View source for Brown algae

{{short description|Large group of multicellular algae, comprising the class Phaeophyceae}}
{{Use dmy dates|date=October 2023}}
{{Automatic taxobox
| fossil_range = [[Late Jurassic]] to present {{fossil range|150|0|ref=<ref name=Medlin1997/><ref name="Lim1986"/>}}
| image = Kelp-forest-Monterey.jpg
| image_caption = Giant kelp (''[[Macrocystis pyrifera]]'')
| image_alt = Giant kelp (Macrocystis pyrifera)
| display_parents = 3
| taxon = Phaeophyceae
| authority = [[Frans Reinhold Kjellman|Kjellman]], 1891<ref>{{cite book
|last=Kjellman |first=F. R.
|year=1891
|chapter=Phaeophyceae (Fucoideae)
|editor1-last=Engler |editor1-first=A.
|editor2-last=Prantl |editor2-first=K.
|title=Die natürlichen Pflanzenfamilien
|volume=1 |issue=2 |pages=176–192
|location=Leipzig
|publisher=[[Wilhelm Engelmann]]}}</ref>
| subdivision_ranks = Orders
| subdivision = See [[#Classification|classification]]
| synonyms =
* Fucophyceae <small>[[Eugenius Warming|Warming]], 1884</small>
* Melanophyceae <small>[[Gottlob Ludwig Rabenhorst|Rabenhorst]], 1863</small>
* Phaeophyta <small></small>
}}

'''Brown algae''' ({{singular}}: '''alga''') are a large group of [[multicellular]] [[algae]] comprising the [[class (biology)|class]] '''Phaeophyceae'''. They include many seaweeds located in colder waters of the [[Northern Hemisphere]]. Brown [[algae]] are the major [[seaweeds]] of the temperate and polar regions. Many brown algae, such as members of the order [[Fucales]], commonly grow along rocky seashores. Most brown algae live in marine environments, where they play an important role both as food and as a potential [[habitat]]. For instance, ''[[Macrocystis]]'', a [[kelp]] of the order [[Laminariales]], may reach {{cvt|60|m|ft}} in length and forms prominent underwater [[kelp forest]]s that contain a high level of biodiversity.<ref name="Cock-2011">{{Cite journal|last1=Cock|first1=J. Mark|last2=Peters|first2=Akira F.|last3=Coelho|first3=Susana M.|date=9 August 2011|title=Brown algae|journal=Current Biology|volume=21|issue=15|pages=R573–R575|doi=10.1016/j.cub.2011.05.006|pmid=21820616|doi-access=free}}</ref> Another example is ''[[Sargassum]]'', which creates unique floating mats of seaweed in the tropical waters of the [[Sargasso Sea]] that serve as the habitats for many species. Some members of the class, such as kelps, are used by humans as food.

Between 1,500 and 2,000 [[species]] of brown algae are known worldwide.<ref>{{Cite book |url=https://books.google.com/books?id=s1P855ZWc0kC&pg=166 |title=Algae: an introduction to phycology |last1=Hoek |first1=Christiaan |last2=den Hoeck |first2=Hoeck Van |last3=Mann |first3=David |last4=Jahns |first4=H.M. |date=1995 |publisher=Cambridge University Press |isbn=9780521316873 |oclc=443576944 |page=166 |access-date=26 April 2018 |archive-date=16 October 2023 |archive-url=https://web.archive.org/web/20231016191928/https://books.google.com/books?id=s1P855ZWc0kC&pg=166#v=onepage&q&f=false |url-status=live }}</ref> Some species, such as ''[[Ascophyllum nodosum]]'', have become subjects of extensive research in their own right due to their commercial importance. They also have environmental significance through [[carbon fixation]].<ref name="Cock-2011" />

Brown algae belong to the [[Stramenopile]]s, a [[clade]] of [[Eukaryote|eukaryotic]] organisms that are distinguished from [[Viridiplantae|green plants]] by having [[chloroplast]]s surrounded by four membranes, suggesting that they were acquired secondarily from a [[symbiotic relationship]] between a basal eukaryote and a red or green alga. Most brown algae contain the pigment [[fucoxanthin]], which is responsible for the distinctive greenish-brown color that gives them their name. Brown algae are unique among Stramenopiles in developing into multicellular forms with [[Cellular differentiation|differentiated]] [[Tissue (biology)|tissues]], but they reproduce by means of [[Flagellum|flagellated]] [[spore]]s and [[gamete]]s that closely resemble cells of single-celled Stramenopiles. Genetic studies show their closest relatives to be the [[yellow-green algae]].

==Morphology==
Brown algae exist in a wide range of sizes and forms. The smallest members of the group grow as tiny, feathery tufts of threadlike [[cell (biology)|cell]]s no more than a few centimeters (a few inches) long.<ref name="Connor1989">
{{cite book
|last1=Connor |first1=J.
|last2=Baxter |first2=C.
|year=1989
|title=Kelp Forests
|publisher=[[Monterey Bay Aquarium]]
|isbn=978-1-878244-01-7
}}</ref> Some species have a stage in their life cycle that consists of only a few cells, making the entire alga microscopic. Other groups of brown algae grow to much larger sizes. The [[Ascophyllum|rockweed]]s and leathery [[kelp]]s are often the most conspicuous algae in their habitats.<ref name="Dittmer1964">
{{cite book
|last=Dittmer |first=H. J.
|year=1964
|title=Phylogeny and Form in the Plant Kingdom
|url=https://archive.org/details/phylogenyforminp0000ditt |url-access=registration |pages=[https://archive.org/details/phylogenyforminp0000ditt/page/115 115–137]
|location=Princeton, NJ
|publisher=[[D. Van Nostrand Company]]
|isbn=978-0-88275-167-2
}}</ref> Kelps can range in size from the {{convert|2|ft|cm|-1|order=flip|adj=mid|-tall|sp=us}} sea palm ''[[Postelsia]]'' to the giant kelp ''[[Macrocystis pyrifera]]'', which grows to over {{cvt|150|ft|m|-1|order=flip}} long<ref name="Abbott 76">
{{cite book
|last1=Abbott |first1=I. A.
|last2=Hollenberg |first2=G. J.
|year=1976
|title=Marine Algae of California
|location=California
|publisher=[[Stanford University Press]]
|isbn=978-0-8047-0867-8
}}</ref><ref name="Cribb1953">
{{cite journal
|last=Cribb |first=A. B.
|year=1953
|title=''Macrocystis pyrifera'' (L.) Ag. in Tasmanian waters
|journal=[[Australian Journal of Marine and Freshwater Research]]
|volume=5 |issue=1 |pages=1–34
|doi=10.1071/MF9540001
}}</ref> and is the largest of all the algae. In form, the brown algae range from small crusts or cushions<ref name="Jones62">{{cite journal
 |last    = Jones
 |first   = W. E.
 |year    = 1962
 |title   = A key to the genera of the British seaweeds
 |url     = https://cdn.fieldstudiescouncil.net/fsj/vol1.4_23.pdf
 |journal = [[Field Studies (journal)|Field Studies]]
 |volume  = 1
 |issue   = 4
 |pages   = 1–32
}}</ref> to leafy free-floating mats formed by species of ''[[Sargassum]]''. They may consist of delicate felt-like strands of cells, as in ''[[Ectocarpus]]'', or of {{convert|1|ft|cm|order=flip|sp=us|adj=mid|-long}} flattened branches resembling a fan, as in ''[[Padina (alga)|Padina]]''.

Regardless of size or form, two visible features set the Phaeophyceae apart from all other algae. First, members of the group possess a characteristic color that ranges from an [[Olive drabs|olive green]] to various shades of [[brown]]. The particular shade depends upon the amount of [[fucoxanthin]] present in the alga.<ref name="Bold1987"/> Second, all brown algae are [[Multicellular organism|multicellular]]. There are no known species that exist as single cells or as colonies of cells,<ref name="Bold1987"/> and the brown algae are the only major group of [[seaweed]]s that does not include such forms. However, this may be the result of classification rather than a consequence of evolution, as all the groups hypothesized to be the closest relatives of the browns include single-celled or colonial forms.{{citation needed|date=January 2024}} They can change color depending on salinity, ranging from reddish to brown.

