View source for Algae

{{Short description|Diverse group of photosynthetic eukaryotic organisms}}
{{hatgrp|
{{other uses}}
{{redirect|Alga}}
}}
{{Use dmy dates|date=October 2024}}
{{paraphyletic group
| color_as = Eukaryota
| subheader = An informal term for a diverse group of photosynthetic eukaryotes
| fossil_range = {{long fossil range|Mesoproterozoic|present|ref = <ref>{{cite journal |first=N. J. |last=Butterfield |year=2000 |title=''Bangiomorpha pubescens'' n. gen., n. sp.: Implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes |journal=[[Paleobiology (journal)|Paleobiology]] |volume=26 |issue=3 |pages=386–404 |url=http://paleobiol.geoscienceworld.org/cgi/content/abstract/26/3/386 |doi=10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2 |bibcode=2000Pbio...26..386B |s2cid=36648568 |issn=0094-8373 |url-status=live |archive-url=https://web.archive.org/web/20070307035241/http://paleobiol.geoscienceworld.org/cgi/content/abstract/26/3/386 |archive-date=7 March 2007}}</ref><ref name="Gibson-2018">
{{cite journal |author=T.M. Gibson |year=2018 |title=Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis |url=https://pubs.geoscienceworld.org/gsa/geology/article/46/2/135/524864/Precise-age-of-Bangiomorpha-pubescens-dates-the |journal=[[Geology (journal)|Geology]] |volume=46 |issue=2 |pages=135–138 |doi=10.1130/G39829.1 |bibcode=2018Geo....46..135G }}</ref>}}
| image = NSW seabed 1.JPG
| image_caption = A variety of algae growing on the sea bed in shallow waters
| image2 = Водоросли пресноводного водоема 2.jpg
| image2_caption = A variety of microscopic unicellular and colonial freshwater algae
| auto = yes
| parent = Eukaryota
| includes = * [[Archaeplastida]]
** [[Viridiplantae]]/[[green algae]]
*** [[Mesostigmatophyceae]]
*** [[Chlorokybophyceae]]
*** [[Chlorophyta]]
*** [[Charophyta]]
** [[Rhodophyta]] (red algae)
** [[Glaucophyta]]
* [[Chlorarachniophytes]]
* [[Euglenid]]s
* [[Heterokont]]s
** [[Bacillariophyceae]] (Diatoms)
** [[Axodine]]s
** [[Bolidomonas]]
** [[Eustigmatophyceae]]
** [[Phaeophyceae]] (brown algae)
** [[Chrysophyceae]] (golden algae)
** [[Raphidophyceae]]
** [[Synurophyceae]]
** [[Xanthophyceae]] (yellow-green algae)
* [[Cryptophyta]]
* [[Dinoflagellata]]
* [[Haptophyta]]
| excludes_text = Typically excluded
| excludes = * [[Cyanobacteria]] (blue-green algae)
* [[Embryophyta]] (land plants)
}}

'''Algae''' ({{IPAc-en|UK|ˈ|æ|l|ɡ|i:}} {{respell|AL|ghee}}, {{IPAc-en|US|ˈ|æ|l|dʒ|i:}} {{respell|AL|jee}};<ref>{{Cite web |url=https://dictionary.cambridge.org/dictionary/english/algae |title=ALGAE &#124; English meaning - Cambridge Dictionary |access-date=6 April 2023}}</ref> {{Singular}}: '''alga''' {{IPAc-en|ˈ|æ|l|ɡ|ə}} {{respell|AL|gə}}) is an informal term for any [[organism]]s of a large and diverse group of [[photosynthesis|photosynthetic]] [[eukaryote]]s, which include [[species]] from multiple distinct [[clade]]s. Such organisms range from [[unicellular]] [[microalgae]] such as ''[[Chlorella]]'', ''[[Prototheca]]'' and the [[diatom]]s, to [[multicellular]] [[macroalgae]] such as the [[Macrocystis pyrifera|giant kelp]], a large [[brown algae|brown alga]] which may grow up to {{convert|50|m}} in length. Most algae are aquatic organisms and lack many of the distinct cell and tissue types, such as [[stoma]]ta, [[xylem]] and [[phloem]] that are found in [[embryophyte|land plants]]. The largest and most complex marine algae are called [[seaweed]]s. In contrast, the most complex freshwater forms are the ''[[Charophyta]]'', a [[phylum|division]] of [[green algae]] which includes, for example, ''[[Spirogyra]]'' and [[stonewort]]s. Algae that are carried passively by water are [[plankton]], specifically [[phytoplankton]].

Algae constitute a [[polyphyletic]] group<ref name="Nabors-2004" /> since they do not include a [[common ancestor]], and although their [[chlorophyll]]-bearing [[plastid]]s seem to have a single origin (from [[symbiogenesis]] with [[cyanobacteria]]),<ref name="Keeling-2004">{{cite journal |title=Diversity and evolutionary history of plastids and their hosts |first=Patrick J. |last=Keeling |journal=American Journal of Botany |year=2004 |volume=91 |pages=1481–1493 |doi=10.3732/ajb.91.10.1481 |issue=10 |pmid=21652304 |doi-access=free }}</ref> they were acquired in different ways. Green algae are a prominent examples of algae that have primary [[chloroplast]]s derived from [[endosymbiont]] cyanobacteria. [[Diatom]]s and brown algae are examples of algae with secondary chloroplasts derived from endosymbiotic [[red algae]], which they acquired via [[phagocytosis]].<ref>{{cite journal |first1=J. D. |last1=Palmer |first2=D. E. |last2=Soltis |first3=M. W. |last3=Chase |year=2004 |title=The plant tree of life: an overview and some points of view |journal=American Journal of Botany |volume=91 |issue=10 |pages=1437–1445 |doi=10.3732/ajb.91.10.1437 |pmid=21652302 |doi-access=free}}</ref> Algae exhibit a wide range of reproductive strategies, from simple [[asexual reproduction|asexual]] cell division to complex forms of [[sexual reproduction]] via [[spore]]s.<ref>Smithsonian National Museum of Natural History; Department of Botany. {{cite web |url=http://botany.si.edu/projects/algae/introduction.htm |title=Algae Research |access-date=25 August 2010 |url-status=live |archive-url=https://web.archive.org/web/20100702180840/http://botany.si.edu/projects/algae/introduction.htm |archive-date=2 July 2010}}</ref>

Algae lack the various structures that characterize [[plant]]s (which evolved from freshwater green algae), such as the phyllids (leaf-like structures) and [[rhizoid]]s of [[bryophyte]]s ( [[non-vascular plant]]s), and the [[root]]s, [[leaf|leaves]] and other [[xylem]]ic/[[phloem]]ic [[organ (biology)|organ]]s found in [[tracheophyte]]s ([[vascular plant]]s). Most algae are [[autotroph]]ic, although some are [[mixotroph]]ic, deriving energy both from photosynthesis and uptake of organic carbon either by [[osmotrophy]], [[Myzocytosis|myzotrophy]] or [[phagocytosis|phagotrophy]]. Some unicellular species of green algae, many [[golden algae]], [[euglenid]]s, [[dinoflagellate]]s, and other algae have become [[heterotroph]]s (also called colorless or apochlorotic algae), sometimes [[parasitic]], relying entirely on external energy sources and have limited or no photosynthetic apparatus.<ref>Pringsheim, E. G. 1963. ''Farblose Algen. Ein beitrag zur Evolutionsforschung''. Gustav Fischer Verlag, Stuttgart. 471 pp., [[species:Algae#Pringsheim (1963)]].</ref><ref>{{cite journal |last1=Tartar |first1=A. |last2=Boucias |first2=D. G. |last3=Becnel |first3=J. J. |last4=Adams |first4=B. J. |year=2003 |title=Comparison of plastid 16S rRNA (rrn 16) genes from Helicosporidium spp.: Evidence supporting the reclassification of Helicosporidia as green algae (Chlorophyta) |journal=International Journal of Systematic and Evolutionary Microbiology |volume=53 |pages=1719–1723 |doi=10.1099/ijs.0.02559-0 |pmid=14657099 |issue=Pt 6 |doi-access=free}}</ref><ref>{{cite journal |last1=Figueroa-Martinez |first1=F. |last2=Nedelcu |first2=A. M. |last3=Smith |first3=D. R. |last4=Reyes-Prieto |first4=A. |year=2015 |title=When the lights go out: the evolutionary fate of free-living colorless green algae |journal=New Phytologist |volume=206 |issue=3 |pages=972–982 |doi=10.1111/nph.13279 |pmid=26042246 |pmc=5024002}}</ref> Some other heterotrophic organisms, such as the [[apicomplexans]], are also derived from cells whose ancestors possessed  chlorophyllic plastids, but are not traditionally considered as algae. Algae have photosynthetic machinery ultimately derived from cyanobacteria that produce [[oxygen]] as a [[byproduct]] of splitting [[water molecule]]s, unlike other organisms that conduct [[anoxygenic photosynthesis]] such as [[purple sulfur bacteria|purple]] and [[green sulfur bacteria]]. Fossilized filamentous algae from the [[Vindhya]] basin have been dated to 1.6 to 1.7&nbsp;billion years ago.<ref>{{cite journal |pmid=19416859 |year=2009 |last1=Bengtson |first1=S. |last2=Belivanova |first2=V. |last3=Rasmussen |first3=B. |last4=Whitehouse |first4=M. |title=The controversial 'Cambrian' fossils of the Vindhyan are real but more than a billion years older |volume=106 |issue=19 |pages=7729–7734 |doi=10.1073/pnas.0812460106 |pmc=2683128 |journal=Proceedings of the National Academy of Sciences of the United States of America |bibcode=2009PNAS..106.7729B |doi-access=free }}</ref>

