View source for Azolla

{{Short description|Genus of aquatic plants}}
{{Use mdy dates|date=November 2022}}
{{Automatic taxobox
| fossil_range = {{fossil range|Maastrichtian|Recent|[[Maastrichtian]]-[[Holocene]]}}
| image = Azolla caroliniana0.jpg
| image_caption = ''Azolla caroliniana''
| taxon = Azolla
| authority = [[Jean-Baptiste Lamarck|Lam.]]<ref name=trop1>In: ''Encyclopédie Méthodique, Botanique'' 1(1): 343. 1783. {{cite web |url=http://www.tropicos.org/Name/40023609 |title=Name - ''Azolla'' Lam. |quote=Annotation: a sp. nov. reference for ''Azolla filiculoides''<br />Type Specimens [[Holotype|HT]]: ''Azolla filiculoides'' |work=Tropicos |publisher=[[Missouri Botanical Garden]] |location=[[Saint Louis, Missouri]] |access-date=February 19, 2010}}</ref>
| type_species = ''[[Azolla filiculoides]]''<ref name=trop1/>
| type_species_authority = [[Jean-Baptiste Lamarck|Lam.]]
| subdivision_ranks = Species
| subdivision = See text
| synonyms =
* ''Carpanthus'' <small>Rafinesque</small>
* ''Rhizosperma'' <small>Meyen</small>
}}

'''''Azolla''''' (common called '''mosquito fern''', '''water fern''', and '''fairy moss''') is a [[genus]] of seven species of [[aquatic plant|aquatic]] [[fern]]s in the family [[Salviniaceae]]. They are extremely reduced in form and specialized, having a significantly different appearance to other ferns and more resembling some [[moss]]es or even [[Lemnoideae|duckweeds]]. ''[[Azolla filiculoides]]'' is one of two fern species for which a reference genome has been published.<ref>{{cite journal |last1=Li |first1=Fay-Wei |last2=Brouwer |first2=Paul |last3=Carretero-Paulet |first3=Lorenzo |last4=Cheng |first4=Shifeng |last5=de Vries |first5=Jan |last6=Delaux |first6=Pierre-Marc |last7=Eily |first7=Ariana |last8=Koppers |first8=Nils |last9=Kuo |first9=Li-Yaung |date=2018-07-02 |title=Fern genomes elucidate land plant evolution and cyanobacterial symbioses |journal=Nature Plants |volume=4 |issue=7 |pages=460–472 |language=En |doi=10.1038/s41477-018-0188-8 |pmid=29967517 |pmc=6786969 |bibcode=2018NatPl...4..460L |issn=2055-0278}}</ref> It is believed that this genus grew so prolifically during the [[Eocene]] (and thus absorbed such a large amount of carbon) that it triggered a global cooling event that has lasted to the present.<ref name="Speelman-2009">{{cite journal |last1=Speelman |first1=E. N. |last2=Van Kempen |first2=M. M. L. |last3=Barke |first3=J. |last4=Brinkhuis |first4=H. |last5=Reichart |first5=G. J. |last6=Smolders |first6=A. J. P. |last7=Roelofs |first7=J. G. M. |last8=Sangiorgi |first8=F. |last9=De Leeuw |first9=J. W. |last10=Lotter |first10=A. F. |last11=Sinninghe Damsté |first11=J. S. |date=March 2009 |title=The Eocene Arctic Azolla bloom: environmental conditions, productivity and carbon drawdown |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1472-4669.2009.00195.x |journal=Geobiology |language=en |volume=7 |issue=2 |pages=155–170 |doi=10.1111/j.1472-4669.2009.00195.x|pmid=19323694 |bibcode=2009Gbio....7..155S |s2cid=13206343 |url-access=subscription }}</ref>

''Azolla'' may establish as an [[invasive plant]] in areas where it is not native. In such a situation, it can alter [[aquatic ecosystem]]s and [[biodiversity]] substantially.{{Why|date=May 2025|reason=What exactly causes this?}}<ref name="Weber-2017">{{cite book |last1=Weber |first1=Ewald |title=Invasive Plant Species of the World: A Reference Guide to Environmental Weeds |date=2017 |page=65 |isbn=978-1-78064-386-1 |url=https://books.google.com/books?id=Ns_UDgAAQBAJ}}</ref>