===Visible structures===
[[File:Laminaria hyperborea.jpg|thumb|240px|Two specimens of ''[[Laminaria hyperborea]]'', each showing the rootlike '''holdfast''' at lower left, a divided '''blade''' at upper right, and a stemlike '''stipe''' connecting the blade to the holdfast.]]
{{further|Thallus|Holdfast (biology)|Stipe (botany)|Lamina (algae)|Pneumatocyst}}

Whatever their form, the body of all brown algae is termed a ''[[thallus]]'', indicating that it lacks the complex [[xylem]] and [[phloem]] of [[vascular plant]]s. This does not mean that brown algae completely lack specialized structures. But, because some botanists define "true" stems, leaves, and roots by the presence of these tissues, their absence in the brown algae means that the stem-like and leaf-like structures found in some groups of brown algae must be described using different terminology.<ref name="Raven2005">
{{cite book
|last1=Raven |first1=P. H.
|last2=Evert |first2=R. F.
|last3=Eichhorn |first3=S. E.
|year=2005
|title=Biology of Plants
|url=https://archive.org/details/biologyofplants00rave_0 |url-access=registration |edition=7th |pages=[https://archive.org/details/biologyofplants00rave_0/page/316 316]–321, 347
|location=New York
|publisher=[[W. H. Freeman and Company]]
|isbn=978-0-7167-1007-3
}}</ref> Although not all brown algae are structurally complex, those that are typically possess one or more characteristic parts.

A ''[[holdfast (biology)|holdfast]]'' is a rootlike structure present at the base of the algae. Like a root system in plants, a holdfast serves to anchor the alga in place on the ''substrate'' where it grows, and thus prevents the alga from being carried away by the current. Unlike a root system, the holdfast generally does not serve as the primary organ for water uptake, nor does it take in nutrients from the substrate. The overall physical appearance of the holdfast differs among various brown algae and among various substrates. It may be heavily branched, or it may be cup-like in appearance. A single alga typically has just one holdfast, although some species have more than one stipe growing from their holdfast.

A ''[[stipe (botany)|stipe]]'' is a stalk or stemlike structure present in an alga. It may grow as a short structure near the base of the alga (as in ''[[Laminaria]]''), or it may develop into a large, complex structure running throughout the algal body (as in ''[[Sargassum]]'' or ''[[Macrocystis]]''). In the most structurally differentiated brown algae (such as ''[[Fucus]]''), the tissues within the stipe are divided into three distinct layers or regions. These regions include a central pith, a surrounding cortex, and an outer epidermis, each of which has an analog in the stem of a vascular plant. In some brown algae, the pith region includes a core of elongated cells that resemble the [[phloem]] of vascular plants both in structure and function. In others (such as ''[[Nereocystis]]''), the center of the stipe is hollow and filled with gas that serves to keep that part of the alga buoyant. The stipe may be relatively flexible and elastic in species like ''[[Macrocystis pyrifera]]'' that grow in strong currents, or may be more rigid in species like ''[[Postelsia palmaeformis]]'' that are exposed to the atmosphere at low tide.

Many algae have a flattened portion that may resemble a leaf, and this is termed a ''blade'', ''[[lamina (algae)|lamina]]'', or ''frond''. The name ''blade'' is most often applied to a single undivided structure, while ''frond'' may be applied to all or most of an algal body that is flattened, but this distinction is not universally applied. The name ''lamina'' refers to that portion of a structurally differentiated alga that is flattened. It may be a single or a divided structure, and may be spread over a substantial portion of the alga. In [[Fucus|rockweeds]], for example, the lamina is a broad wing of tissue that runs continuously along both sides of a branched ''midrib''. The midrib and lamina together constitute almost all of a rockweed, so that the lamina is spread throughout the alga rather than existing as a localized portion of it.

[[File:Bladder Wrack (Fucus vesiculosus) - geograph.org.uk - 224125.jpg|thumb|left|240px|Species like ''[[Fucus vesiculosus]]'' produce numerous gas-filled '''pneumatocysts''' (air bladders) to increase buoyancy.]]
In some brown algae, there is a single lamina or blade, while in others there may be many separate blades. Even in those species that initially produce a single blade, the structure may tear with rough currents or as part of maturation to form additional blades. These blades may be attached directly to the stipe, to a holdfast with no stipe present, or there may be an air bladder between the stipe and blade. The surface of the lamina or blade may be smooth or wrinkled; its tissues may be thin and flexible or thick and leathery. In species like ''[[Egregia menziesii]]'', this characteristic may change depending upon the turbulence of the waters in which it grows.<ref name="Connor1989" /> In other species, the surface of the blade is coated with slime to discourage the attachment of [[epiphyte]]s or to deter [[herbivore]]s. Blades are also often the parts of the alga that bear the reproductive structures.

Gas-filled floats called ''[[pneumatocyst]]s'' provide [[buoyancy]] in many [[kelp]]s and members of the [[Fucales]]. These bladder-like structures occur in or near the ''lamina'', so that it is held nearer the water surface and thus receives more light for photosynthesis. Pneumatocysts are most often spherical or [[ellipsoid]]al, but can vary in shape among different species. Species such as ''[[Nereocystis|Nereocystis luetkeana]]'' and ''[[Pelagophycus|Pelagophycus porra]]'' bear a single large pneumatocyst between the top of the stipe and the base of the blades. In contrast, the giant kelp ''Macrocystis pyrifera'' bears many blades along its stipe, with a pneumatocyst at the base of each blade where it attaches to the main stipe. Species of ''[[Sargassum]]'' also bear many blades and pneumatocysts, but both kinds of structures are attached separately to the stipe by short stalks. In species of ''[[Fucus]]'', the pneumatocysts develop within the lamina itself, either as discrete spherical bladders or as elongated gas-filled regions that take the outline of the lamina in which they develop.

===Growth===
[[File:Capo Gallo Dicotoma.jpg|thumb|240px|Growth in ''[[Dictyota dichotoma]]'' occurs at each frond tip, where new cells are produced.]]

The brown algae include the largest and fastest growing of seaweeds.<ref name="Connor1989" /> Fronds of ''[[Macrocystis]]'' may grow as much as {{convert|50|cm|in|sigfig=1|abbr=on}} per day, and the stipes can grow {{convert|6|cm|in|sigfig=2|abbr=on}} in a single day.<ref name="Round1981">
{{cite book
|last=Round |first=F. E.
|year=1981
|title=The Ecology of Algae
|page=103
|location=Cambridge
|publisher=[[Cambridge University Press]]
|isbn=978-0-521-26906-3
}}</ref>

Growth in most brown algae occurs at the tips of structures as a result of divisions in a single ''apical cell'' or in a row of such cells. They are single cellular organisms.<ref name="Dittmer1964" /> As this apical cell divides, the new cells that it produces develop into all the tissues of the alga. Branchings and other lateral structures appear when the apical cell divides to produce two new apical cells. However, a few groups (such as ''[[Ectocarpus]]'') grow by a diffuse, unlocalized production of new cells that can occur anywhere on the thallus.<ref name="Bold1987" />

===Tissue organization===

The simplest brown algae are filamentous—that is, their cells are elongate and have septa cutting across their width. They branch by getting wider at their tip, and then dividing the widening.<ref>{{cite book
|last=Wynne
|first=M. J.
|year=1981
|chapter=The Biology of seaweeds
|editor1-last=Lobban
|editor1-first=C. S.
|editor2-last=Wynne
|editor2-first=M. J.
|title=Phaeophyta: Morphology and Classification
|chapter-url=https://books.google.com/books?id=QG4tqjFPWJ0C&pg=PA52
|page=52
|series=Botanical Monographs
|volume=17
|publisher=[[University of California Press]]
|isbn=978-0-520-04585-9
|access-date=11 May 2020
|archive-date=16 October 2023
|archive-url=https://web.archive.org/web/20231016193130/https://books.google.com/books?id=QG4tqjFPWJ0C&pg=PA52#v=onepage&q&f=false
|url-status=live
}}</ref>