Because of the wide range of algae types, they have increasingly different industrial and traditional applications in human society. Traditional [[seaweed farming]] practices have existed for thousands of years and have strong traditions in [[East Asia]] food cultures. More modern [[algaculture]] applications extend the [[Edible seaweed|food traditions]] for other applications, including cattle feed, using algae for [[bioremediation]] or pollution control, transforming sunlight into [[algae fuel]]s or other chemicals used in industrial processes, and in medical and scientific applications. A 2020 review found that these applications of algae could play an important role in [[carbon sequestration]] to [[Climate change mitigation|mitigate climate change]] while providing lucrative value-added products for global economies.<ref>{{Cite book |last1=Paul |first1=Vishal |last2=Chandra Shekharaiah |first2=P. S. |last3=Kushwaha |first3=Shivbachan |last4=Sapre |first4=Ajit |last5=Dasgupta |first5=Santanu |last6=Sanyal |first6=Debanjan |title=Renewable Energy and Climate Change |chapter=Role of Algae in CO2 Sequestration Addressing Climate Change: A Review |date=2020 |editor-last=Deb |editor-first=Dipankar |editor2-last=Dixit |editor2-first=Ambesh |editor3-last=Chandra |editor3-first=Laltu |chapter-url=https://link.springer.com/chapter/10.1007/978-981-32-9578-0_23 |series=Smart Innovation, Systems and Technologies |volume=161 |language=en |location=Singapore |publisher=Springer |pages=257–265 |doi=10.1007/978-981-32-9578-0_23 |isbn=978-981-329-578-0 |s2cid=202902934}}</ref>

==Etymology and study==
The singular {{lang|la|alga}} is the Latin word for 'seaweed' and retains that meaning in English.<ref>{{cite book |chapter=alga, algae |title=Webster's Third New International Dictionary of the English Language Unabridged with Seven Language Dictionary |volume=1 |date=1986 |publisher=Encyclopædia Britannica, Inc}}</ref> The [[etymology]] is obscure. Although some speculate that it is related to Latin {{lang|la|algēre}}, 'be cold',<ref>{{cite book |first=Eric |last=Partridge |chapter=algae |title=Origins |url=https://archive.org/details/originsshortetym0000part |url-access=registration |date=1983|publisher=Greenwich House |isbn=9780517414255 }}</ref> no reason is known to associate seaweed with temperature. A more likely source is {{lang|la|alliga}}, 'binding, entwining'.<ref>{{cite book |title=A Latin Dictionary |chapter=Alga |first1=Charlton T. |last1=Lewis |first2=Charles |last2=Short |location=Oxford |publisher=Clarendon Press |date=1879 |chapter-url= https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0059%3Aalphabetic+letter%3DA%3Aentry+group%3D37%3Aentry%3Dalga |access-date=31 December 2017}}</ref>

The [[Ancient Greek]] word for 'seaweed' was {{lang|el|φῦκος}} ({{lang|el-Latn|phŷkos}}), which could mean either the seaweed (probably red algae) or a red dye derived from it. The Latinization, {{lang|la|fūcus}}, meant primarily the cosmetic rouge. The etymology is uncertain, but a strong candidate has long been some word related to the [[Biblical Hebrew|Biblical]] {{lang|he|פוך}} ({{lang|he-Latn|pūk}}), 'paint' (if not that word itself), a [[Kohl (cosmetics)|cosmetic eye-shadow]] used by the [[ancient Egypt]]ians and other inhabitants of the eastern Mediterranean. It could be any color: black, red, green, or blue.<ref>{{cite book |first1=Thomas Kelly |last1=Cheyne |first2=John Sutherland |last2=Black |title=Encyclopædia biblica: A critical dictionary of the literary, political and religious history, the archæology, geography, and natural history of the Bible |url= https://books.google.com/books?id=GccVAAAAYAAJ&pg=PA3525 |date=1902 |publisher=Macmillan Company |page=3525}}</ref>

The study of algae is most commonly called [[phycology]] ({{Etymology|gre|phykos|seaweed}}); the term [[wikt:algology|algology]] is falling out of use.<ref>{{Citation |title=Basic characteristics of the algae |date=2008 |url=https://www.cambridge.org/core/books/phycology/basic-characteristics-of-the-algae/64674E3DEFD655BDAB55324B95265EEC |work=Phycology |pages=3–30 |editor-last=Lee |editor-first=Robert Edward |access-date=2023-09-13 |edition=4 |place=Cambridge |publisher=Cambridge University Press |doi=10.1017/CBO9780511812897.002 |isbn=978-1-107-79688-1}}</ref>

==Classifications==
[[File:Gephyrocapsa oceanica color.jpg|thumb|False-color [[scanning electron micrograph]] of the unicellular [[coccolithophore]] ''[[Gephyrocapsa]] oceanica'']]
One definition of algae is that they "have [[chlorophyll]] as their primary [[photosynthetic pigment]] and lack a sterile covering of cells around their [[Gamete|reproductive cells]]".<ref>{{cite book |last=Lee |first=R. E. |date=2008 |title=Phycology |url=https://archive.org/details/phycology00leer_0 |url-access=registration |publisher=Cambridge University Press |isbn=9780521367448 }}</ref> On the other hand, the colorless ''Prototheca'' under ''[[Chlorophyta]]'' are all devoid of any chlorophyll. Although [[cyanobacteria]] are often referred to as "blue-green algae", most authorities exclude all [[prokaryote]]s, including cyanobacteria, from the definition of algae.<ref name="Nabors-2004">{{cite book |last=Nabors |first=Murray W. |title=Introduction to Botany |date=2004 |publisher=Pearson Education, Inc |location=San Francisco |isbn=978-0-8053-4416-5}}</ref><ref>{{cite book |editor-last=Allaby |editor-first=M. |date=1992 |title=The Concise Dictionary of Botany |publisher=Oxford University Press |chapter=Algae}}</ref>

The algae contain [[chloroplast]]s that are similar in structure to cyanobacteria. Chloroplasts contain circular [[DNA]] like that in cyanobacteria and are interpreted as representing reduced endosymbiotic [[cyanobacteria]]. However, the exact origin of the [[Chloroplast|chloroplasts]] is different among separate lineages of algae, reflecting their acquisition during different endosymbiotic events. The table below describes the composition of the three major groups of algae. Their lineage relationships are shown in the figure in the upper right. Many of these groups contain some members that are no longer photosynthetic. Some retain plastids, but not chloroplasts, while others have lost plastids entirely.<ref>{{cite journal |last1=Sato |first1=Naoki |title=Are Cyanobacteria an Ancestor of Chloroplasts or Just One of the Gene Donors for Plants and Algae? |journal=Genes |date=27 May 2021 |volume=12 |issue=6 |pages=823 |doi=10.3390/genes12060823 |doi-access=free |pmid=34071987 |pmc=8227023 |issn=2073-4425}}</ref>

[[Phylogeny]] based on [[plastid]]<ref>{{cite journal |last1=Bhattacharya |first1=D. |last2=Medlin |first2=L.|author-link2=Linda Medlin |year=1998 |journal=Plant Physiology |volume=116 |issue=1 |pages=9–15 |url= http://www.plantphysiol.org/cgi/reprint/116/1/9.pdf |doi=10.1104/pp.116.1.9 |title=Algal Phylogeny and the Origin of Land Plants |url-status=live |archive-url= https://web.archive.org/web/20090207051438/http://www.plantphysiol.org/cgi/reprint/116/1/9.pdf |archive-date=7 February 2009 |pmc=1539170}}</ref> not nucleocytoplasmic genealogy:

{{clade
|1=[[Cyanobacteria]]
|2={{clade
   |1=[[Glaucophyte]]s
   |2={{clade
      |sublabel1=''with rhodoplasts''
      |1={{clade
         |1=[[Rhodophytes]]
         |2=[[Stramenopile]]s
         |3={{clade
            |1=[[Cryptomonad|Cryptophytes]]
            |2=[[Haptophytes]]
            }}
         }}
      |sublabel2=''with chloroplasts''
      |2={{clade
         |1=[[Euglenophyte]]s
         |2={{clade
            |1=[[Chlorarachniophytes]]
            |2={{clade
               |1=[[Chlorophyte]]s
               |2={{clade
                  |1=[[Charophyte]]s
                  |2=[[Embryophytes|Land plants (Embryophyta)]]
                  }}
               }}
            }}
         }}
      }}
   }}
}}

{|class="wikitable"
|-
! Supergroup affiliation !! Members !! [[Endosymbiont]] !! Summary
|-
|[[Primoplantae]]/<br />[[Archaeplastida]]
|
* [[Chlorophyta]]
* [[Rhodophyta]]
* [[Glaucophyta]]
|[[Cyanobacteria]]
|These algae have "primary" [[chloroplast]]s, i.e. the chloroplasts are surrounded by two membranes and probably developed through a single endosymbiotic event. The chloroplasts of red algae have [[chlorophyll]]s ''a'' and ''c'' (often), and [[phycobilin]]s, while those of green algae have chloroplasts with chlorophyll ''a'' and ''b'' without phycobilins. Land plants are pigmented similarly to green algae and probably developed from them, thus the [[Chlorophyta]] is a sister taxon to the plants; sometimes the Chlorophyta, the [[Charophyta]], and land plants are grouped together as the [[Viridiplantae]].
|-
|[[Excavata]] and [[Rhizaria]]
|
*[[Chlorarachniophyte]]s
*[[Euglenid]]s
|[[Green algae]]
|
These groups have green chloroplasts containing chlorophylls ''a'' and ''b''.<ref>{{cite book |title=Biology |edition=8 |last1=Losos |first1=Jonathan B. |last2=Mason |first2=Kenneth A. |last3=Singer |first3=Susan R. |publisher=McGraw-Hill |date=2007 |isbn=978-0-07-304110-0}}</ref> Their chloroplasts are surrounded by four and three membranes, respectively, and were probably retained from ingested green algae.