==Phylogeny==
Phylogeny of ''Azolla''<ref>{{cite journal |last1=Nitta |first1=Joel H. |last2=Schuettpelz |first2=Eric |last3=Ramírez-Barahona |first3=Santiago |last4=Iwasaki |first4=Wataru |display-authors=et al. |year=2022 |title=An Open and Continuously Updated Fern Tree of Life |journal=Frontiers in Plant Science |volume=13 |issue= |page= 909768| doi=10.3389/fpls.2022.909768 |pmid= 36092417|pmc= 9449725|bibcode= 2022FrPS...1309768N|doi-access=free}}</ref><ref>{{cite web|last1= |first1= |last2= |display-authors=et al. |year=2023 |title=Tree viewer: interactive visualization of FTOL |url=https://fernphy.github.io/viewer.html |version=FTOL v1.4.0 [GenBank release 253] |access-date=8 March 2023}}</ref>
{{Clade | style=font-size:100%;line-height:80%
|label1=''Azolla''
|1={{clade
  |1=''[[Azolla nilotica|A. nilotica]]'' <small>[[Decne.]] ex [[Mett.]]</small> (Nile Azolla)
  |2={{clade
    |1={{clade
      |1=''[[Azolla filiculoides|A. filiculoides]]'' <small>[[Jean-Baptiste Lamarck|Lam.]]</small> (Large mosquito fern)
      |2=''[[Azolla rubra|A. rubra]]'' <small>[[R.Br.]]</small>
       }}
    |2={{clade
      |1=''[[Azolla caroliniana|A. caroliniana]]'' <small>Willdenow 1810</small> (Eastern/Carolinian mosquito fern)
      |2={{clade
        |1=''[[Azolla cristata]]'' <small>[[Kaulf.]]</small> (Mexican mosquito fern)
        |2=''[[Azolla pinnata|A. pinnata]]'' <small>R.Br.</small> (Ferny/Pacific Azolla; Feathered mosquitofern)
         }}
       }}
     }}
   }}
}}

Other species include:<ref>{{cite journal |last1=Evrard |first1=C. |last2=Van Hove |first2=C. |title=Taxonomy of the American ''Azolla'' species (Azollaceae): A critical review |journal=Systematics and Geography of Plants |date=2004 |volume=74 |pages=301–318}}</ref><ref name=trop2>{{cite web |url=http://www.tropicos.org/NameSubordinateTaxa.aspx?nameid=40023609 |title=Name - ''Azolla'' Lam. subordinate taxa |work=Tropicos |publisher=Missouri Botanical Garden |location=Saint Louis, Missouri |access-date=February 19, 2010}}</ref><ref name=ipni>{{cite web |url=http://www.ipni.org:80/ipni/advPlantNameSearch.do?find_genus=Azolla&find_rankToReturn=spec&output_format=normal&query_type=by_query&back_page=plantsearch |title=Query Results for Genus ''Azolla'' |work=[[International Plant Names Index|IPNI]] |access-date=February 19, 2010}}</ref><ref name=nobanis>{{cite web |url=http://www.nobanis.org/files/factsheets/Azolla_filiculoides.pdf |title=NOBANIS -- Invasive Alien Species Fact Sheet -- ''Azolla filiculoides'' |author=Hussner, A. |location=[[Heinrich Heine Universität]], [[Düsseldorf]] |year=2006 |work=Online Database of the North European and Baltic Network on Invasive Alien Species |access-date=February 19, 2010}}</ref>