These filaments may be haplostichous or polystichous, multiaxial or monoaxial forming or not a [[pseudoparenchyma]].<ref name="Textbook of Algae">
{{cite book
|last1=Sharma |first1=O. P
|year=1986
|title=Textbook of Algae
|url=https://books.google.com/books?id=hOa74Hm4zDIC&pg=PA298
|page=298
|publisher=[[Tata McGraw-Hill]]
|isbn=978-0-07-451928-8
}}</ref><ref>{{Cite book|last=Graham; Wilcox; Graham|title=Algae, 2nd Edition|publisher=Pearson|year=2009|isbn=9780321603128}}</ref>   Besides fronds, there are the large in size [[parenchyma]]<nowiki/>tic kelps with three-dimensional development and growth and different tissues ([[wiktionary:meristoderm|meristoderm]], [[Cortex (botany)|cortex]] and [[wiktionary:medulla|medulla]]) which could be consider the trees of the sea.<ref>Fritsch, F. E. 1945. The Structure And Reproduction Of The Algae. Cambridge University Press, Cambridge.</ref><ref>{{Cite journal|last1=Charrier|first1=Bénédicte|last2=Le Bail|first2=Aude|last3=de Reviers|first3=Bruno|date=August 2012|title=Plant Proteus: brown algal morphological plasticity and underlying developmental mechanisms|url=http://dx.doi.org/10.1016/j.tplants.2012.03.003|journal=Trends in Plant Science|volume=17|issue=8|pages=468–477|doi=10.1016/j.tplants.2012.03.003|pmid=22513108|issn=1360-1385|access-date=13 November 2020|archive-date=16 October 2023|archive-url=https://web.archive.org/web/20231016193229/https://www.cell.com/trends/plant-science/fulltext/S1360-1385(12)00055-6?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS1360138512000556%3Fshowall%3Dtrue|url-status=live|url-access=subscription}}</ref> There are also the [[Fucales]] and [[Dictyotales]] smaller than kelps but still parenchymatic with the same kind of distinct tissues.

The [[cell wall]] consists of two layers; the inner layer bears the strength, and consists of [[cellulose]]; the outer wall layer is mainly [[algin]], and is gummy when wet but becomes hard and brittle when it dries out.<ref name="Textbook of Algae"/> Specifically, the brown algal cell wall consists of several components with alginates and [[Sulfation|sulphated]] [[fucose|fucan]] being its main ingredients, up to 40% each of them.<ref>{{Cite journal |doi=10.1016/j.tplants.2018.10.013 |title=Gazing at Cell Wall Expansion under a Golden Light |year=2019 |last1=Charrier |first1=Bénédicte |last2=Rabillé |first2=Hervé |last3=Billoud |first3=Bernard |journal=Trends in Plant Science |volume=24 |issue=2 |pages=130–141 |pmid=30472067 |s2cid=53725259 |url=https://hal.archives-ouvertes.fr/hal-02183603/file/PDF%20article%20accept%C3%A9.pdf |access-date=17 September 2022 |archive-date=8 January 2022 |archive-url=https://web.archive.org/web/20220108014851/https://hal.archives-ouvertes.fr/hal-02183603/file/PDF%20article%20accept%C3%A9.pdf |url-status=live }}</ref> Cellulose, a major component from most plant cell walls, is present in a very small percentage, up to 8%. Cellulose and alginate biosynthesis pathways seem to have been acquired from other organisms through endosymbiotic and horizontal gene transfer respectively, while the sulphated polysaccharides are of ancestral origin.<ref>{{Cite journal |doi=10.1111/j.1469-8137.2010.03374.x |doi-access=free|title=The cell wall polysaccharide metabolism of the brown alga Ectocarpus siliculosus. Insights into the evolution of extracellular matrix polysaccharides in Eukaryotes |year=2010 |last1=Michel |first1=Gurvan |last2=Tonon |first2=Thierry |last3=Scornet |first3=Delphine |last4=Cock |first4=J. Mark |last5=Kloareg |first5=Bernard |journal=New Phytologist |volume=188 |issue=1 |pages=82–97 |pmid=20618907 }}</ref> Specifically, the cellulose synthases seem to come from the red alga endosymbiont of the photosynthetic stramenopiles ancestor, and the ancestor of brown algae acquired the key enzymes for alginates biosynthesis from an [[actinobacterium]]. The presence and fine control of alginate structure in combination with the cellulose which existed before it, gave potentially the brown algae the ability to develop complex structurally multicellular organisms like the kelps.<ref>{{Cite journal |doi=10.1093/aob/mcu096 |doi-access=free|title=Chemical and enzymatic fractionation of cell walls from Fucales: Insights into the structure of the extracellular matrix of brown algae |year=2014 |last1=Deniaud-Bouët |first1=Estelle |last2=Kervarec |first2=Nelly |last3=Michel |first3=Gurvan |last4=Tonon |first4=Thierry |last5=Kloareg |first5=Bernard |last6=Hervé |first6=Cécile |journal=Annals of Botany |volume=114 |issue=6 |pages=1203–1216 |pmid=24875633 |pmc=4195554 }}</ref>

==Evolutionary history==

Genetic and ultrastructural evidence place the Phaeophyceae among the [[heterokont]]s (Stramenopiles),<ref name="pmid16248873">{{cite journal
|last=Adl
|first=S. M.
|year=2005
|title=The new higher level classification of eukaryotes with emphasis on the taxonomy of protists
|url=http://www.vliz.be/imisdocs/publications/233133.pdf
|journal=[[Journal of Eukaryotic Microbiology]]
|volume=52
|issue=5
|pages=399–451
|doi=10.1111/j.1550-7408.2005.00053.x
|pmid=16248873
|display-authors=etal
|doi-access=free
|access-date=27 August 2012
|archive-date=29 May 2013
|archive-url=https://web.archive.org/web/20130529063517/http://www.vliz.be/imisdocs/publications/233133.pdf
|url-status=live
}}</ref> a large assemblage of organisms that includes both [[photosynthesis|photosynthetic]] members with [[plastid]]s (such as the [[diatom]]s) as well as non-photosynthetic groups (such as the [[slime net]]s and [[Oomycete|water mold]]s). Although some heterokont relatives of the brown algae lack plastids in their cells, scientists believe this is a result of evolutionary loss of that organelle in those groups rather than independent acquisition by the several photosynthetic members.<ref name="Lane2008">{{cite journal
|last1=Lane
|first1=C. E.
|last2=Archibald
|first2=J. M.
|year=2008
|title=The eukaryotic tree of life: Endosymbiosis takes its TOL
|url=http://www.vliz.be/imisdocs/publications/135930.pdf
|journal=[[Trends in Ecology and Evolution]]
|volume=23
|issue=5
|pages=268–275
|doi=10.1016/j.tree.2008.02.004
|pmid=18378040
|citeseerx=10.1.1.555.2930
|access-date=27 August 2012
|archive-date=20 June 2013
|archive-url=https://web.archive.org/web/20130620111315/http://www.vliz.be/imisdocs/publications/135930.pdf
|url-status=live
}}</ref> Thus, all heterokonts are believed to descend from a single [[heterotrophic]] ancestor that became photosynthetic when it acquired plastids through [[endosymbiosis]] of another unicellular eukaryote.<ref name="Hoek">{{cite book|title=Algae: An Introduction to Phycology|last1=van den Hoek|first1=C.|last2=Mann|first2=D. G.|last3=Jahns|first3=H. M.|publisher=[[Cambridge University Press]]|year=1995|isbn=978-0-521-31687-3|location=Cambridge|pages=165–218}}</ref>