'''Chlorarachniophytes''', which belong to the phylum [[Cercozoa]], contain a small [[nucleomorph]], which is a [[relict]] of the algae's [[cell nucleus|nucleus]].

'''Euglenids''', which belong to the phylum [[Euglenozoa]], live primarily in fresh water and have chloroplasts with only three membranes. The endosymbiotic green algae may have been acquired through [[myzocytosis]] rather than [[phagocytosis]].<ref>{{cite journal |last1=Archibald |first1=J. M. |last2=Keeling |first2=P. J. |title=Recycled plastids: A 'green movement' in eukaryotic evolution |journal=Trends in Genetics |volume=18 |issue=11 |date=November 2002 |pages=577–584 |doi=10.1016/S0168-9525(02)02777-4 |pmid=12414188}}</ref>

(Another group with green algae endosymbionts is the dinoflagellate genus '''[[Lepidodinium]]''', which has replaced its original endosymbiont of red algal origin with one of green algal origin. A nucleomorph is present, and the host genome still have several red algal genes acquired through endosymbiotic gene transfer. Also the euglenid and chlorarachniophyte genome contain genes of apparent red algal ancestry)<ref>{{cite journal|doi=10.1016/j.pisc.2015.07.002 | volume=6 | title=Euglena in time: Evolution, control of central metabolic processes and multi-domain proteins in carbohydrate and natural product biochemistry|year=2015|journal=Perspectives in Science|pages=84–93 | last1 = O'Neill | first1 = Ellis C. | last2 = Trick | first2 = Martin | last3 = Henrissat | first3 = Bernard | last4 = Field | first4 = Robert A.| bibcode=2015PerSc...6...84O | doi-access = free }}</ref><ref>{{Cite journal |last1=Ponce-Toledo |first1=Rafael I. |last2=López-García |first2=Purificación |last3=Moreira |first3=David |date=October 2019 |title=Horizontal and endosymbiotic gene transfer in early plastid evolution |journal=New Phytologist |language=en |volume=224 |issue=2 |pages=618–624 |doi=10.1111/nph.15965 |issn=0028-646X |pmc=6759420 |pmid=31135958}}</ref><ref>{{Cite journal |last1=Ponce-Toledo |first1=Rafael I |last2=Moreira |first2=David |last3=López-García |first3=Purificación |last4=Deschamps |first4=Philippe |date=2018-06-19 |title=Secondary Plastids of Euglenids and Chlorarachniophytes Function with a Mix of Genes of Red and Green Algal Ancestry |url=https://doi.org/10.1093/molbev/msy121 |journal=Molecular Biology and Evolution |volume=35 |issue=9 |pages=2198–2204 |doi=10.1093/molbev/msy121 |issn=0737-4038 |pmc=6949139 |pmid=29924337}}</ref>
|-
|[[Halvaria]] and [[Hacrobia]]
|
* [[Heterokont]]s
* [[Dinoflagellate]]s
* [[Haptophyta]]
* [[Cryptomonad]]s
|[[Red algae]]
|
These groups have chloroplasts containing chlorophylls ''a'' and ''c'', and phycobilins. The shape can vary; they may be of discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon shaped. They have one or more pyrenoids to preserve protein and starch. The latter chlorophyll type is not known from any prokaryotes or primary chloroplasts, but genetic similarities with red algae suggest a relationship there.<ref>{{cite journal |last1=Janson |first1=Sven |last2=Graneli |first2=Edna |title=Genetic analysis of the psbA gene from single cells indicates a cryptomonad origin of the plastid in Dinophysis (Dinophyceae) |journal=Phycologia |date=September 2003 |volume=42 |issue=5 |pages=473–477 |issn=0031-8884 |doi=10.2216/i0031-8884-42-5-473.1|bibcode=2003Phyco..42..473J |s2cid=86730888 }}</ref>

In the first three of these groups ([[Chromista]]), the chloroplast has four membranes, retaining a [[nucleomorph]] in [[cryptomonad]]s, and they likely share a common pigmented ancestor, although other evidence casts doubt on whether the [[heterokont]]s, [[Haptophyta]], and [[cryptomonad]]s are in fact more closely related to each other than to other groups.<ref>{{cite journal |title=Evaluating Support for the Current Classification of Eukaryotic Diversity |first1=Laura |last1=Wegener Parfrey|author-link1=Laura Wegener Parfrey |first2=Erika |last2=Barbero |first3=Elyse |last3=Lasser |first4=Micah |last4=Dunthorn |first5=Debashish |last5=Bhattacharya|author-link6=David J. Patterson |first6=David J. |last6=Patterson|author-link7=Laura A. Katz |first7=Laura A |last7=Katz |doi=10.1371/journal.pgen.0020220 |journal=PLOS Genetics |date=December 2006 |volume=2 |issue=12 |pages=e220 |pmid=17194223 |pmc=1713255 |doi-access=free }}</ref><ref>{{cite journal |last1=Burki |first1=F. |last2=Shalchian-Tabrizi |first2=K. |last3=Minge |first3=M. |last4=Skjæveland |first4=Å. |last5=Nikolaev |first5=S. I. |year=2007 |title=Phylogenomics Reshuffles the Eukaryotic Supergroups |journal=PLOS ONE |volume=2 |issue=8 |page=e790 |doi=10.1371/journal.pone.0000790 |pmid=17726520 |pmc=1949142 |editor-last=Butler |editor-first=Geraldine |bibcode=2007PLoSO...2..790B |display-authors=etal|doi-access=free }}</ref>

The typical dinoflagellate chloroplast has three membranes, but considerable diversity exists in chloroplasts within the group, and a number of endosymbiotic events apparently occurred.<ref name="Keeling-2004" /> The [[Apicomplexa]], a group of closely related parasites, also have plastids called [[apicoplast]]s, which are not photosynthetic, but appear to have a common origin with [[dinoflagellate]] chloroplasts.<ref name="Keeling-2004" />
|}

[[File:Gmelin - Historia Fucorum (Titelblatt).png|thumb|upright |Title page of [[Samuel Gottlieb Gmelin|Gmelin]]'s ''Historia Fucorum'', dated 1768]]

[[Carl Linnaeus|Linnaeus]], in ''[[Species Plantarum]]'' (1753),<ref>{{cite book |last=Linnæus |first=Caroli |date=1753 |title=Species Plantarum |volume=2 |page=1131 |url= https://www.biodiversitylibrary.org/item/13830#page/573/mode/1up |publisher=Impensis Laurentii Salvii}}</ref> the starting point for modern [[botanical nomenclature]], recognized 14 genera of algae, of which only four are currently considered among algae.<ref>{{Cite book |url= https://books.google.com/books?id=hOa74Hm4zDIC&pg=PA22 |title=Textbook of Algae |isbn=9780074519288 |last1=Sharma |first1=O. P. |date=1 January 1986 |page=22|publisher=Tata McGraw-Hill }}</ref> In ''[[10th edition of Systema Naturae|Systema Naturae]]'', Linnaeus described the genera ''[[Volvox]]'' and ''[[Corallina]]'', and a species of ''[[Acetabularia]]'' (as ''[[Madrepora]]''), among the animals.

In 1768, [[Samuel Gottlieb Gmelin]] (1744–1774) published the ''Historia Fucorum'', the first work dedicated to marine algae and the first book on [[marine biology]] to use the then new binomial nomenclature of Linnaeus. It included elaborate illustrations of seaweed and marine algae on folded leaves.<ref>{{cite book |last=Gmelin |first=S. G. |date=1768 |url= https://books.google.com/books?id=YUAAAAAAQAAJ&q=%22Historia+Fucorum%22 |via=Google Books |title=Historia Fucorum |publisher=Ex typographia Academiae scientiarum |location=St. Petersburg}}</ref><ref>{{cite book |last1=Silva |first1=P. C. |last2=Basson |first2=P. W. |last3=Moe |first3=R. L. |date=1996 |url= https://books.google.com/books?id=vuWEemVY8WEC&q=%22Historia+Fucorum%22+binomial+nomenclature&pg=PA2 |via=Google Books |title=Catalogue of the Benthic Marine Algae of the Indian Ocean|publisher=University of California Press |isbn=9780520915817 }}</ref>