At least six [[extinct]] species are known from the [[fossil record]]:
*''[[Azolla intertrappea]]'' <small>[[Birbal Sahni|Sahni]] & H.S. Rao, 1934</small> ([[Eocene]], [[India]])<ref name="Arnold1955">{{cite journal |last1=Arnold |first1=C.A. |year=1955 |title=A Tertiary ''Azolla'' from British Columbia |journal=Contributions from the Museum of. Paleontology, University of Michigan |volume=12 |issue=4 |pages=37–45 |url=http://deepblue.lib.umich.edu/bitstream/2027.42/48289/2/ID129.pdf}}</ref>
*''[[Azolla berryi]]'' <small>[[Rowland W. Brown|Brown]], 1934</small> ([[Eocene]], [[Green River Formation]], [[Wyoming]])<ref name="Arnold1955"/>
*''[[Azolla prisca]]'' <small>[[Marjorie Elizabeth Jane Chandler|Chandler]] & [[Eleanor Mary Reid|Reid]], 1926</small> ([[Oligocene]], [[London Clay]], [[Isle of Wight]])<ref name="Arnold1955"/>
*''[[Azolla tertiaria]]'' <small>[[Edward W. Berry|Berry]], 1927</small> ([[Pliocene]], [[Esmeralda Formation]], [[Nevada]])<ref name="Arnold1955"/>
*''[[Azolla primaeva]]'' <small>([[David P. Penhallow|Penhallow]]) [[Chester A. Arnold|Arnold]], 1955</small> ([[Eocene]], [[Allenby Formation]], [[British Columbia]])<ref name="Arnold1955"/>
*''[[Azolla boliviensis]]'' <small>[[Vivi Vajda|Vajda]] & [[Stephen McLoughlin|McLoughlin]], 2005</small> ([[Maastrichtian]] – [[Paleocene]], [[Eslaboacuten Formation]] and [[Flora Formation]] [[Bolivia]])<ref name="Vajda2005">{{cite journal |last1=Vajda |first1=V |author2=McLoughlin, S. |year=2005 |title=A new Maastrichtian-Paleocene ''Azolla'' species from Bolivia, with a comparison of the global record of coeval ''Azolla'' microfossils |journal=Alcheringa: An Australasian Journal of Palaeontology |volume=29 |issue=2 |pages=305–329 |doi=10.1080/03115510508619308 |bibcode=2005Alch...29..305V |s2cid=128643041}}</ref>

==Ecology==
[[File:Azolla-13.3348RGB.tif|alt=Confocal microscopy image of an Azolla filiculoides root cross section showing several layers of cells in a concentric pattern stained in cyan and magenta|thumb|''Azolla filiculoides'' root cross section]]
[[File:Canning rv azolla 10 gnangarra.jpg|thumb|Azolla covering the [[Canning River (Western Australia)]]]]

''Azolla'' is a highly [[plant growth|productive plant]] that can double its [[biomass (ecology)|biomass]] in 1.9 days,<ref>{{cite journal |journal=Aquatic Botany |volume=15 |issue=2 |pages=175–185 |date=1983 |title=The growth of four species of Azolla as affected by temperature |author=Iwao Watanabe, Nilda S.Berja |doi=10.1016/0304-3770(83)90027-X|bibcode=1983AqBot..15..175W }}</ref> depending on growing conditions. The plant can yield can reach 8–10 tonnes fresh matter per hectare in Asian [[paddy field]]s. 37.8 tonnes fresh weight/ha (2.78&nbsp;t/ha dry weight) has been reported for ''A. pinnata'' in India.<ref>{{cite web |title=Hasan, M. R.; Chakrabarti, R., 2009. Use of algae and aquatic macrophytes as feed in small-scale aquaculture: A review. FAO Fisheries and Aquaculture technical paper, 531. FAO, Rome, Italy |url=https://www.fao.org/3/i1141e/i1141e00.htm |website=fao.org/ |access-date=18 August 2014}}</ref>