The closest relatives of the brown algae include unicellular and filamentous species, but no unicellular species of brown algae are known. However, most scientists assume that the Phaeophyceae evolved from unicellular ancestors.<ref>
{{cite book
|last=Niklas |first=K. J.
|year=1997
|title=The Evolutionary Biology of Plants
|page=156
|location=Chicago
|publisher=[[University of Chicago Press]]
|isbn=978-0-226-58082-1
}}</ref> [[DNA]] sequence comparison also suggests that the brown algae evolved from the filamentous [[Phaeothamniophyceae]],<ref name="Lee2008">
{{Cite book
|last=Lee
|first=R. E.
|year=2008
|title=Phycology
|edition=4th
|publisher=[[Cambridge University Press]]
|isbn=978-0-521-63883-8
|url-access=registration
|url=https://archive.org/details/phycology00robe
}}</ref> [[Xanthophyceae]],<ref name="Ariztia1991">
{{cite journal
|last1=Ariztia |first1=E. V.
|last2=Andersen |first2=R. A.
|last3=Sogin |first3=M. L.
|year=1991
|title=A new phylogeny of chromophyte algae using 16S-like rRNA sequences from ''Mallomonas papillosa'' (Synurophyceae) and ''Tribonema aequale'' (Xanthophyceae)
|journal=[[Journal of Phycology]]
|volume=27 |issue=3 |pages=428–436
|doi=10.1111/j.0022-3646.1991.00428.x
|s2cid=84693030
}}</ref> or the [[Chrysophyceae]]<ref name="Taylor1993">
{{cite book
|last1=Taylor |first1=T. N.
|last2=Taylor |first2=E. L.
|year=1993
|title=The Biology and Evolution of Fossil Plants
|pages=128–131
|location=Englewood Cliffs, NJ
|publisher=[[Prentice Hall]]
|isbn=978-0-13-651589-0
}}</ref> between 150<ref name="Medlin1997">{{Cite book
|last1=Medlin
|first1=L. K.
|last2=Kooistra
|first2=Wiebe H. C. F.
|last3=Potter
|first3=Daniel
|last4=Saunders
|first4=Gary W.
|last5=Andersen
|first5=Robert A.
|title=Origins of Algae and their Plastids
|chapter=Phylogenetic relationships of the 'golden algae' (Haptophytes, heterokont chromophytes) and their plastids
|display-authors=1
|year=1997
|url=http://epic.awi.de/2100/1/Med1997c.pdf
|volume=11
|pages=187–219
|hdl=10013/epic.12690
|doi=10.1007/978-3-7091-6542-3_11
|series=Plant Systematics and Evolution
|isbn=978-3-211-83035-2
|access-date=27 August 2012
|archive-date=5 October 2013
|archive-url=https://web.archive.org/web/20131005084158/http://epic.awi.de/2100/1/Med1997c.pdf
|url-status=live
}}</ref> and 200 million years ago.<ref name="Lim1986">{{Cite journal
|last1=Lim
|first1=B.-L.
|last2=Kawai
|first2=H.
|last3=Hori
|first3=H.
|last4=Osawa
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|title=Molecular evolution of 5S ribosomal RNA from red and brown algae
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|year=1986
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|issue=2
|pages=169–176
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|access-date=27 August 2012
|archive-date=10 January 2015
|archive-url=https://web.archive.org/web/20150110191502/https://www.jstage.jst.go.jp/article/ggs1921/61/2/61_2_169/_pdf
|url-status=live
}}</ref> In many ways, the evolution of the brown algae parallels that of the [[green algae]] and [[red algae]],<ref name="Dittmer1964" /> as all three groups possess complex multicellular species with an [[alternation of generations]]. Analysis of 5S [[Ribosomal RNA|rRNA]] sequences reveals much smaller evolutionary distances among genera of the brown algae than among genera of red or green algae,<ref name="Lim1986"/><ref>
{{cite journal
|last1=Hori |first1=H.
|last2=Osawa |first2=S.
|year=1987
|title=Origin and evolution of organisms as deduced from 5S ribosomal RNS sequences
|journal=[[Molecular Biology and Evolution]]
|volume=4 |issue=5 |pages=445–472
|pmid=2452957
|doi=10.1093/oxfordjournals.molbev.a040455
|doi-access=free
}}</ref> which suggests that the brown algae have diversified much more recently than the other two groups.

===Fossils===
The occurrence of Phaeophyceae as [[fossil]]s is rare due to their generally soft-bodied nature,<ref name="Arnold1947">
{{cite book
|last=Arnold |first=C. A.
|year=1947
|title=An Introduction to Paleobotany
|url=https://archive.org/details/in.ernet.dli.2015.50241 |page=[https://archive.org/details/in.ernet.dli.2015.50241/page/n64 48]
|location=New York; London
|publisher=[[McGraw-Hill]]
|isbn=978-1-4067-1861-4
}}</ref> and scientists continue to debate the identification of some finds.<ref>{{cite journal
|last1=Coyer
|first1=J. A.
|last2=Smith
|first2=G. J.
|last3=Andersen
|first3=R. A.
|year=2001
|title=Evolution of ''Macrocystis'' spp. (Phaeophyta) as determined by ITS1 and ITS2 sequences
|url=http://129.125.2.51/fmns-research/marbee/publications/CoyerJPhyc01.pdf
|journal=[[Journal of Phycology]]
|volume=37
|issue=4
|pages=574–585
|doi=10.1046/j.1529-8817.2001.037001574.x
|s2cid=84074495
|access-date=27 August 2012
|archive-date=16 October 2023
|archive-url=https://web.archive.org/web/20231016194154/https://www.rug.nl/research/fse/
|url-status=live
}}</ref> Part of the problem with identification lies in the [[convergent evolution]] of morphologies between many brown and red algae.<ref name="Fry1955">
{{cite journal
|last1=Fry |first1=W. L.
|last2=Banks |first2=H. P.
|year=1955
|title=Three new genera of algae from the Upper Devonian of New York
|journal=[[Journal of Paleontology]]
|volume=29 |issue=1
|pages=37–44
|jstor=1300127
}}</ref> Most fossils of soft-tissue algae preserve only a flattened outline, without the microscopic features that permit the major groups of multicellular algae to be reliably distinguished. Among the brown algae, only species of the genus ''[[Padina (alga)|Padina]]'' deposit significant quantities of minerals in or around their cell walls.<ref name="Prescott1968">
{{cite book
|last=Prescott |first=G. W.
|year=1968
|title=The Algae: A Review
|url=https://archive.org/details/algaereview0000pres |url-access=registration |pages=[https://archive.org/details/algaereview0000pres/page/207 207]–231, 371–372
|location=Boston
|publisher=[[Houghton Mifflin Company]]
|isbn=978-3-87429-244-3
}}</ref> Other algal groups, such as the [[red algae]] and [[green algae]], have a number of [[calcareous]] members. Because of this, they are more likely to leave evidence in the fossil record than the soft bodies of most brown algae and more often can be precisely classified.<ref name="Simpson1953">
{{cite book
|last=Simpson |first=G. G.
|year=1953
|title=Life of the Past: An Introduction to Paleontology
|url=https://archive.org/details/lifeofpastintr00simp |url-access=registration |pages=[https://archive.org/details/lifeofpastintr00simp/page/158 158–159]
|location=New Haven
|publisher=[[Yale University Press]]
}}</ref>