[[William Henry Harvey|W. H. Harvey]] (1811–1866) and [[Lamouroux]] (1813)<ref name="Medlin-1997">{{cite journal |first1=Linda K. |last1=Medlin |first2=Wiebe H. C. F. |last2=Kooistra |first3=Daniel |last3=Potter |first4=Gary W. |last4=Saunders |first5=Robert A. |last5=Anderson |year=1997 |url=http://epic.awi.de/2100/1/Med1997c.pdf |title=Phylogenetic relationships of the 'golden algae' (haptophytes, heterokont chromophytes) and their plastids |url-status=live |archive-url= https://web.archive.org/web/20131005084158/http://epic.awi.de/2100/1/Med1997c.pdf |archive-date=5 October 2013 |journal=Plant Systematics and Evolution |page=188}}</ref> were the first to divide macroscopic algae into four divisions based on their pigmentation. This is the first use of a biochemical criterion in plant systematics. Harvey's four divisions are: red algae (Rhodospermae), brown algae (Melanospermae), green algae (Chlorospermae), and Diatomaceae.<ref>{{cite book |last=Dixon |first=P. S. |title=Biology of the Rhodophyta |date=1973 |publisher=Oliver & Boyd |location=Edinburgh |isbn=978-0-05-002485-0 |page=232}}</ref><ref>{{cite book |last=Harvey |first=D. |date=1836 |chapter-url= http://img.algaebase.org/pdf/562E38EB0a0fc2A17Eukv24B7E9F/18893.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://img.algaebase.org/pdf/562E38EB0a0fc2A17Eukv24B7E9F/18893.pdf |archive-date=2022-10-09 |url-status=live |access-date=31 December 2017 |title=''Flora hibernica'' comprising the Flowering Plants Ferns Characeae Musci Hepaticae Lichenes and Algae of Ireland arranged according to the natural system with a synopsis of the genera according to the Linnaean system |chapter=Algae |editor-last=Mackay |editor-first=J. T. |pages=157–254}}.</ref>

At this time, microscopic algae were discovered and reported by a different group of workers (e.g., [[Otto Friedrich Müller|O. F. Müller]] and [[Christian Gottfried Ehrenberg|Ehrenberg]]) studying the [[Infusoria]] (microscopic organisms). Unlike [[macroalgae]], which were clearly viewed as plants, [[microalgae]] were frequently considered animals because they are often motile.<ref name="Medlin-1997" /> Even the nonmotile (coccoid) microalgae were sometimes merely seen as stages of the lifecycle of plants, macroalgae, or animals.<ref>Braun, A. ''[https://www.biodiversitylibrary.org/bibliography/2057#/summary Algarum unicellularium genera nova et minus cognita, praemissis observationibus de algis unicellularibus in genere (New and less known genera of unicellular algae, preceded by observations respecting unicellular algae in general)] {{webarchive |url= https://web.archive.org/web/20160420033958/http://www.biodiversitylibrary.org/bibliography/2057 |date=20 April 2016}}.'' Lipsiae, Apud W. Engelmann, 1855. Translation at: Lankester, E. & Busk, G. (eds.). ''Quarterly Journal of Microscopical Science'', 1857, vol. 5, [http://jcs.biologists.org/content/s1-5/17/13.full.pdf+html (17), 13–16] {{webarchive |url= https://web.archive.org/web/20160304130906/http://jcs.biologists.org/content/s1-5/17/13.full.pdf+html |date=4 March 2016}}; [http://jcs.biologists.org/content/s1-5/18/90.full.pdf+html (18), 90–96] {{webarchive |url= https://web.archive.org/web/20160305133158/http://jcs.biologists.org/content/s1-5/18/90.full.pdf+html |date=5 March 2016}}; [http://jcs.biologists.org/content/s1-5/19/143.full.pdf+html (19), 143–149] {{webarchive |url= https://web.archive.org/web/20160304113651/http://jcs.biologists.org/content/s1-5/19/143.full.pdf+html |date=4 March 2016}}.</ref><ref>Siebold, C. Th. v. "[https://www.biodiversitylibrary.org/item/49155#page/5/mode/1up Ueber einzellige Pflanzen und Thiere (On unicellular plants and animals)] {{webarchive |url= https://web.archive.org/web/20141126005532/http://www.biodiversitylibrary.org/item/49155 |date=26 November 2014}}". In: Siebold, C. Th. v. & Kölliker, A. (1849). ''Zeitschrift für wissenschaftliche Zoologie'', Bd. 1, p. 270. Translation at: Lankester, E. & Busk, G. (eds.). ''Quarterly Journal of Microscopical Science'', 1853, vol. 1, [http://jcs.biologists.org/content/s1-1/2/111.full.pdf+html (2), 111–121] {{webarchive |url= https://web.archive.org/web/20160304114623/http://jcs.biologists.org/content/s1-1/2/111.full.pdf+html |date=4 March 2016}}; [http://jcs.biologists.org/content/s1-1/3/195.full.pdf+html (3), 195–206] {{webarchive |url= https://web.archive.org/web/20160304115243/http://jcs.biologists.org/content/s1-1/3/195.full.pdf+html |date=4 March 2016}}.</ref>

Although used as a taxonomic category in some pre-Darwinian classifications, e.g., Linnaeus (1753),<ref name="Ragan-2010">{{cite journal |last1=Ragan |first1= Mark |date=2010-06-03 |title=On the delineation and higher-level classification of algae |url= https://www.tandfonline.com/doi/abs/10.1080/09670269810001736483 |journal=European Journal of Phycology |volume=33 |issue=1 |pages=1–15 |doi=10.1080/09670269810001736483 |access-date=2024-02-16}}</ref> de Jussieu (1789),<ref name="de Jussieu-1789">{{cite book |last=de Jussieu |first=Antoine Laurent |date= 1789 |title=Genera plantarum secundum ordines naturales disposita |url= https://archive.org/details/generaplantarums00juss/page/n3/mode/2up  |publisher= Parisiis, Apud Viduam Herissant et Theophilum Barrois |page=6}}</ref> Lamouroux (1813), Harvey (1836), Horaninow (1843),  Agassiz (1859),  Wilson & Cassin (1864),<ref name="Ragan-2010" /> in further classifications, the "algae" are seen as an artificial, polyphyletic group.<ref name="Khan-2020b">{{cite journal |last1=Khan |first1=Amna Komal |last2=Kausar |first2=Humera |last3=Jaferi |first3=Syyada Samra |last4=Drouet |first4=Samantha |last5=Hano |first5=Christophe |last6=Abbasi |first6=Bilal Haider |last7=Anjum |first7= Sumaira |display-authors=3 |date=2020-11-06 |title="An Insight into the Algal Evolution and Genomics |journal=Biomolecules |volume=10 |issue=11 |pages=1524  |doi=10.3390/biom10111524 |doi-access=free |pmid=33172219 |pmc=7694994 }}</ref>

Throughout the 20th century, most classifications treated the following groups as divisions or classes of algae: [[cyanophyte]]s, [[rhodophyte]]s, [[chrysophyte]]s, [[xanthophyte]]s, [[diatom|bacillariophytes]], [[phaeophyte]]s, [[Dinoflagellate#History|pyrrhophytes]] ([[Cryptomonad|cryptophytes]] and [[dinophyte]]s), [[euglenophyte]]s, and [[chlorophyte]]s. Later, many new groups were discovered (e.g., [[Bolidophyceae]]), and others were splintered from older groups: [[charophyte]]s and [[glaucophyte]]s (from chlorophytes), many [[heterokontophyte]]s (e.g., [[Synurophyceae|synurophytes]] from chrysophytes, or [[eustigmatophyte]]s from xanthophytes), [[haptophyte]]s (from chrysophytes), and [[chlorarachniophyte]]s (from xanthophytes).{{citation needed|date=August 2023}}

With the abandonment of plant-animal dichotomous classification, most groups of algae (sometimes all) were included in [[Protist]]a, later also abandoned in favour of [[Eukaryota]]. However, as a legacy of the older plant life scheme, some groups that were also treated as [[protozoa]]ns in the past still have duplicated classifications (see [[ambiregnal protist]]s).{{citation needed|date=August 2023}}

Some parasitic algae (e.g., the green algae ''[[Prototheca]]'' and ''[[Helicosporidium]]'', parasites of metazoans, or ''[[Cephaleuros]]'', parasites of plants) were originally classified as [[fungi]], [[sporozoan]]s, or [[protist]]ans of ''[[incertae sedis]]'',<ref>{{cite book |last1=Williams |first1=B. A. |last2=Keeling |first2=P. J. |date=2003 |chapter=Cryptic organelles in parasitic protists and fungi |editor-last=Littlewood |editor-first=D. T. J. |title=The Evolution of Parasitism |publisher=Elsevier Academic Press |location=London |page=46 |isbn=978-0-12-031754-7 |chapter-url= https://books.google.com/books?id=_fAQGEJobT0C&pg=PA46}}</ref> while others (e.g., the green algae ''[[Phyllosiphon]]'' and ''[[Rhodochytrium]]'', parasites of plants, or the red algae ''[[Pterocladiophila]]'' and ''Gelidiocolax mammillatus'', parasites of other red algae, or the dinoflagellates ''[[Oodinium]]'', parasites of fish) had their relationship with algae conjectured early. In other cases, some groups were originally characterized as parasitic algae (e.g., ''[[Chlorochytrium]]''), but later were seen as [[endophytic]] algae.<ref>Round (1981). pp.&nbsp;398–400, {{Cite book |url= https://books.google.com/books?id=Rm08AAAAIAAJ&pg=PA398 |title=The Ecology of Algae |access-date=6 February 2015 |isbn=9780521269063 |last1=Round |first1=F. E. |date=8 March 1984|publisher=CUP Archive }}.</ref> Some filamentous bacteria (e.g., ''[[Beggiatoa]]'') were originally seen as algae. Furthermore, groups like the [[apicomplexan]]s are also parasites derived from ancestors that possessed plastids, but are not included in any group traditionally seen as algae.{{citation needed|date=August 2023}}