''Azolla'' floats on the surface of water by means of numerous small, closely overlapping scale-like [[Leaf|leaves]], with their roots hanging in the water. They form a [[symbiosis|symbiotic]] relationship with the [[cyanobacterium]] ''[[Anabaena|Anabaena azollae]]'',{{refn|1=Currently ''[https://lpsn.dsmz.de/species/trichormus-azollae Trichormus azollae]'' {{au|(Strasburger 1884) Komárek and Anagnostidis 1989}}. Synonyms: ''Nostoc azollae'' {{au|Strasburger 1883}}, ''Anabaena azollae'' {{au|Strasburger 1884}}, ''Desikacharya azollae'' {{au|(Strasburger 1884) Saraf et al. 2019}}.{{cn|date=January 2025}}|group=note}} which lives outside the cells of its host and which [[nitrogen fixation|fixes]] [[atmospheric]] [[nitrogen]].<ref>{{cite journal | pmc=2900214 | year=2010 | last1=Ran | first1=L. | last2=Larsson | first2=J. | last3=Vigil-Stenman | first3=T. | last4=Nylander | first4=J. A. | last5=Ininbergs | first5=K. | last6=Zheng | first6=W. W. | last7=Lapidus | first7=A. | last8=Lowry | first8=S. | last9=Haselkorn | first9=R. | last10=Bergman | first10=B. | title=Genome Erosion in a Nitrogen-Fixing Vertically Transmitted Endosymbiotic Multicellular Cyanobacterium | journal=PLOS ONE | volume=5 | issue=7 | pages=e11486 | doi=10.1371/journal.pone.0011486 | pmid=20628610 | bibcode=2010PLoSO...511486R | doi-access=free }}</ref> The typical [[limiting factor]] on its growth is [[phosphorus]]; thus, an abundance of phosphorus—due for example to [[eutrophication]] or chemical runoff—often leads to ''Azolla'' blooms. Unlike all other known plants, its symbiotic microorganism [[Vertical transmission|transfers directly]] from one generation to the next. ''A.&nbsp;azollae'' is completely dependent on its host, as several of its genes have either been lost or transferred to the [[cell nucleus|nucleus]] in Azolla's cells.<ref>{{Cite web|url=https://www.geolsoc.org.uk/Geoscientist/Archive/June-2014/The-Arctic-Azolla-event|title=The Geological Society of London - The Arctic Azolla event|website=www.geolsoc.org.uk|date=April 8, 2025 }}</ref>

The nitrogen-fixing capability of ''Azolla'' has led to widespread use as a [[biofertiliser]], especially in parts of [[southeast Asia]]. The plant has been used to bolster agricultural productivity in China for over a thousand years. When [[paddy field|rice paddies]] are flooded in the spring, they can be planted with ''Azolla'', which then quickly multiplies to cover the water, suppressing weeds. The rotting plant material resulting from the die-off of this ''Azolla'' releases nitrogen into the water for the rice plants, providing up to nine tonnes of [[protein]] per hectare per year.<ref>{{Cite web|url=http://www.fao.org/ag/AGA/AGAP/FRG/afris/DATA/558.htm|title=FAO figures}}</ref>

''Azolla'' are [[weed]]s in many parts of the world, entirely covering some bodies of water. The myth that no [[mosquito]] can penetrate the coating of fern to lay its eggs in the water gives the plant its common name "mosquito fern";<ref name="Mosquito Fern">{{cite web |url=http://www.americaswetlandresources.com/wildlife_ecology/plants_animals_ecology/plants/mosquitofern.html |title=Mosquito Fern |access-date=2007-11-10 |work=America's Wetland Resource Center |publisher=Loyola University, New Orleans |archive-date=May 16, 2006 |archive-url=https://web.archive.org/web/20060516014940/http://www.americaswetlandresources.com/wildlife_ecology/plants_animals_ecology/plants/mosquitofern.html |url-status=dead }}</ref> however, ''Azolla'' may deter the survival of some of mosquito larvae.<ref name=":0" />