Fossils comparable in morphology to brown algae are known from strata as old as the Upper [[Ordovician]],<ref name="Fry1983">
{{cite journal
|last=Fry |first=W. L.
|year=1983
|title=An algal flora from the Upper Ordovician of the Lake Winnipeg region, Manitoba, Canada
|journal=[[Review of Palaeobotany and Palynology]]
|volume=39 |issue=3–4 |pages=313–341
|doi=10.1016/0034-6667(83)90018-0
|bibcode=1983RPaPa..39..313F
}}</ref> but the [[Taxonomy (biology)|taxonomic]] affinity of these impression fossils is far from certain.<ref>{{cite web
|last1=Speer
|first1=B. R.
|last2=Waggoner
|first2=B. M.
|year=2000
|title=Phaeophyta: Fossil Record
|url=http://www.ucmp.berkeley.edu/chromista/browns/phaeofr.html
|access-date=24 April 2007
|archive-date=9 June 2007
|archive-url=https://web.archive.org/web/20070609225826/http://www.ucmp.berkeley.edu/chromista/browns/phaeofr.html
|url-status=live
}}</ref> Claims that earlier [[Ediacaran]] fossils are brown algae<ref name="Loeblich1974">
{{Cite journal
|last=Loeblich |first=A. R.
|year=1974
|title=Protistan Phylogeny as Indicated by the Fossil Record
|journal=[[Taxon (journal)|Taxon]]
|volume=23 |issue=2/3 |pages=277–290
|doi=10.2307/1218707
|jstor=1218707 
}}</ref> have since been dismissed.<ref name=Lee2008/> While many [[carbon]]aceous fossils have been described from the [[Precambrian]], they are typically preserved as flattened outlines or fragments measuring only millimeters long.<ref name="Hofmann1985">
{{cite book
|last=Hofmann |first=H. J.
|year=1985
|chapter=Precambrian Carbonaceous Megafossils
|editor=D. F. Toomey |editor2=M. H. Nitecki
|title=Paleoalgology: Contemporary Research and Applications
|pages=20–33
|location=Berlin
|publisher=[[Springer-Verlag]]
}}</ref> Because these fossils lack features diagnostic for identification at even the highest level, they are assigned to fossil [[Form classification|form taxa]] according to their shape and other gross morphological features.<ref name="Hofmann1994">
{{cite book
|last=Hofmann |first=H. J.
|year=1994
|chapter=Proterozoic carbonaceous compressions ("metaphytes" and "worms")
|editor-last=Bengtson |editor-first=S.
|title=Life on Earth
|pages=342–357
|series=Nobel Symposium
|volume=84
|location=New York
|publisher=[[Columbia University Press]]
}}</ref> A number of [[Devonian]] fossils termed ''fucoids'', from their resemblance in outline to species in the genus ''[[Fucus]]'', have proven to be inorganic rather than true fossils.<ref name="Arnold1947"/> The Devonian megafossil ''[[Prototaxites]]'', which consists of masses of filaments grouped into trunk-like axes, has been considered a possible brown alga.<ref name="Bold1987">
{{cite book
|last1=Bold |first1=H. C.
|last2=Alexopoulos |first2=C. J.
|last3=Delevoryas |first3=T.
|year=1987
|title=Morphology of Plants and Fungi
|edition=5th |pages=112–131, 174–186
|location=New York
|publisher=[[Harper & Row Publishers]]
|isbn=978-0-06-040839-8
}}</ref> However, modern research favors reinterpretation of this fossil as a terrestrial [[fungus]] or fungal-like organism.<ref name="Hueber2001">
{{cite journal
|last1=Hueber |first1=F. M.
|year=2001
|title=Rotted wood-alga-fungus: the history and life of ''Prototaxites'' Dawson 1859
|journal=[[Review of Palaeobotany and Palynology]]
|volume=116 |issue=1 |pages=123–158
|doi=10.1016/S0034-6667(01)00058-6
|bibcode=2001RPaPa.116..123H
}}</ref> Likewise, the fossil ''[[Protosalvinia]]'' was once considered a possible brown alga, but is now thought to be an early [[land plant]].<ref name="Taylor 1987">
{{cite journal
 |last1        = Taylor
 |first1       = W. A.
 |last2       = Taylor
 |first2      = T. N.
 |year        = 1987
 |title       = Spore wall ultrastructure of ''Protosalvinia''
 |url         = http://paleobotany.bio.ku.edu/taylorPDFs%5C%5B1987%5D%20Taylor%20and%20Taylor-Spore%20wall%20ultrastructure%20of%20Protosalvinia.pdf
 |journal     = [[American Journal of Botany]]
 |volume      = 74
 |issue       = 3
 |pages       = 437–443
 |doi         = 10.2307/2443819
 |jstor       = 2443819
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20100617124912/http://paleobotany.bio.ku.edu/taylorPDFs/%5B1987%5D%20Taylor%20and%20Taylor-Spore%20wall%20ultrastructure%20of%20Protosalvinia.pdf
 |archive-date = 17 June 2010
}}</ref>

A number of [[Paleozoic]] fossils have been tentatively classified with the brown algae, although most have also been compared to known red algae species. ''[[Phascolophyllaphycus]]'' possesses numerous elongate, inflated blades attached to a stipe. It is the most abundant of algal fossils found in a collection made from [[Carboniferous]] strata in [[Illinois]].<ref name="Leary1986">
{{cite journal
|last=Leary |first=R. L.
|year=1986
|title=Three new genera of fossil noncalcareous algae from Valmeyeran (Mississippian) strata of Illinois
|journal=[[American Journal of Botany]]
|volume=73 |issue=3 |pages=369–375
|doi=10.2307/2444080
|jstor=2444080
}}</ref> Each hollow blade bears up to eight [[pneumatocyst]]s at its base, and the stipes appear to have been hollow and inflated as well. This combination of characteristics is similar to certain modern genera in the order [[Laminariales]] (kelps). Several fossils of ''[[Drydenia]]'' and a single specimen of ''[[Hungerfordia (alga)|Hungerfordia]]'' from the Upper Devonian of [[New York (state)|New York]] have also been compared to both brown and red algae.<ref name="Fry1955"/> Fossils of ''Drydenia'' consist of an elliptical blade attached to a branching filamentous holdfast, not unlike some species of ''[[Laminaria]]'', ''[[Porphyra]]'', or ''[[Gigartina]]''. The single known specimen of ''Hungerfordia'' branches dichotomously into lobes and resembles genera like ''[[Chondrus]]'' and ''[[Fucus]]''<ref name="Fry1955"/> or ''[[Dictyota]]''.<ref name="Bold1978">
{{cite book
|last1=Bold
|first1=H. C.
|last2=Wynne
|first2=M. J.
|year=1978
|title=Introduction to the Algae
|edition=2nd
|page=[https://archive.org/details/introductiontoal0000bold/page/27 27]
|publisher=[[Prentice-Hall]]
|isbn=978-0-13-477786-3
|url-access=registration
|url=https://archive.org/details/introductiontoal0000bold/page/27
}}</ref>

The earliest known fossils that can be assigned reliably to the Phaeophyceae come from [[Miocene]] [[diatomite]] deposits of the [[Monterey Formation]] in [[California]].<ref name="Hoek"/> Several soft-bodied brown macroalgae, such as ''[[Julescraneia]]'', have been found.<ref>
{{cite journal
|last1=Parker |first1=B. C.
|last2=Dawson |first2=E. Y.
|year=1965
|title=Non-calcareous marine algae from California Miocene deposits
|journal=[[Nova Hedwigia]]
|volume=10 |pages=273–295; plates 76–96
}}</ref>

== Classification ==

===Phylogeny===
Based on the work of Silberfeld, Rousseau & de Reviers 2014.<ref name="BioOne">{{Cite journal|last1=Silberfeld |first1=Thomas |last2=Rousseau |first2=Florence| last3=de Reviers |first3=Bruno |year=2014| title=An Updated Classification of Brown Algae (Ochrophyta, Phaeophyceae) | journal=Cryptogamie, Algologie| volume=35 | issue=2| pages=117–156| doi=10.7872/crya.v35.iss2.2014.117|s2cid=86227768 }}</ref>
{{clade| style=font-size:100%;line-height:80%
|1={{Clade
  |1={{Clade|label1=[[Discosporangiales]]
  |1={{Clade
    |1=[[Choristocarpaceae]]
    |2=[[Discosporangiaceae]]
     }}}}
  |2={{Clade
    |1={{Clade|label1=[[Ishigeales]]
    |1={{Clade
      |1=[[Ishigeaceae]]
      |2=[[Petrodermataceae]]
       }}}}
    |2={{Clade
      |label1=[[Dictyotophycidae]]
      |1={{Clade
        |1=[[Onslowiales]]
        |2={{Clade
          |1=[[Dictyotales]]
          |2={{Clade
            |1=[[Syringodermatales]]
            |label2=[[Sphacelariales]]
            |2={{Clade
              |1=[[Lithodermataceae]]
              |2={{Clade
                |1=[[Phaeostrophiaceae]]
                |2={{Clade
                  |1=[[Stypocaulaceae]]
                  |2={{Clade
                    |1=[[Cladostephaceae]]
                    |2=[[Sphacelariaceae]]
                     }}
                   }}
                 }}
               }}
             }}
           }}
         }}
      |label2=[[Fucophycidae]]
      |2={{Clade
        |1=[[Bachelotiaceae]]
        |2={{Clade
          |1=[[Desmarestiales]]
          |2={{Clade
            |1={{Clade
              |1=[[Sporochnales]] 
              |2=[[Ascoseirales]]
               }}
            |2={{Clade
              |1={{Clade
                |1=[[Ralfsiales]]
                |2={{Clade
                  |label1=[[Tilopteridales]]
                  |1={{Clade
                    |1=[[Cutleriaceae]]
                    |2={{Clade
                      |1=[[Tilopteridaceae]]
                      |2=[[Phyllariaceae]]
                       }}
                     }}
                  |2={{Clade
                    |1=[[Nemodermatales]]
                    |label2=[[Fucales]]
                    |2={{Clade
                      |1=[[Sargassaceae]]
                      |2={{Clade
                        |1=[[Durvillaeaceae]]
                        |2={{Clade
                          |1=[[Himanthaliaceae]]
                          |2=[[Fucaceae]] 
                           }}
                         }}
                       }}
                     }}
                   }}
                 }}
              |2={{Clade
                |1={{Clade|label1=[[Scytothamnales]]
                |1={{Clade
                  |1=[[Asteronemataceae]]
                  |2=[[Splachnidiaceae]]
                   }}}}
                |2={{Clade
                  |label1=[[Laminariales]]
                  |1={{Clade
                    |1={{Clade
                      |1=[[Phaeosiphoniellaceae]] 
                      |2={{Clade
                        |1=[[Akkesiphycaceae]] 
                        |2=[[Pseudochordaceae]]
                         }}
                       }}
                    |2={{Clade
                      |1=[[Chordaceae]]
                      |2={{Clade
                        |1=[[Alariaceae]]
                        |2={{Clade
                          |1=[[Agaraceae]]
                          |2=[[Laminariaceae]]
                           }}
                         }}
                       }}
                     }}
                  |2={{Clade
                    |1=[[Asterocladales]]
                    |label2=[[Ectocarpales]]
                    |2={{Clade
                      |1=[[Adenocystaceae]]
                      |2={{Clade
                        |1={{Clade
                          |1=[[Scytosiphonaceae]]
                          |2={{Clade
                            |1=[[Petrospongiaceae]] 
                            |2=[[Ectocarpaceae]] 
                             }}
                           }}
                        |2={{Clade
                          |1=[[Acinetosporaceae]] 
                          |2=[[Chordariaceae]] 
                           }}
                         }}
                       }}
                     }}
                   }}
                 }}
               }}
             }}
           }}
         }}
       }}
     }}
   }}
}}