==Evolution==
Algae are [[polyphyletic]] thus their origin cannot be traced back to single hypothetical [[common ancestor]]. It is thought that they came into existence when photosynthetic [[coccus|coccoid]] [[cyanobacteria]] got [[phagocytosis|phagocytized]] by a [[unicellular]] [[heterotrophic]] eukaryote (a [[protist]]),<ref name="Reyes-Prieto-2007">{{cite journal|last1=Reyes-Prieto|first1=Adrian|last2=Weber|first2=Andreas P.M.|last3=Bhattacharya|first3=Debashish|year=2007|title=The Origin and Establishment of the Plastid in Algae and Plants|url=https://www.annualreviews.org/doi/10.1146/annurev.genet.41.110306.130134|journal=[[Annual Review of Genetics]]|volume=41|issue=|pages=147–168  |doi=10.1146/annurev.genet.41.110306.130134|pmid=17600460|access-date=2023-12-03}}</ref> giving rise to double-membranous primary [[plastid]]s. Such [[symbiogenesis|symbiogenic]] events (primary symbiogenesis) are believed to have occurred more than 1.5 billion years ago during the [[Calymmian]] [[period (geology)|period]], early in [[Boring Billion]], but it is difficult to track the key events because of so much time gap.<ref name="Khan-2020a">{{cite journal|last1=Khan|first1=Amna Komal|last2=Kausar|first2=Humera|last3=Jaferi|first3=Syyada Samra|last4=Drouet|first4=Samantha|last5=Hano|first5=Christophe|last6=Abbasi|first6=Bilal Haider|last7=Anjum|first7=Sumaira|title=An Insight into the Algal Evolution and Genomics|journal=Biomolecules|date=2020-11-06|volume=10|issue=11|page=1524|doi=10.3390/biom10111524|pmid=33172219 |pmc=7694994 |doi-access=free }}</ref> Primary symbiogenesis gave rise to three [[division (biology)|division]]s of [[archaeplastid]]s, namely the [[Viridiplantae]] ([[green algae]] and later [[plant]]s), [[Rhodophyta]] ([[red algae]]) and [[Glaucophyta]] ("grey algae"), whose plastids further spread into other protist lineages through eukaryote-eukaryote [[predation]], engulfments and subsequent endosymbioses (secondary and tertiary symbiogenesis).<ref name="Khan-2020a"/> This process of serial cell "capture" and "enslavement" explains the diversity of photosynthetic eukaryotes.<ref name="Reyes-Prieto-2007"/>

Recent [[genomic]] and [[phylogenomic]] approaches have significantly clarified plastid [[genome evolution]], the [[horizontal gene transfer|horizontal movement]] of [[endosymbiont]] [[genes]] to the "host" [[cell nucleus|nuclear]] [[genome]], and plastid spread throughout the eukaryotic [[tree of life (biology)|tree of life]].<ref name="Reyes-Prieto-2007"/>

===Relationship to land plants===
Fossils of isolated [[spore]]s suggest [[land plant]]s may have been around as long as 475&nbsp;[[million years ago]] (mya) during the [[Late Cambrian]]/[[Early Ordovician]] period,<ref>{{cite news |title=When plants conquered land |first=Ivan |last=Noble |date=18 September 2003 |url= http://news.bbc.co.uk/1/hi/sci/tech/3117034.stm |publisher=BBC |url-status=live |archive-url= https://web.archive.org/web/20061111170428/http://news.bbc.co.uk/1/hi/sci/tech/3117034.stm |archive-date=11 November 2006}}</ref><ref>{{cite journal |last1=Wellman |first1=C. H. |last2=Osterloff |first2=P. L. |last3=Mohiuddin |first3=U. |year=2003 |title=Fragments of the earliest land plants |journal=Nature |volume=425 |issue=6955 |pages=282–285 |doi=10.1038/nature01884 |pmid=13679913 |bibcode=2003Natur.425..282W |s2cid=4383813 |url= http://eprints.whiterose.ac.uk/106/ |url-status=live |archive-url= https://web.archive.org/web/20170830194441/http://eprints.whiterose.ac.uk/106/ |archive-date=30 August 2017}}</ref> from [[sessility (motility)|sessile]] shallow [[freshwater]] [[charophyte]] algae much like ''[[Chara (alga)|Chara]]'',<ref name="Kenrick-1997">{{cite book |last1=Kenrick |first1=P. |last2=Crane |first2=P.R. |title=The origin and early diversification of land plants. A cladistic study |isbn=978-1-56098-729-1 |year=1997 |publisher=Smithsonian Institution Press |location=Washington}}</ref> which likely got stranded ashore when [[riverine]]/[[lacustrine]] [[water level]]s dropped during [[dry season]]s.<ref name="Raven-2001">{{cite journal |author=Raven, J.A. |author2=Edwards, D. |year=2001 |title=Roots: evolutionary origins and biogeochemical significance |journal=Journal of Experimental Botany |volume=52 |issue=90001 |pages=381–401 |doi=10.1093/jexbot/52.suppl_1.381 |pmid=11326045 |doi-access=free}}</ref> These charophyte algae probably already developed filamentous [[thalli]] and [[holdfast (biology)|holdfast]]s that superficially resembled [[plant stem]]s and [[root]]s, and probably had an isomorphic [[alternation of generations]]. They perhaps evolved some 850&nbsp;mya<ref name="Knauth-2009">{{cite journal |first1=L. Paul |last1=Knauth |first2=Martin J. |last2=Kennedy |date=2009 |title=The late Precambrian greening of the Earth |journal=Nature |volume=460 |issue=7256 |pages=728–732 |doi=10.1038/nature08213 |pmid=19587681 |bibcode=2009Natur.460..728K |s2cid=4398942 }}</ref> and might even be as early as 1&nbsp;[[Gya]] during the late phase of the [[Boring Billion]].<ref name="Strother-2011">{{cite journal |first1=Paul K. |last1=Strother |first2=Leila |last2=Battison |first3= Martin D. |last3=Brasier |first4=Charles H. |last4=Wellman |date=2011 |title=Earth's earliest non-marine eukaryotes |journal=Nature |volume=473 |issue=7348 |pages=505–509 |doi=10.1038/nature09943 |pmid=21490597 |bibcode=2011Natur.473..505S |s2cid=4418860 }}</ref>

==Morphology==
[[File:Kelp-forest-Monterey.jpg|thumb|upright|The [[kelp forest]] exhibit at the Monterey Bay Aquarium: A three-dimensional, multicellular thallus]]

A range of algal [[Morphology (biology)|morphologies]] is exhibited, and [[Convergent evolution|convergence]] of features in unrelated groups is common. The only groups to exhibit three-dimensional multicellular [[Thallus|thalli]] are the [[Red algae|reds]] and [[Brown algae|browns]], and some [[Chlorophyta|chlorophytes]].<ref name="Xiao-2004">{{cite journal |last1=Xiao |first1=S. |last2=Knoll |first2=A. H. |last3=Yuan |first3=X. |last4=Pueschel |first4=C. M. |year=2004 |title=Phosphatized multicellular algae in the Neoproterozoic Doushantuo Formation, China, and the early evolution of florideophyte red algae |journal=American Journal of Botany |volume=91 |issue=2 |pages=214–227 |doi=10.3732/ajb.91.2.214 |pmid=21653378 |doi-access=free }}</ref> Apical growth is constrained to subsets of these groups: the [[Florideophyceae|florideophyte]] reds, various browns, and the charophytes.<ref name="Xiao-2004" /> The form of charophytes is quite different from those of reds and browns, because they have distinct nodes, separated by internode 'stems'; whorls of branches reminiscent of the [[horsetail]]s occur at the nodes.<ref name="Xiao-2004" /> [[Conceptacle]]s are another [[polyphyletic]] trait; they appear in the [[coralline algae]] and the [[Hildenbrandiales]], as well as the browns.<ref name="Xiao-2004" />

Most of the simpler algae are unicellular [[flagellate]]s or [[amoeboid]]s, but colonial and nonmotile forms have developed independently among several of the groups. Some of the more common organizational levels, more than one of which may occur in the [[biological life cycle|lifecycle]] of a species, are
* [[Colony (biology)|Colonial]]: small, regular groups of motile cells
* Capsoid: individual non-motile cells embedded in [[mucilage]]
* Coccoid: individual non-motile cells with cell walls
* Palmelloid: nonmotile cells embedded in mucilage
* Filamentous: a string of connected nonmotile cells, sometimes branching
* Parenchymatous: cells forming a thallus with partial differentiation of tissues

In three lines, even higher levels of organization have been reached, with full tissue differentiation. These are the brown algae,<ref>{{cite web |url= http://www.ucmp.berkeley.edu/chromista/phaeophyta.html |title=Introduction to the Phaeophyta: Kelps and brown "Algae" |first=Ben |last=Waggoner |publisher=University of California Museum of Palaeontology (UCMP) |date=1994–2008 |access-date=19 December 2008 |archive-url= https://web.archive.org/web/20081221171218/http://www.ucmp.berkeley.edu/chromista/phaeophyta.html |archive-date=21 December 2008 |url-status=dead}}</ref>—some of which may reach 50&nbsp;m in length ([[kelp]]s)<ref>{{cite book |last=Thomas |first=D. N. |title=Seaweeds |date=2002 |publisher=The Natural History Museum |location=London |isbn=978-0-565-09175-0}}</ref>—the red algae,<ref>{{cite web |url= http://www.ucmp.berkeley.edu/protista/rhodophyta.html |title=Introduction to the Rhodophyta, the red 'algae' |first=Ben |last=Waggoner |publisher=University of California Museum of Palaeontology (UCMP) |date=1994–2008 |access-date=19 December 2008 |archive-url= https://web.archive.org/web/20081218211021/http://www.ucmp.berkeley.edu/protista/rhodophyta.html |archive-date=18 December 2008 |url-status=dead}}</ref> and the green algae.<ref>{{cite web |url= http://www.ucmp.berkeley.edu/greenalgae/greenalgae.html |title=Introduction to the Green Algae |work=berkeley.edu |url-status=dead |archive-url= https://web.archive.org/web/20070213103838/http://www.ucmp.berkeley.edu/greenalgae/greenalgae.html |archive-date=13 February 2007 |access-date=15 February 2007}}</ref> The most complex forms are found among the charophyte algae (see [[Charales]] and [[Charophyta]]), in a lineage that eventually led to the higher land plants. The innovation that defines these nonalgal plants is the presence of female reproductive organs with protective cell layers that protect the zygote and developing embryo. Hence, the land plants are referred to as the [[Embryophyte]]s.