Most species can produce large amounts of [[3-deoxyanthocyanin|deoxyanthocyanin]]s in response to various stresses,<ref>{{cite journal |doi=10.1007/BF02857915 |last1=Wagner |first1=G.M. |year=1997 |title=''Azolla'': a review of its biology and utilization |journal=Bot. Rev. |volume=63 |issue=1 |pages=1–26 |bibcode=1997BotRv..63....1W |s2cid=347780}}</ref> including bright sunlight and extreme temperatures,<ref>{{cite journal |doi=10.1007/BF02859886 |last1=Moore |first1=A. W. |year=1969 |title=''Azolla'': Biology and agronomic significance |journal=Bot. Rev. |volume=35 |issue=1 |pages=17–35 |bibcode=1969BotRv..35...17M |s2cid=42431293}}</ref><ref>{{cite journal |last1=Zimmerman |first1=William J. |year=1985 |title=Biomass and Pigment Production in Three Isolates of ''Azolla'' II. Response to Light and Temperature Stress |journal=Ann. Bot. |volume=56 |issue=5 |pages=701–709 |doi=10.1093/oxfordjournals.aob.a087059}}</ref> causing the water surface to appear to be covered with an intensely red carpet. [[Herbivore]] feeding induces accumulation of deoxyanthocyanins and leads to a reduction in the proportion of [[polyunsaturated fat]]ty acids in the [[fronds]], thus lowering their palatability and nutritive value.<ref>{{cite journal |doi=10.1016/S0304-3770(02)00077-3 |last1=Cohen |first1=M.F. |last2=Meziane |first2=T. |last3=Tsuchiya |first3=M. |last4=Yamasaki |first4=H. |year=2002 |title=Feeding deterrence of ''Azolla'' in relation to deoxyanthocyanin and fatty acid composition |url=http://www.bashanfoundation.org/michael/michaelfeeding.pdf |journal=Aquatic Botany |volume=74 |issue=2 |pages=181–187 |bibcode=2002AqBot..74..181C |access-date=2010-01-22 |archive-url=https://web.archive.org/web/20081122000150/http://www.bashanfoundation.org/michael/michaelfeeding.pdf |archive-date=2008-11-22 |url-status=dead}}</ref>

''Azolla'' cannot survive winters with prolonged freezing, so is often grown as an [[ornamental plant]] at high latitudes where it cannot establish itself firmly enough to become a weed. It is also not tolerant of [[salinity]]; normal plants cannot survive in greater than 1–1.6‰, and even conditioned organisms die if grown in water with a salinity above 5.5‰.<ref name=Brinkhuis2006>{{cite journal
|author=Brinkhuis, H.
|author2=Schouten, S.|author3=Collinson, M.E.|author4=Sluijs, A.|author5=Sinninghe Damsté, J.S.|author6=Dickens, G.R.|author7=Huber, M.|author8=Cronin, T.M.|author9=Onodera, J.|author10=Takahashi, K.
|year=2006
|title=Episodic fresh surface waters in the Eocene Arctic Ocean
|journal=Nature
|volume=441
|pages=606–9
|url=http://www.nature.com/search/executeSearch?sp-q-9=NATURE&sp-q=Episodic+fresh+surface+waters+in+the+Eocene+Arctic+Ocean&sp-c=10&sp-x-9=cat&sp-s=date&submit=go&sp-a=sp1001702d&sp-sfvl-field=subject%7Cujournal&sp-x-1=ujournal&sp-p-1=phrase&sp-p=all
|access-date=2007-10-17
|doi=10.1038/nature04692
|pmid=16752440
|issue=7093
|bibcode=2006Natur.441..606B|display-authors=etal|hdl=11250/174278|s2cid=4412107|hdl-access=free}}</ref><ref name="Speelman-2009"/>

===''Azolla filiculoides''===
''Azolla filiculoides'' (red azolla) is the only member of the family Azollaceae found in [[Tasmania]], where it is a common native aquatic plant. It is often found behind farm dams and other still waterbodies. The plants are small (usually only a few cm across) and float, but they are fast growing, and can be abundant and form large mats. The plants are typically red, and have small, water repellent leaves.

==Reproduction==
{{more citations needed section|date=January 2025}}
[[File:Azolla megaspore Postglacial Galapagos Islands TEM longitudinal section 1.jpg|thumb|Transmission electron micrograph of a megaspore of the genus ''Azolla'' from postglacial sediments of Laguna El Junco, Galápagos Island of San Cristobal<ref name="Kem">{{cite journal |last1=Kempf |first1=E.K. |year=1976 |title=Low Magnifications - A Marginal Area of Electron Microscopy |journal=ZEISS Information |volume=21 |issue=83 |pages=57–60}}</ref>]]
''Azolla'' reproduces sexually, and [[asexual reproduction|asexual]]ly by splitting.