===Taxonomy===
{{Further|List of brown algal genera}}

This is a list of the [[order (biology)|order]]s in the class Phaeophyceae:<ref name="BioOne"/><ref>{{cite web| last1=Guiry| first1=M. D.| last2=Guiry| first2=G. M.| year=2009| title=AlgaeBase| url=http://www.algaebase.org/| publisher=[[National University of Ireland]]| access-date=31 December 2012| archive-date=12 October 2009| archive-url=https://web.archive.org/web/20091012183548/http://www.algaebase.org/| url-status=live}}</ref>

* Class '''Phaeophyceae''' <small>Hansgirg 1886</small> [Fucophyceae; Melanophycidae <small>Rabenhorst 1863 stat. nov. Cavalier-Smith 2006</small>]
** Subclass [[Discosporangiophycidae]] <small>Silberfeld, Rousseau & Reviers 2014</small>
*** Order [[Discosporangiales]] <small>Schmidt 1937 emend. Kawai et al. 2007</small>
**** Family [[Choristocarpaceae]] <small>Kjellman 1891</small>
**** Family [[Discosporangiaceae]] <small>Schmidt 1937</small>
** Subclass [[Ishigeophycidae]] <small>Silberfeld, Rousseau & Reviers 2014</small>
*** Order [[Ishigeales]] <small>Cho & [[Sung Min Boo|Boo]] 2004</small>
**** Family [[Ishigeaceae]] <small>Okamura 1935</small>
**** Family [[Petrodermataceae]] <small>Silberfeld, Rousseau & Reviers 2014</small>
** Subclass [[Dictyotophycidae]] <small>Silberfeld, Rousseau & Reviers 2014</small>
*** Order [[Dictyotales]] <small>Bory de Saint-Vincent 1828 ''ex'' Phillips ''et al.''</small>
**** Family [[Dictyotaceae]] <small>Lamouroux ex Dumortier 1822</small> [Scoresbyellaceae <small>Womersley 1987</small>; Dictyopsidaceae]
*** Order [[Onslowiales]] <small>Draisma & Prud'homme van Reine 2008</small>
**** Family [[Onslowiaceae]] <small>Draisma & Prud'homme van Reine 2001</small>
*** Order [[Sphacelariales]] <small>[[Walter Migula|Migula]] 1909</small>
**** Family [[Cladostephaceae]] <small>Oltmanns 1922</small>
**** Family [[Lithodermataceae]] <small>Hauck 1883</small>
**** Family [[Phaeostrophiaceae]] <small>Kawai et al. 2005</small>
**** Family [[Sphacelariaceae]] <small>Decaisne 1842</small>
**** Family [[Sphacelodermaceae]] <small>Draisma, Prud'homme & Kawai 2010</small>
**** Family [[Stypocaulaceae]] <small>Oltmanns 1922</small>
*** Order [[Syringodermatales]] <small>Henry 1984</small>
**** Family [[Syringodermataceae]] <small>Henry 1984</small>
** Subclass [[Fucophycidae]] <small>Cavalier-Smith 1986</small>
*** Order [[Ascoseirales]] <small>[[Vsevolod Alexeevič Petrov|Petrov]]1964 emend. Moe & Henry 1982</small>
**** Family [[Ascoseiraceae]] <small>Skottsberg 1907</small>
*** Order [[Asterocladales]] <small>[[Thomas Silberfeld|T.Silberfeld]] et al. 2011</small>
**** Family [[Asterocladaceae]] <small>Silberfeld et al. 2011</small>
*** Order [[Desmarestiales]] <small>[[William Albert Setchell|Setchell]] & [[Nathaniel Lyon Gardner|Gardner]] 1925</small>
**** Family [[Arthrocladiaceae]] <small>Chauvin 1842</small>
**** Family [[Desmarestiaceae]] <small>(Thuret) Kjellman 1880</small>
*** Order [[Ectocarpales]] <small>Bessey 1907 emend. Rousseau & Reviers 1999a</small> [Chordariales <small>Setchell & Gardner 1925</small>; Dictyosiphonales <small>Setchell & Gardner 1925</small>; Scytosiphonales <small>Feldmann 1949</small>]
**** Family [[Acinetosporaceae]] <small>Hamel ex Feldmann 1937</small> [Pylaiellaceae; Pilayellaceae]
**** Family [[Adenocystaceae]] <small>Rousseau et al. 2000 emend. Silberfeld et al. 2011</small> [Chordariopsidaceae]
**** Family [[Chordariaceae]] <small>Greville 1830 emend. Peters & Ramírez 2001</small> [Myrionemataceae]
**** Family [[Ectocarpaceae]] <small>Agardh 1828 emend. Silberfeld et al. 2011</small>
**** Family [[Petrospongiaceae]] <small>Racault et al. 2009</small>
**** Family [[Scytosiphonaceae]] <small>Ardissone & Straforello 1877</small> [Chnoosporaceae <small>Setchell & Gardner 1925</small>]
*** Order [[Fucales]] <small>Bory de Saint-Vincent 1827</small> [Notheiales <small>Womersley 1987</small>; Durvillaeales <small>Petrov 1965</small>]
**** Family [[Bifurcariopsidaceae]] <small>Cho et al. 2006</small>
**** Family [[Durvillaeaceae]] <small>(Oltmanns) De Toni 1891</small>
**** Family [[Fucaceae]] <small>Adanson 1763</small>
**** Family [[Himanthaliaceae]] <small>(Kjellman) De Toni 1891</small>
**** Family [[Hormosiraceae]] <small>Fritsch 1945</small>
**** Family [[Notheiaceae]] <small>Schmidt 1938</small>
**** Family [[Sargassaceae]] <small>Kützing 1843</small> [Cystoseiraceae <small>De Toni 1891</small>]
**** Family [[Seirococcaceae]] <small>Nizamuddin 1987</small>
**** Family [[Xiphophoraceae]] <small>Cho et al. 2006</small>
*** Order [[Laminariales]] <small>[[Walter Migula|Migula]] 1909</small> [Phaeosiphoniellales <small>Silberfeld, Rousseau & Reviers 2014 ord. nov. prop.</small>]
**** Family [[Agaraceae]] <small>Postels & Ruprecht 1840</small> [Costariaceae]
**** Family [[Akkesiphycaceae]] <small>Kawai & Sasaki 2000</small>
**** Family [[Alariaceae]] <small>Setchell & Gardner 1925</small>
**** Family [[Aureophycaceae]] <small>Kawai & Ridgway 2013</small>
**** Family [[Chordaceae]] <small>Dumortier 1822</small>
**** Family [[Laminariaceae]] <small>Bory de Saint-Vincent 1827</small> [Arthrothamnaceae <small>Petrov 1974</small>]
**** Family [[Lessoniaceae]] <small>Setchell & Gardner 1925</small>
**** Family [[Pseudochordaceae]] <small>Kawai & Kurogi 1985</small>
*** Order [[Nemodermatales]] <small>Parente et al. 2008</small>
**** Family [[Nemodermataceae]] <small>Kuckuck ex Feldmann 1937</small>
*** Order [[Phaeosiphoniellales]] <small>Silberfeld, Rousseau & Reviers 2014</small>
**** Family [[Phaeosiphoniellaceae]] <small>Phillips et al. 2008</small>
*** Order [[Ralfsiales]] <small>Nakamura ex Lim & Kawai 2007</small>
**** Family [[Mesosporaceae]] <small>Tanaka & Chihara 1982</small>
**** Family [[Neoralfsiaceae]] <small>Lim & Kawai 2007</small>
**** Family [[Ralfsiaceae]] <small>Farlow 1881</small> [Heterochordariaceae <small>Setchell & Gardner 1925</small>]
*** Order [[Scytothamnales]] <small>Peters & Clayton 1998 emend. Silberfeld et al. 2011</small>
**** Family [[Asteronemataceae]] <small>Silberfeld et al. 2011</small>
**** Family [[Bachelotiaceae]] <small>Silberfeld et al. 2011</small>
**** Family [[Splachnidiaceae]] <small>Mitchell & Whitting 1892</small> [Scytothamnaceae <small>Womersley 1987</small>]
*** Order [[Sporochnales]] <small>[[Camille Sauvageau|Sauvageau]] 1926</small>
**** Family [[Sporochnaceae]] <small>Greville 1830</small>
*** Order [[Tilopteridales]] <small>Bessey 1907 emend. Phillips et al. 2008</small> [Cutleriales <small>Bessey 1907</small>]
**** Family [[Cutleriaceae]] <small>Griffith & Henfrey 1856</small>
**** Family [[Halosiphonaceae]] <small>Kawai & Sasaki 2000</small>
**** Family [[Phyllariaceae]] <small>Tilden 1935</small>
**** Family [[Stschapoviaceae]] <small>Kawai 2004</small>
**** Family [[Tilopteridaceae]] <small>Kjellman 1890</small>