===Turfs===
The term algal turf is commonly used but poorly defined. Algal turfs are thick, carpet-like beds of seaweed that retain [[sediment]] and compete with foundation species like [[coral reef|corals]] and [[kelp forest#Kelp|kelps]], and they are usually less than 15&nbsp;cm tall. Such a turf may consist of one or more species, and will generally cover an area in the order of a square metre or more. Some common characteristics are listed:<ref name="Connell-2014" >{{cite journal|url=https://www.researchgate.net/publication/285992380 |last1=Connell |first1=Sean |last2=Foster |first2=M.S. |last3=Airoldi |first3=Laura |date=9 January 2014 |title= What are algal turfs? Towards a better description of turfs |journal=Marine Ecology Progress Series |volume=495 |pages=299–307 |doi=10.3354/meps10513 |bibcode=2014MEPS..495..299C |doi-access=free }}</ref>
* Algae that form aggregations that have been described as turfs include diatoms, cyanobacteria, chlorophytes, phaeophytes and rhodophytes. Turfs are often composed of numerous species at a wide range of spatial scales, but monospecific turfs are frequently reported.<ref name="Connell-2014" />
* Turfs can be morphologically highly variable over geographic scales and even within species on local scales and can be difficult to identify in terms of the constituent species.<ref name="Connell-2014" />
* Turfs have been defined as short algae, but this has been used to describe height ranges from less than 0.5&nbsp;cm to more than 10&nbsp;cm. In some regions, the descriptions approached heights which might be described as canopies (20 to 30&nbsp;cm).<ref name="Connell-2014" />

{{expand section|date=February 2021}}

==Physiology==
Many algae, particularly species of the [[Characeae]],<ref>{{Cite book |last=Tazawa |first=Masashi |chapter-url= https://books.google.com/books?id=iMxH0-q42PkC&pg=PA31 |access-date=7 October 2012 |volume=72 |date=2010 |publisher=Springer |isbn=978-3-642-13145-5 |pages=5–34 |doi=10.1007/978-3-642-13145-5_1 |title=Progress in Botany 72 |chapter=Sixty Years Research with Characean Cells: Fascinating Material for Plant Cell Biology}}</ref> have served as model experimental organisms to understand the mechanisms of the water permeability of membranes, [[osmoregulation]], [[turgor regulation]],{{clarify|reason=how is turgor regulation distinct from osmoregulation|date=June 2021}} [[salt tolerance]], [[cytoplasmic streaming]], and the generation of [[action potentials]]. [[Plant hormones]] are found not only in higher plants, but in algae, too.<ref>{{cite journal |last1=Tarakhovskaya |first1=E. R. |last2=Maslov |first2=Yu. I. |last3=Shishova |first3=M. F. |date=April 2007 |title=Phytohormones in algae |journal=Russian Journal of Plant Physiology |volume=54 |issue=2 |pages=163–170 |doi=10.1134/s1021443707020021|s2cid=27373543 }}</ref>

==Symbiotic algae==
Some species of algae form [[symbiosis|symbiotic relationships]] with other organisms. In these symbioses, the algae supply photosynthates (organic substances) to the host organism providing protection to the algal cells. The host organism derives some or all of its energy requirements from the algae. Examples are:

===Lichens===
{{Main|Lichen}}
[[File:Lichens near Clogher Head (stevefe).jpg|thumb|Rock lichens in Ireland]]
[[Lichen]]s are defined by the [[International Association for Lichenology]] to be "an association of a fungus and a photosynthetic [[symbiont]] resulting in a stable vegetative body having a specific structure".<ref>{{cite book |last1=Brodo |first1=Irwin M. |last2=Sharnoff |first2=Sylvia Duran |last3=Sharnoff |first3=Stephen |last4=Laurie-Bourque |first4=Susan |title=Lichens of North America |date=2001 |publisher=Yale University Press |location=New Haven |isbn=978-0-300-08249-4 |page=8}}</ref> The fungi, or mycobionts, are mainly from the [[Ascomycota]] with a few from the [[Basidiomycota]]. In nature, they do not occur separate from lichens. It is unknown when they began to associate.<ref>{{cite book |last=Pearson |first=Lorentz C. |title=The Diversity and Evolution of Plants |date=1995 |publisher=CRC Press |isbn=978-0-8493-2483-3 |page=221}}</ref> One or more<ref>{{Cite journal |last=Tuovinen |first=Veera |last2=Ekman |first2=Stefan |last3=Thor |first3=Göran |last4=Vanderpool |first4=Dan |last5=Spribille |first5=Toby |last6=Johannesson |first6=Hanna |date=2019-01-17 |title=Two Basidiomycete Fungi in the Cortex of Wolf Lichens |url=https://linkinghub.elsevier.com/retrieve/pii/S0960982218316543 |journal=Current Biology |volume=29 |issue=3 |pages=476–483.e5 |doi=10.1016/j.cub.2018.12.022 |issn=0960-9822}}</ref> mycobiont associates with the same phycobiont species, from the green algae, except that alternatively, the mycobiont may associate with a species of cyanobacteria (hence "photobiont" is the more accurate term). A photobiont may be associated with many different mycobionts or may live independently; accordingly, lichens are named and classified as fungal species.<ref>Brodo et al. (2001), p. 6: "A species of lichen collected anywhere in its range has the same lichen-forming fungus and, generally, the same photobiont. (A particular photobiont, though, may associate with scores of different lichen fungi)."</ref> The association is termed a morphogenesis because the lichen has a form and capabilities not possessed by the symbiont species alone (they can be experimentally isolated). The photobiont possibly triggers otherwise latent genes in the mycobiont.<ref>Brodo et al. (2001), p. 8.</ref>

[[Trentepohlia (alga)|Trentepohlia]] is an example of a common green alga genus worldwide that can grow on its own or be lichenised. Lichen thus share some of the habitat and often similar appearance with specialized species of algae (''[[aerophyte]]s'') growing on exposed surfaces such as tree trunks and rocks and sometimes discoloring them.

===Coral reefs===
{{Main|Coral|Coral reef|Symbiodinium}}
[[File:Coral Reef.jpg|thumb|Floridian coral reef]] [[Coral reef]]s are accumulated from the [[calcareous]] exoskeletons of [[marine invertebrate]]s of the order [[Scleractinia]] (stony [[coral]]s). These animals [[Metabolism|metabolize]] sugar and oxygen to obtain energy for their cell-building processes, including [[secretion]] of the exoskeleton, with water and [[carbon dioxide]] as byproducts. Dinoflagellates (algal protists) are often [[endosymbiont]]s in the cells of the coral-forming marine invertebrates, where they accelerate host-cell metabolism by generating sugar and oxygen immediately available through photosynthesis using incident light and the carbon dioxide produced by the host. Reef-building stony corals ([[hermatypic coral]]s) require endosymbiotic algae from the genus ''[[Symbiodinium]]'' to be in a healthy condition.<ref>{{cite book |last=Taylor |first=Dennis L. |editor-last=Goff |editor-first=Lynda J. |title=Algal Symbiosis: A Continuum of Interaction Strategies |date=1983 |publisher=CUP Archive |isbn=978-0-521-25541-7 |pages=[https://archive.org/details/algalsymbiosisco0000unse/page/19 19]–20 |contribution=The coral-algal symbiosis |url-access=registration |url= https://archive.org/details/algalsymbiosisco0000unse}}</ref> The loss of ''Symbiodinium'' from the host is known as [[coral bleaching]], a condition which leads to the deterioration of a reef.