Like all ferns, sexual reproduction leads to [[spore]] formation, but unlike other members of this group, ''Azolla'' is [[heterosporous]], producing spores of two kinds. During the summer months, numerous spherical structures called [[sporocarp (ferns)|sporocarps]] form on the undersides of the branches. The male sporocarp is greenish or reddish and looks like the egg mass of an insect or spider. It is two millimeters in diameter, and bears numerous male [[sporangia]]. Male spores (microspores) are extremely small and are produced inside each [[microsporangium]]. Microspores tend to adhere in clumps called massulae.<ref name=Arnold1955/>

Female sporocarps are much smaller, containing one sporangium and one functional spore. Since an individual female spore is considerably larger than a male spore, it is termed a [[megaspore]].

''Azolla'' has microscopic male and female gametophytes that develop inside the male and female spores. The female [[gametophyte]] protrudes from the megaspore and bears a small number of [[archegonia]], each containing a single egg. The microspore forms a male gametophyte with a single [[antheridium]] which produces eight swimming sperm.<ref>{{cite book |author1=Scagel, Robert F. |author2=Bandoni, Robert J. |author3=Rouse, Glenn E. |author4=Schofield, W.B. |author5=Stein, Janet R. |author6=Taylor, T.M. |year=1965 |title=An Evolutionary Survey of the Plant Kingdom |place=Belmont, California |publisher=Wadsworth Publishing}} 658&nbsp;pp.</ref> The barbed [[glochidia]] on the male spore clusters cause them to cling to the female megaspores, thus facilitating fertilization.

==Applications==

===Food and animal feed===
In addition to its traditional cultivation as a bio-fertilizer for wetland paddies, ''Azolla'' is finding increasing use for sustainable production of [[livestock feed]].<ref>{{cite magazine |author1=Pillai, P. Kamalasanana |author2=Premalatha, S. |author3=Rajamony, S. |title=Azolla – a sustainable feed substitute for livestock |magazine=[[Farming Matters]] magazine |department=Small animals in focus |series=Azolla livestock feed |url=http://www.agriculturesnetwork.org/magazines/global/small-animals-in-focus/azolla-livestock-feed |access-date=2008-01-14}}</ref> ''Azolla'' is rich in protein, essential amino acids, vitamins, and minerals. Studies describe feeding ''Azolla'' to [[dairy cattle]], pigs, ducks, and chickens, with reported increases in [[Dairy|milk production]], weight of broiler chickens and egg production of layers, as compared to conventional feed. One [[FAO]] study describes how ''Azolla'' integrates into a tropical biomass agricultural system, reducing the need for food supplements.<ref>{{cite journal |author1=Preston, T.R. |author2=Murgueitio, E. |year=1992–1993 |title=Sustainable intensive livestock systems for the humid tropics |journal=World Animal Review |series=Sustainable animal production |volume=71 |issn=1014-6954 |publisher=[[UN]] [[FAO]] |url=http://www.fao.org/docrep/u7600t/u7600T04.htm |access-date=2011-09-28}}</ref> 