[[File:Laminaria Life Cycle.png|thumb|The life cycle of a representative species Laminaria. Most Brown Algae follow this form of sexual reproduction.]] [[File:Fucus conceptacle XS3.jpg|thumb|A closeup of a Fucus's conceptacle, showing the gametes coming together to form a fertilized zygote.]]

==Life cycle==
{{more citations needed|section|date=March 2025}}
Most brown algae, with the exception of the [[Fucales]], perform [[sexual reproduction]] through [[sporic meiosis]].<ref>{{Cite book|url=https://books.google.com/books?id=qYTwAAAAMAAJ|title=Introduction to the algae: structure and reproduction|last1=Bold|first1=Harold Charles|last2=Wynne|first2=Michael James|date=1 January 1985|publisher=Prentice-Hall|isbn=9780134777467|language=en|access-date=11 May 2020|archive-date=16 October 2023|archive-url=https://web.archive.org/web/20231016194152/https://books.google.com/books?id=qYTwAAAAMAAJ|url-status=live}}</ref> Between generations, the algae go through separate [[sporophyte]] ([[diploid]]) and [[gametophyte]] ([[haploid]]) phases. The sporophyte stage is often the more visible of the two, though some species of brown algae have similar diploid and haploid phases. Free floating forms of brown algae often do not undergo [[sexual reproduction]] until they attach themselves to substrate. The haploid generation consists of male and female [[gametophyte]]s.<ref>{{Cite book|title=Plant Reproductive Ecology : Patterns and Strategies.|last=Lesley.|first=Lovett-Doust, Jon and|date=1 January 1990|publisher=Oxford University Press|isbn=9780198021926|oclc=437173314}}</ref> The fertilization of egg cells varies between species of brown algae, and may be [[Isogamy|isogamous]], [[Oogamy|oogamous]], or [[Anisogamy|anisogamous]].  Fertilization may take place in the water with eggs and motile sperm, or within the [[oogonium]] itself.

Certain species of brown algae can also perform [[asexual reproduction]] through the production of motile diploid [[zoospore]]s. These zoospores form in plurilocular [[sporangium]], and can mature into the sporophyte phase immediately.

In a representative species ''[[Laminaria]]'', there is a conspicuous [[diploid]] generation and smaller haploid generations. Meiosis takes place within several [[Locule|unilocular]] [[sporangium]] along the algae's blade, each one forming either haploid male or female [[zoospore]]s. The spores are then released from the sporangia and grow to form male and female gametophytes. The female gametophyte produces an egg in the oogonium, and the male gametophyte releases motile sperm that fertilize the egg. The fertilized zygote then grows into the mature diploid sporophyte.

In the order [[Fucales]], sexual reproduction is [[oogamous]], and the mature diploid is the only form for each generation. Gametes are formed in specialized [[conceptacle]]s that occur scattered on both surfaces of the receptacle, the outer portion of the blades of the parent plant. Egg cells and motile sperm are released from separate sacs within the conceptacles of the parent algae, combining in the water to complete fertilization. The fertilized zygote settles onto a surface and then differentiates into a leafy [[thallus]] and a finger-like holdfast. Light regulates differentiation of the zygote into blade and holdfast.

[[File:Saccharina latissima NOAA.jpg|thumb|''[[Saccharina latissima]]'' on a beach.]]

== Ecology ==
Brown algae have adapted to a wide variety of marine ecological niches including the tidal splash zone, rock pools, the whole intertidal zone and relatively deep near shore waters. They are an important constituent of some brackish water ecosystems, and have colonized freshwater on a minimum of six known occasions.<ref>{{cite journal | pmid=28509401 | doi=10.1111/jpy.12547 | volume=53 | issue=4 | title=Transitions between marine and freshwater environments provide new clues about the origins of multicellular plants and algae | year=2017 | journal=J Phycol | pages=731–745 | last1=Dittami | first1=SM | last2=Heesch | first2=S | last3=Olsen | first3=JL | last4=Collén | first4=J | s2cid=43325392 | url=https://hal.sorbonne-universite.fr/hal-01528781/file/Dittami_Transitions_between.pdf | access-date=11 May 2020 | archive-date=26 February 2020 | archive-url=https://web.archive.org/web/20200226204029/https://hal.sorbonne-universite.fr/hal-01528781/file/Dittami_Transitions_between.pdf | url-status=live }}</ref> A large number of Phaeophyceae are intertidal or upper littoral,<ref name=Lee2008/> and they are predominantly cool and cold water organisms that benefit from nutrients in up welling cold water currents and inflows from land; ''[[Sargassum]]'' being a prominent exception to this generalisation.

Brown algae growing in brackish waters are almost solely asexual.<ref name=Lee2008/>

==Chemistry==
{{Algal carbon isotopes}}
Brown algae have a {{delta|13|C|link}} value in the range of &minus;30.0‰ to &minus;10.5‰, in contrast with red algae and greens. This reflects their different metabolic pathways.<ref name=Fletcher2004>
{{Cite journal
|last1=Fletcher |first1=B. J.
|last2=Beerling |first2=D. J.
|author-link2 = David Beerling
|last3=Chaloner |first3=W. G.
|year=2004
|title=Stable carbon isotopes and the metabolism of the terrestrial Devonian organism Spongiophyton
|journal=[[Geobiology (journal)|Geobiology]]
|volume=2 | issue=2 | pages=107–119
|doi=10.1111/j.1472-4677.2004.00026.x
|bibcode=2004Gbio....2..107F
|s2cid=85079041
}}</ref>

They have [[cellulose]] walls with [[alginic acid]] and also contain the polysaccharide [[fucoidan]] in the amorphous sections of their cell walls. A few species (of ''[[Padina (alga)|Padina]]'') calcify with [[aragonite]] needles.<ref name=Lee2008/>