===Sea sponges===
{{Main|Sea sponge}}
[[Endosymbiont]]ic green algae live close to the surface of some sponges, for example, breadcrumb sponges (''[[Halichondria panicea]]''). The alga is thus protected from predators; the sponge is provided with oxygen and sugars which can account for 50 to 80% of sponge growth in some species.<ref>{{cite journal |url= http://uwsp.edu/cnr/UWEXlakes/laketides/vol26-4/vol26-4.pdf |title=Are There Sponges in Your Lake? |first=Susan |last=Knight |journal=Lake Tides |via=UWSP.edu |volume=26 |issue=4 |pages=4–5 |publisher=Wisconsin Lakes Partnership |date=Fall 2001 |access-date=4 August 2007 |url-status=dead |archive-url= https://web.archive.org/web/20070702204058/http://www.uwsp.edu/cnr/UWEXlakes/laketides/vol26-4/vol26-4.pdf |archive-date=2 July 2007}}</ref>

==Life cycle==
{{details|Conceptacle}}
[[Red algae|Rhodophyta]], [[Chlorophyta]], and [[Heterokont]]ophyta, the three main algal [[phylum|divisions]], have life cycles which show considerable variation and complexity. In general, an asexual phase exists where the seaweed's cells are [[Ploidy|diploid]], a sexual phase where the cells are [[haploid]], followed by fusion of the male and female [[gamete]]s. Asexual reproduction permits efficient population increases, but less variation is possible. Commonly, in sexual reproduction of unicellular and colonial algae, two specialized, sexually compatible, haploid gametes make physical contact and fuse to form a [[zygote]]. To ensure a successful mating, the development and release of gametes is highly synchronized and regulated; pheromones may play a key role in these processes.<ref>{{cite journal |last1=Frenkel |first1=J. |last2=Vyverman |first2=W. |last3=Pohnert |first3=G. |title=Pheromone signaling during sexual reproduction in algae |journal=Plant J. |volume=79 |issue=4 |pages=632–644 |year=2014 |pmid=24597605 |doi=10.1111/tpj.12496|doi-access=free }}</ref> Sexual reproduction allows for more variation and provides the benefit of efficient recombinational repair of DNA damages during [[meiosis]], a key stage of the sexual cycle.<ref>{{Cite journal |last1=Bernstein |first1=Harris |last2=Byerly |first2=Henry C. |last3=Hopf |first3=Frederic A. |last4=Michod |first4=Richard E. |date=1985-09-20 |title=Genetic Damage, Mutation, and the Evolution of Sex |url=https://www.science.org/doi/10.1126/science.3898363 |journal=Science |language=en |volume=229 |issue=4719 |pages=1277–1281 |bibcode=1985Sci...229.1277B |doi=10.1126/science.3898363 |issn=0036-8075 |pmid=3898363}}</ref> However, sexual reproduction is more costly than asexual reproduction.<ref>{{cite journal |last=Otto |first=S. P. |title=The evolutionary enigma of sex |journal=Am. Nat. |volume=174 |issue=Suppl 1 |pages=S1–S14 |year=2009 |pmid=19441962 |doi=10.1086/599084 |s2cid=9250680 |url= https://www.researchgate.net/publication/24427058 |url-status=live |archive-url= https://web.archive.org/web/20170409111359/https://www.researchgate.net/publication/24427058 |archive-date=9 April 2017}}</ref> Meiosis has been shown to occur in many different species of algae.<ref>{{cite journal |last1=Heywood |first1=P. |last2=Magee |first2=P. T. |title=Meiosis in protists: Some structural and physiological aspects of meiosis in algae, fungi, and protozoa |journal=Bacteriol Rev |volume=40 |issue=1 |pages=190–240 |year=1976 |pmid=773364 |pmc=413949 |doi=10.1128/MMBR.40.1.190-240.1976}}</ref>

==Numbers==
[[File:Taiwan 2009 East Coast ShihTiPing Giant Stone Steps Algae FRD 6581.jpg|thumb|Algae on coastal rocks at [[:zh:石梯坪|Shihtiping]] in [[Taiwan]]]]
The ''Algal Collection of the US National Herbarium'' (located in the [[National Museum of Natural History]]) consists of approximately 320,500 dried specimens, which, although not exhaustive (no exhaustive collection exists), gives an idea of the order of magnitude of the number of algal species (that number remains unknown).<ref>{{cite web |title=Algae Herbarium |publisher=National Museum of Natural History, Department of Botany |year=2008 |url= http://botany.si.edu/projects/algae/herbarium.htm |access-date=19 December 2008 |archive-url= https://web.archive.org/web/20081201112552/http://botany.si.edu/projects/algae/herbarium.htm |archive-date=1 December 2008 |url-status=live}}</ref> Estimates vary widely. For example, according to one standard textbook,<ref>John (2002), p. 1.</ref> in the [[British Isles]] the ''UK Biodiversity Steering Group Report'' estimated there to be 20,000 algal species in the UK. Another checklist reports only about 5,000 species. Regarding the difference of about 15,000 species, the text concludes: "It will require many detailed field surveys before it is possible to provide a reliable estimate of the total number of species&nbsp;..."

Regional and group estimates have been made, as well:
* 5,000–5,500 species of red algae worldwide
* "some 1,300 in Australian Seas"<ref>Huisman (2000), p. 25.</ref>
* 400 seaweed species for the western coastline of South Africa,<ref>Stegenga (1997).</ref> and 212 species from the coast of KwaZulu-Natal.<ref>{{cite book |last=Clerck |first=Olivier |title=Guide to the seaweeds of KwaZulu-Natal |date=2005 |publisher=National Botanic Garden of Belgium |isbn=978-90-72619-64-8}}</ref> Some of these are duplicates, as the range extends across both coasts, and the total recorded is probably about 500 species. Most of these are listed in [[List of seaweeds of South Africa (disambiguation)|List of seaweeds of South Africa]]. These exclude [[phytoplankton]] and crustose corallines.
* 669 marine species from California (US)<ref>Abbott and Hollenberg (1976), p. 2.</ref>
* 642 in the check-list of Britain and Ireland<ref>Hardy and Guiry (2006).</ref>
and so on, but lacking any scientific basis or reliable sources, these numbers have no more credibility than the British ones mentioned above. Most estimates also omit microscopic algae, such as phytoplankton.

The most recent estimate suggests 72,500 algal species worldwide.<ref>{{cite journal |last=Guiry |first=Michael D. |author-link=Michael D. Guiry |year=2012 |title=How Many Species of Algae Are There? |url= https://www.researchgate.net/publication/263561612 |journal=Journal of Phycology |volume=48 |issue=5 |pages=1057–1063 |doi=10.1111/j.1529-8817.2012.01222.x |pmid=27011267|bibcode=2012JPcgy..48.1057G |s2cid=30911529 }}</ref>

==Distribution==
The distribution of algal species has been fairly well studied since the founding of [[phytogeography]] in the mid-19th century.<ref name="Round-1981">{{cite book |last=Round |first=F. E. |date=1981 |title=The ecology of algae |chapter=Chapter 8, Dispersal, continuity and phytogeography |pages=357–361 |publisher=CUP Archive |chapter-url= https://books.google.com/books?id=Rm08AAAAIAAJ&pg=PA398 |via=Google Books |isbn=9780521269063}}</ref> Algae spread mainly by the dispersal of [[spore]]s analogously to the dispersal of [[cryptogam]]ic [[plant]]s by [[spore]]s. Spores can be found in a variety of environments: fresh and marine waters, air, soil, and in or on other organisms.<ref name="Round-1981" /> Whether a spore is to grow into an adult organism depends on the species and the environmental conditions where the spore lands.

The spores of freshwater algae are dispersed mainly by running water and wind, as well as by living carriers.<ref name="Round-1981" /> However, not all bodies of water can carry all species of algae, as the chemical composition of certain water bodies limits the algae that can survive within them.<ref name="Round-1981" /> Marine spores are often spread by ocean currents. Ocean water presents many vastly different habitats based on temperature and nutrient availability, resulting in phytogeographic zones, regions, and provinces.<ref>Round (1981), p. 362.</ref>

To some degree, the distribution of algae is subject to floristic discontinuities caused by geographical features, such as [[Antarctica]], long distances of ocean or general land masses. It is, therefore, possible to identify species occurring by locality, such as "[[Pacific]] algae" or "[[North Sea]] algae". When they occur out of their localities, hypothesizing a transport mechanism is usually possible, such as the hulls of ships. For example, ''[[Ulva reticulata]]'' and ''[[U. fasciata]]'' travelled from the mainland to [[Hawaii]] in this manner.

Mapping is possible for select species only: "there are many valid examples of confined distribution patterns."<ref>Round (1981), p. 357.</ref> For example, ''[[Clathromorphum]]'' is an arctic genus and is not mapped far south of there.<ref>Round (1981), p. 371.</ref> However, scientists regard the overall data as insufficient due to the "difficulties of undertaking such studies."<ref>Round (1981), p. 366.</ref>

==Ecology==
[[File:Phytoplankton Lake Chuzenji.jpg|thumb|left|Phytoplankton, [[Lake Chūzenji]]]]
Algae are prominent in bodies of water, common in terrestrial environments, and are found in unusual environments, such as on [[Snow algae|snow]] and [[Ice algae|ice]]. Seaweeds grow mostly in shallow marine waters, under {{convert|100|m|ft|abbr=on}} deep; however, some such as ''[[Navicula]] pennata'' have been recorded to a depth of {{convert|360|m|ft|abbr=on}}.<ref>Round (1981), p. 176.</ref> A type of algae, ''Ancylonema nordenskioeldii'', was found in [[Greenland]] in areas known as the 'Dark Zone', which caused an increase in the rate of melting ice sheet.<ref>{{cite web |url=https://www.space.com/40266-greenland-dark-zone-gets-darker.html |title=Greenland Has a Mysterious 'Dark Zone' — And It's Getting Even Darker |website=Space.com |date=10 April 2018 }}</ref> The same algae was found in the [[Italian Alps]], after pink ice appeared on parts of the Presena glacier.<ref>{{cite web |url=https://news.sky.com/story/alpine-glacier-turning-pink-due-to-algae-that-accelerates-climate-change-scientists-say-12022244 |title=Alpine glacier turning pink due to algae that accelerates climate change, scientists say |website=Sky News |date=6 July 2020 }}</ref>

The various sorts of algae play significant roles in aquatic ecology. Microscopic forms that live suspended in the water column ([[phytoplankton]]) provide the food base for most marine [[food chain]]s. In very high densities ([[algal bloom]]s), these algae may discolor the water and outcompete, poison, or [[asphyxiate]] other life forms.