==== Concerns related to BMAA ====
Concerns about [[biomagnification]] exist because the plant may contain the neurotoxin [[BMAA]] that remains present in the bodies of animals consuming it, and BMAA has been documented as passing along the [[food chain]].<ref name=Banack>{{cite journal |author1=Banack, S.A. |author2=Cox, P.A. |year=2003 |title=Biomagnification of cycad neurotoxins in flying foxes: Implications for ALS-PDC in Guam |journal=Neurology |volume=61 |issue=3 |pages=387–389 |doi=10.1212/01.wnl.0000078320.18564.9f |pmid=12913204 |s2cid=38943437}}</ref> ''Azolla'' may contain this substance that is a possible cause of neurodegenerative diseases, including causing ALS, Alzheimer's, and Parkinson's.<ref>{{cite web |author=Sjodin, Erik |date=December 2014 |title=Azolla, BMAA, and neurodegenerative diseases |url=https://www.academia.edu/10074317 |access-date=2015-01-08}}</ref><ref>{{cite magazine |title=Are toxins in seafood causing ALS, Alzheimer's, and Parkinson's? |date=May 2011 |magazine=[[Discover Magazine]] |url=http://discovermagazine.com/2011/may/22-seafood-toxins-causing-als-alzheimers-parkinsons |access-date=2019-09-13}}</ref><ref>{{cite news |author=Williams, Amy Bennett |date=7 August 2019 |title=Documentary about algae and public health debuts to sold-out crowd |newspaper=Fort Myers News-Press |url=https://www.news-press.com/story/tech/science/environment/2019/08/07/documentary-algae-and-public-health-debuts-sold-out-crowd/1926083001/}}</ref> ''Azolla'' has been suggested as a foodstuff for human consumption; however, no long-term studies of the safety of eating ''Azolla'' have been made on humans.<ref>{{cite book |last=Sjodin |first=Erik |year=2012 |title=The Azolla Cooking and Cultivation Project |publisher=Erik Sjödin |isbn=978-91-980686-0-3}}</ref> Previous studies attributed neurotoxin production to ''Anabaena flos-aquae'' species, which is also a type of nitrogen-fixing cyanobacteria.<ref>{{cite journal |last=Agnihotri |first=Vijai K. |year=2014 |title=Anabaena flos-aquae |journal=Critical Reviews in Environmental Science and Technology |volume=44 |issue=18 |pages=1995–2037 |s2cid=84472933 |doi=10.1080/10643389.2013.803797|bibcode=2014CREST..44.1995A }}</ref> Studies published in 2024 have found that “the Azolla–Nostoc azollae superorganism does not contain BMAA or their isomers DAB and AEG and that Azolla and N. azollae do not synthesize other common cyanotoxins.” <ref>{{cite journal |last=Bujak |first=Jonathan P. |year=2024 |title=Azolla as a Safe Food: Suppression of Cyanotoxin-Related Genes and Cyanotoxin Production in Its Symbiont, Nostoc azollae |journal=Plants |volume=13 |issue=19 |page=2707 | doi=10.3390/plants13192707 |doi-access=free |pmid=39409577 |pmc=11479175 |bibcode=2024Plnts..13.2707B }}</ref> Further research may be needed to ascertain whether ''A.&nbsp;azollae'' is a healthy foodstuff for humans. 

====Companion plant====
''Azolla'' has been used for at least one thousand years in rice paddies as a [[companion plant]], to fix nitrogen and to block out light to prevent competition from other plants. Rice is planted when tall enough to poke through the ''Azolla'' layer. Mats of mature ''Azolla'' can also be used as a weed-suppressing [[mulch]].

Rice farmers used ''Azolla'' as a rice biofertilizer 1500 years ago. The earliest known written record of this practice is in a book written by Jia Ssu Hsieh (Jia Si Xue) in 540&nbsp;AD on ''The Art of Feeding the People'' (Chih Min Tao Shu). By the end of the [[Ming dynasty]] in the early 17th century, ''Azolla''<nowiki/>'s use as a green compost was documented in local records.<ref>{{cite web |title=The East discovers Azolla |url=http://theazollafoundation.org/azolla/azollas-use-in-the-east/ |website=Azolla Foundation |access-date=18 August 2014}}</ref>

===Larvicide===
The myth that no mosquito can penetrate the coating of fern to lay its eggs in the water gives the plant its common name "mosquito fern".<ref name="Mosquito Fern"/> ''Azolla'' have been used to control mosquito larvae in rice fields. The plant grows in a thick mat on the surface of the water, making it more difficult for the larvae to reach the surface to breathe, effectively choking the larvae.<ref name=":0">{{cite journal |last1=Myer |first1=Landon |last2=Okech |first2=Bernard A. |last3=Mwobobia |first3=Isaac K. |last4=Kamau |first4=Anthony |last5=Muiruri |first5=Samuel |last6=Mutiso |first6=Noah |last7=Nyambura |first7=Joyce |last8=Mwatele |first8=Cassian |last9=Amano |first9=Teruaki |title=Use of Integrated Malaria Management Reduces Malaria in Kenya |journal=PLOS ONE |volume=3 |pages=e4050 |year=2008 |doi=10.1371/journal.pone.0004050 |editor1-last=Myer |editor1-first=Landon |issue=12 |pmid=19115000 |pmc=2603594 |bibcode=2008PLoSO...3.4050O |doi-access=free}}</ref>