In addition to alginates, fucoidan and cellulose, the carbohydrate composition of brown algae consists of [[mannitol]], [[laminarin]] and [[glucan]].<ref>{{cite journal | pmc=8623139 | year=2021 | last1=Li | first1=Y. | last2=Zheng | first2=Y. | last3=Zhang | first3=Y. | last4=Yang | first4=Y. | last5=Wang | first5=P. | last6=Imre | first6=B. | last7=Wong | first7=A. C. | last8=Hsieh | first8=Y. S. | last9=Wang | first9=D. | title=Brown Algae Carbohydrates: Structures, Pharmaceutical Properties, and Research Challenges | journal=Marine Drugs | volume=19 | issue=11 | page=620 | doi=10.3390/md19110620 | pmid=34822491 | doi-access=free }}</ref>

The photosynthetic system of brown algae is made of a [[P700]] complex containing [[chlorophyll a]]. Their plastids also contain [[chlorophyll c]] and carotenoids (the most widespread of those being [[fucoxanthin]]).<ref>{{cite journal | doi = 10.1007/BF00035009 | volume=23 | title=Subunit organization of PSI particles from brown algae and diatoms: polypeptide and pigment analysis | year=1990 | journal=Photosynthesis Research | pages=181–193 | last1 = Berkaloff | first1 = Claire| issue=2 | pmid=24421060 | s2cid=7160955 }}</ref>

Brown algae produce a specific type of tannin called [[phlorotannin]]s in higher amounts than red algae do.

==Importance and uses==
Brown algae include a number of [[edible seaweed]]s. All brown algae contain [[alginic acid]] (alginate) in their cell walls, which is extracted commercially and used as an industrial thickening agent in food and for other uses.<ref>{{Cite web|url=http://www.fao.org/docrep/w6355e/w6355e04.htm|title=Alginic acid|website=www.fao.org|access-date=20 April 2017|archive-date=20 April 2017|archive-url=https://web.archive.org/web/20170420143852/http://www.fao.org/docrep/w6355e/w6355e04.htm|url-status=live}}</ref> One of these products is used in [[lithium-ion batteries]].<ref>{{Cite journal|last1=Kovalenko|first1=Igor|last2=Zdyrko|first2=Bogdan|last3=Magasinski|first3=Alexandre|last4=Hertzberg|first4=Benjamin|last5=Milicev|first5=Zoran|last6=Burtovyy|first6=Ruslan|last7=Luzinov|first7=Igor|author8-link=Gleb Yushin|last8=Yushin|first8=Gleb|date=1 January 2011|title=A Major Constituent of Brown Algae for Use in High-Capacity Li-Ion Batteries|journal=Science|volume=334|issue=6052|pages=75–79|jstor=23059304|doi=10.1126/science.1209150|pmid=21903777|bibcode=2011Sci...334...75K|s2cid=6523029 |doi-access=free}}</ref> Alginic acid is used as a stable component of a battery [[anode]]. This [[polysaccharide]] is a major component of brown algae, and is not found in land plants.

Alginic acid can also be used in [[aquaculture]]. For example, alginic acid enhances the immune system of rainbow trout. Younger fish are more likely to survive when given a diet with alginic acid.<ref>{{Cite journal|last1=Gioacchini|first1=Giorgia|last2=Lombardo|first2=Francesco|last3=Avella|first3=Matteo Alessandro|last4=Olivotto|first4=Ike|last5=Carnevali|first5=Oliana|date=1 April 2010|title=Welfare improvement using alginic acid in rainbow trout (Oncorhynchus mykiss) juveniles|journal=Chemistry and Ecology|volume=26|issue=2|pages=111–121|doi=10.1080/02757541003627738|bibcode=2010ChEco..26..111G |hdl=11566/39838 |s2cid=86175912 |issn=0275-7540|hdl-access=free}}</ref>

Brown algae including [[kelp]] beds also fix a significant portion of the earth's carbon dioxide yearly through photosynthesis.<ref>{{Cite journal|last1=Vásquez|first1=Julio A.|last2=Zuñiga|first2=Sergio|last3=Tala|first3=Fadia|last4=Piaget|first4=Nicole|last5=Rodríguez|first5=Deni C.|last6=Vega|first6=J. M. Alonso|date=1 April 2014|title=Economic valuation of kelp forests in northern Chile: values of goods and services of the ecosystem|journal=Journal of Applied Phycology|language=en|volume=26|issue=2|pages=1081–1088|doi=10.1007/s10811-013-0173-6|s2cid=14492051 |issn=0921-8971}}</ref> Additionally, they can store a great amount of carbon dioxide which can help us in the fight against climate change.<ref>{{cite journal |last1=Krause-Jensen |first1=D.|last2=Duarte|first2=C.|title=Substantial role of macroalgae in marine carbon sequestration. |journal=Nature Geoscience |date=21 February 2020 |volume=9 |issue=10|pages=737–742|doi=10.1038/ngeo2790}}</ref>
[[Sargachromanol|Sargachromanol G]], an extract of [[Sargassum siliquastrum]], has been shown to have anti-inflammatory effects.<ref>{{Cite journal|last1=Yoon|first1=Weon-Jong|last2=Heo|first2=Soo-Jin|last3=Han|first3=Sang-Chul|last4=Lee|first4=Hye-Ja|last5=Kang|first5=Gyeoung-Jin|last6=Kang|first6=Hee-Kyoung|last7=Hyun|first7=Jin-Won|last8=Koh|first8=Young-Sang|last9=Yoo|first9=Eun-Sook|date=1 August 2012|title=Anti-inflammatory effect of sargachromanol G isolated from Sargassum siliquastrum in RAW 264.7 cells|journal=Archives of Pharmacal Research|language=en|volume=35|issue=8|pages=1421–1430|doi=10.1007/s12272-012-0812-5|pmid=22941485|s2cid=39748571 |issn=0253-6269}}</ref>

===Edible brown algae===
{{col-begin}}
{{col-3}}

====[[Kelp]] (Laminariales)====
* [[Arame]] (''Eisenia bicyclis'')
* Badderlocks (''[[Alaria esculenta]]'')
* Cochayuyo (''[[Durvillaea antarctica]]'')
* ''[[Ecklonia cava]]''
* [[Kombu]] (''[[Saccharina japonica]]'')
* Oarweed (''[[Laminaria digitata]]'')
* Sea palm ''[[Postelsia palmaeformis]]''
* Sea whip (''[[Nereocystis luetkeana]]'')
* Sugar kelp (''[[Saccharina latissima]]'')
* [[Wakame]] (''Undaria pinnatifida'')
* Hirome (''[[Undaria undarioides]]'')
{{col-break}}

====[[Fucales]]====
* Bladderwrack (''[[Fucus vesiculosus]]'')
* Channelled wrack (''[[Pelvetia canaliculata]]'')
* [[Hijiki]] or Hiziki (''Sargassum fusiforme'')
* Limu Kala (''[[Sargassum echinocarpum]]'')
* ''[[Sargassum]]''
** ''[[Sargassum cinetum]]''
** ''[[Sargassum vulgare]]''
** ''[[Sargassum swartzii]]''
** ''[[Sargassum myriocysum]]''
* Spiral wrack (''[[Fucus spiralis]]'') 
* Thongweed (''[[Himanthalia elongata]]'')

{{col-break}}

====[[Ectocarpales]]====
* Mozuku (''[[Cladosiphon okamuranus]]'')
{{col end}}

==See also==
* [[Wrack (seaweed)]]

==References==
{{Reflist|2}}

==External links==
{{Commons category|Phaeophyceae}}
{{Wikispecies|Phaeophyceae}}
* [https://web.archive.org/web/20070206131605/http://www.mbari.org/staff/conn/botany/flora/browns.htm Monterey Bay Flora]
* [http://www.ucmp.berkeley.edu/tertiary/mio/monterey.html The Monterey Formation of California], [[University of California Museum of Paleontology]]
* [https://web.archive.org/web/20101230163151/http://seaweed.ucg.ie/algae/phaeophyta.html Phaeophyceae], [[National University of Ireland, Galway]]

{{Chromalveolata}}
{{Taxonbar|from=Q184573}}
{{Authority control}}

[[Category:Brown algae| ]]
[[Category:Biological oceanography]]
[[Category:Tithonian first appearances]]
[[Category:Extant Late Jurassic first appearances]]