carbon. As a result, [[climate change]] and increased CO<sub>2</sub> levels are expected to affect the strain composition of cyanobacterial blooms.<ref name=Sandrini2016 /><ref name=Cavicchioli2019 />

== Gallery ==
<gallery mode="packed" heights="180" style="float:left;">
File:Lago de coatepeque de color.jpg|Cyanobacteria activity turns [[Coatepeque Caldera]] lake a turquoise color
File:2010 Filamentous Cyanobacteria Bloom near Fiji.jpg|Cyanobacterial bloom near [[Fiji]]
File:Sinilevää Köyliönjärvessä 3.jpg|Cyanobacteria in [[Lake Köyliö]].
File:Video- The Cyanobacteria- Oscillatoria and Gleocapsa.webm| Video – ''[[Oscillatoria]]'' and ''Gleocapsa'' – with oscillatory movement as filaments of ''Oscillatoria'' orient towards light
</gallery>

{{clear}}

== See also ==
{{div col|colwidth=18em}}
* [[Archean Eon]]
* [[Bacterial phyla]], other major lineages of Bacteria
* [[Biodiesel]]
* [[Cyanobiont]]
* [[Endosymbiotic theory]]
* [[Geological history of oxygen]]
* [[Hypolith]]
{{div col end}}

== Notes ==
{{NoteFoot}}

== References ==
{{Reflist|30em}}

== Further reading ==
{{refbegin|30em}}
* {{cite book | vauthors = Gillian C |title=Nature's Superfood: the Blue-Green Algae Revolution |publisher=Newleaf |isbn=978-0-7522-0569-4 |year=1997 |edition=first}}
* {{cite book | vauthors = Savage M |author-link=Marshall Savage |title=The Millennial Project: Colonizing the Galaxy in Eight Easy Steps |publisher=Little Brown & Co |isbn=978-0-316-77163-4 |year=1994 |url=https://archive.org/details/millennialprojec00sava}}
* {{cite book |vauthors=Fogg GE, Stewart WD, Fay P, Walsby AE |year=1973 |title=The Blue-green Algae |publisher=[[Academic Press]] |location=London and New York |isbn=978-0-12-261650-1}}
* {{cite web |url=http://www.ucmp.berkeley.edu/bacteria/cyanointro.html |title=Architects of the earth's atmosphere, Introduction to the Cyanobacteria |website=[[University of California, Berkeley]] |date=3 February 2006}}
* {{ cite book |vauthors=Whitton BA |chapter=Phylum Cyanophyta (Cyanobacteria) |title=The Freshwater Algal Flora of the British Isles |location=Cambridge |publisher=[[Cambridge University Press]] |isbn=978-0-521-77051-4 |year=2002}}
* {{cite journal |doi=10.1080/09670262.2010.492914 |vauthors=Pentecost A, Franke U |year=2010 |title=Photosynthesis and calcification of the stromatolitic freshwater cyanobacterium ''Rivularia'' |journal=European Journal of Phycology |volume=45 |issue=4 |pages=345–53 |bibcode=2010EJPhy..45..345P |doi-access=free}}
* {{cite book |doi=10.1007/0-306-46855-7 |title=The Ecology of Cyanobacteria |date=2002 |publisher=Kluwer Academic Publishers |location=Dordrecht |isbn=0-7923-4735-8 |editor-last1=Whitton |editor-last2=Potts |editor-first1=Brian A. |editor-first2=Malcolm }}
* {{cite web |url=http://www.paristechreview.com/2011/12/01/micro-algae-blue-oil/ |title=From Micro-Algae to Blue Oil |work=ParisTech Review |date=December 2011 |access-date=2 March 2012 |archive-url=https://web.archive.org/web/20160417030653/http://www.paristechreview.com/2011/12/01/micro-algae-blue-oil/ |archive-date=17 April 2016 |url-status=dead}}
{{refend}}

== External links ==
{{Commons category|Cyanobacteria}}
* [http://www.thebigger.com/biology/monera/what-are-cyanobacteria-and-what-are-its-types/ What are Cyanobacteria and What are its Types?]
* [https://web.archive.org/web/20190916232500/http://www-cyanosite.bio.purdue.edu/ Webserver for Cyanobacteria Research] (archived 16 September 2019)
* [http://scientistatwork.blogs.nytimes.com/2013/01/31/diving-an-antarctic-time-capsule-filled-with-primordial-life Diving an Antarctic Time Capsule Filled With Primordial Life]

{{Plankton}}
{{Microorganisms}}
{{Bacteria classification}}
{{Life on Earth}}
{{Taxonbar|from1=Q93315|from2=Q25577567}}
{{Authority control}}

[[Category:Cyanobacteria| ]]
[[Category:Phototrophic bacteria]]
[[Category:Photosynthesis]]
[[Category:Gram-negative bacteria]]
[[Category:Environmental chemistry]]
[[Category:Bacteria phyla]]