=== Paleoclimatology and climate change ===
Azolla has been proposed as a [[carbon sequestration]] modality. The proposal draws upon the hypothesized [[Azolla event|''Azolla'' event]] that asserts that 55 million years ago, ''Azolla'' covered the [[Arctic]] – at the time a hot, tropical, freshwater environment – and then sank, permanently sequestering [[Tonne|teratons]] of carbon that would otherwise have contributed to the planet's greenhouse effect. This ended a warming event that reached {{Convert|12-15|C-change|0}} warmer than present-day averages, eventually causing the formation of [[ice sheet]]s in [[Antarctica]] and the current [[greenhouse and icehouse earth|"icehouse period"]].<ref>{{cite web |last=Wang |first=Brian |title=Fix Hothouse Earth Just Like Last Time {{!}} NextBigFuture.com|url=https://www.nextbigfuture.com/2021/08/fix-hothouse-earth-just-like-last-time.html|access-date=2021-08-14|language=en-US}}</ref><ref>{{cite web |date=2018-07-11 |title=Can a tiny fern help fight climate change and cut fertilizer use? |url=https://e360.yale.edu/digest/can-a-tiny-fern-help-fight-climate-change-and-cut-fertilizer-use-azolla |access-date=2021-11-25 |website=Yale E360 |language=en-US}}</ref>

They contribute significantly to decreasing the atmospheric CO<sub>2</sub> levels.<ref name="Speelman-2009"/>

==Invasive species==
This fern has been introduced to other parts of the world, including the United Kingdom, where it has become a pest in some areas. A nominally tropical plant, it has adapted to the colder climate. It can form mats up to {{convert|30|cm}} thick and cover 100% of a water surface, preventing local insects and amphibians from reaching the surface.<ref>{{Cite web|url=https://insideecology.com/2017/11/01/invasive-non-native-species-uk-water-fern/|title=Invasive non-native species (UK) – Water fern|date=November 1, 2017|website=Inside Ecology}}</ref>

''Azolla filiculoides'' is especially invasive because it has the ability to survive temperatures as low as {{convert|-22|C}} and can survive thin layers of ice built up on its growth. ''A. filiculoides'' spreads at rapid pace by way of [[Fragmentation (reproduction)|Fragmentation]] which takes place in the rhizome (root-system). It can double its surface area and/or density in approximately 7-10 days under ideal conditions.<ref name="Weber-2017"/>

''A. filiculoides'' disrupts aquatic ecosystems by altering abiotic conditions in the water. It can displace native plants in places where it is invasive, such as the duckweed ''[[Lemna minor]]'' in Poland. It is difficult to control due to its ability to reestablish itself from spores. <ref name="Weber-2017"/>

==Bioremediation==
{{See also|Bioremediation}}
''Azolla'' can remove [[chromium]], [[nickel]], [[copper]], [[zinc]], and [[lead]] from effluent. It can also remove lead from solutions containing 1–1000 ppm.<ref>{{cite book |title=Bioremediation Technologies: Principles and Practice |author1=Robert L. Irvine |author2=Subhas K. Sikdar |page=102 |url=https://books.google.com/books?id=oLNtgk_VKXsC&q=Bioremediation+of+gypsum&pg=PA81 |isbn=978-1-56676-561-9 |date=1998-01-08|publisher=CRC Press }}</ref>

==Notes== 
{{reflist|group=note}}

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

==External links==
{{commons}}
* [http://theazollafoundation.org/ The Azolla Foundation] website
* [https://web.archive.org/web/20160308121032/http://www.azollaphilippines.com/home.html Azolla Philippines] website
* [https://web.archive.org/web/20100314135728/http://waynesword.palomar.edu/plnov98.htm   Marriage Between A Fern & Cyanobacterium] by W.P. Armstrong

{{Plant classification}}
{{Fern classification}}
{{Taxonbar|from=Q1128633}}
{{Authority control}}

[[Category:Aquatic plants]]
[[Category:Extant Maastrichtian first appearances]]
[[Category:Fern genera]]
[[Category:Invasive plant species in Japan]]
[[Category:Klondike Mountain Formation]]
[[Category:Maastrichtian genus first appearances]]
[[Category:Nitrogen-fixing crops]]
[[Category:Salviniales]]
[[Category:Taxa named by Jean-Baptiste Lamarck]]