Algae can be used as [[indicator organism]]s to monitor pollution in various aquatic systems.<ref name="Omar-2010">{{cite journal |title=Perspectives on the Use of Algae as Biological Indicators for Monitoring and Protecting Aquatic Environments, with Special Reference to Malaysian Freshwater Ecosystems |first=Wan Maznah Wan |last=Omar |pmc=3819078 |journal=Trop Life Sci Res |date=Dec 2010 |volume=21 |issue=2 |pages=51–67 |pmid=24575199}}</ref> In many cases, algal metabolism is sensitive to various pollutants. Due to this, the species composition of algal populations may shift in the presence of chemical pollutants.<ref name="Omar-2010" /> To detect these changes, algae can be sampled from the environment and maintained in laboratories with relative ease.<ref name="Omar-2010" />

On the basis of their habitat, algae can be categorized as: [[Aquatic plant|aquatic]] ([[Phytoplankton|planktonic]], [[Benthic zone|benthic]], [[Marine biology|marine]], [[Freshwater ecosystem|freshwater]], [[Lake ecosystem|lentic]], [[River ecosystem|lotic]]),<ref>Necchi Jr., O. (ed.) (2016). ''River Algae''. Springer, {{Cite book |url= https://books.google.com/books?id=KptPDAAAQBAJ |title=River Algae |isbn=9783319319841 |last1=Necchi |first1=Orlando J. R. |date=2 June 2016|publisher=Springer }}.</ref> [[Terrestrial plant|terrestrial]], [[Aerobiology|aerial]] (subaerial),<ref>{{cite book |last=Johansen |first=J. R. |date=2012 |title=Diatoms of aerial habitats |editor1-last=Smol |editor1-first=J. P. |editor2-last=Stoermer |editor2-first=E. F. |chapter=The Diatoms: Applications for the Environmental and Earth Sciences |edition=2nd |publisher=Cambridge University Press |pages=465–472 |isbn=9781139492621 |chapter-url= https://books.google.com/books?id=SpuPKw7zZGAC&pg=PA465 |via=Google Books}}</ref> [[Lithophyte|lithophytic]], [[Halophyte|halophytic]] (or [[euryhaline]]), [[psammon]], [[Thermophile|thermophilic]], [[Psychrophile|cryophilic]], [[epibiont]] ([[Epiphyte|epiphytic]], [[epizoic]]), [[endosymbiont]] ([[endophytic]], endozoic), [[parasitic]], [[calcareous|calcifilic]] or [[lichen]]ic (phycobiont).<ref>Sharma, O. P. (1986). pp.&nbsp;2–6, [https://books.google.com/books?id=hOa74Hm4zDIC&pg=PA2].</ref>

==Cultural associations==
In [[classical Chinese]], the word {{lang|zh|{{linktext|藻}}}} is used both for "algae" and (in the modest tradition of the [[scholar-official|imperial scholars]]) for "literary talent". The third island in [[Kunming Lake]] beside the [[Summer Palace]] in Beijing is known as the Zaojian Tang Dao (藻鑒堂島), which thus simultaneously means "Island of the Algae-Viewing Hall" and "Island of the Hall for Reflecting on Literary Talent".

== Cultivation ==
{{Excerpt|Algaculture}}

=== Seaweed farming ===
{{Excerpt|seaweed farming}}

=== Bioreactors ===
{{Excerpt|Algae bioreactor|paragraphs=1}}

==Uses==
[[File:Algae Harvester.jpg|thumb|Harvesting algae]]

===Agar===
[[Agar]], a [[gelatin]]ous substance derived from red algae, has a number of commercial uses.<ref>{{cite book |last1=Lewis |first1=J. G. |last2=Stanley |first2=N. F. |last3=Guist |first3=G. G. |editor1-last=Lembi |editor1-first=C. A. |editor2-last=Waaland |editor2-first=J. R. |title=Algae and Human Affairs |date=1988 |publisher=Cambridge University Press |isbn=978-0-521-32115-0 |contribution=9. Commercial production of algal hydrocolloides}}</ref> It is a good medium on which to grow bacteria and fungi, as most microorganisms cannot digest agar.

===Alginates===
[[Alginic acid]], or alginate, is extracted from [[brown algae]]. Its uses range from gelling agents in food, to medical dressings. Alginic acid also has been used in the field of [[biotechnology]] as a [[Biocompatibility|biocompatible medium]] for cell encapsulation and cell immobilization. [[Molecular cuisine]] is also a user of the substance for its gelling properties, by which it becomes a delivery vehicle for flavours.

Between 100,000 and 170,000 wet tons of ''[[Macrocystis]]'' are harvested annually in [[New Mexico]] for [[Alginic acid|alginate]] extraction and [[abalone]] feed.<ref>{{cite web |url= http://www.algaebase.org/generadetail.lasso?genus_id=35715&-session=abv3:51909EC307dcf25DFApmi3530315 |publisher=AlgaeBase |title=Macrocystis C. Agardh 1820: 46 |access-date=28 December 2008 |url-status=live |archive-url= https://web.archive.org/web/20090104145632/http://www.algaebase.org/generadetail.lasso?genus_id=35715&-session=abv3%3A51909EC307dcf25DFApmi3530315 |archive-date=4 January 2009}}</ref><ref>{{cite web |url= http://botany.si.edu/projects/algae/economicuses/brownalgae.htm |work=Algae Research |publisher=Smithsonian National Museum of Natural History |title=Secondary Products of Brown Algae |access-date=29 December 2008 |url-status=live |archive-url= https://web.archive.org/web/20090413034226/http://botany.si.edu/projects/algae/economicuses/brownalgae.htm |archive-date=13 April 2009}}</ref>

===Energy source===
{{Main|Algae fuel|Biological hydrogen production|Biohydrogen|Biodiesel|Ethanol fuel|Butanol fuel|Vegetable oil|Biogas|Hydrothermal Liquefaction}}

To be competitive and independent from fluctuating support from (local) policy on the long run, biofuels should equal or beat the cost level of fossil fuels. Here, algae-based fuels hold great promise,<ref>{{cite journal |last=Chisti |first=Y. |title=Biodiesel from microalgae |journal=Biotechnology Advances |date=May–Jun 2007 |volume=25 |issue=3 |pages=294–306 |pmid=17350212 |doi=10.1016/j.biotechadv.2007.02.001 |s2cid=18234512 |url= https://www.academia.edu/2137836}}</ref><ref>{{cite journal |last1=Yang |first1=Z. K. |last2=Niu |first2=Y. F. |last3=Ma |first3=Y. H. |last4=Xue |first4=J. |last5=Zhang |first5=M. H. |last6=Yang |first6=W. D. |last7=Liu |first7=J. S. |last8=Lu |first8=S. H. |last9=Guan |first9=Y. |last10=Li |first10=H. Y. |title=Molecular and cellular mechanisms of neutral lipid accumulation in diatom following nitrogen deprivation. |journal=Biotechnology for Biofuels |date=4 May 2013 |volume=6 |issue=1 |page=67 |pmid=23642220 |doi=10.1186/1754-6834-6-67 |pmc=3662598 |doi-access=free }}</ref> directly related to the potential to produce more biomass per unit area in a year than any other form of biomass. The break-even point for algae-based biofuels is estimated to occur by 2025.<ref>{{Cite journal |doi=10.1126/science.1189003 |pmid=20705853 |bibcode=2010Sci...329..796W |title=An Outlook on Microalgal Biofuels |journal=Science |volume=329 |issue=5993 |pages=796–799 |last1=Wijffels |first1=René H. |last2=Barbosa |first2=Maria J. |s2cid=206526311 |year=2010 }}</ref>

===Fertilizer===
{{Details|Seaweed fertiliser}}
[[File:Inisheer landscape.jpg|thumb|Seaweed-fertilized gardens on [[Inisheer]]]]
For centuries, seaweed has been used as a fertilizer; [[George Owen of Henllys]] writing in the 16th century referring to drift weed in [[South Wales]]:<ref>{{cite journal |journal=Journal of the Royal Agricultural Society of England |volume=10 |pages=142–143 |title=On the Farming of South Wales: Prize Report |first=Clare Sewell |last=Read |author-link=Clare Sewell Read |year=1849 |url= https://books.google.com/books?id=UJYEAAAAYAAJ&pg=PA142}}</ref>
{{Quote|This kind of ore they often gather and lay on great heapes, where it heteth and rotteth, and will have a strong and loathsome smell; when being so rotten they cast on the land, as they do their muck, and thereof springeth good corn, especially barley&nbsp;... After spring-tydes or great rigs of the sea, they fetch it in sacks on horse backes, and carie the same three, four, or five miles, and cast it on the lande, which doth very much better the ground for corn and grass.}}

Today, algae are used by humans in many ways; for example, as [[fertilizer]]s, [[soil conditioner]]s, and livestock feed.<ref>{{cite book |last=McHugh |first=Dennis J. |title=A Guide to the Seaweed Industry: FAO Fisheries Technical Paper 441 |chapter-url= http://www.fao.org/DOCREP/006/Y4765E/y4765e0c.htm#TopOfPage |date=2003 |publisher=Fisheries and Aquaculture Department, Food and Agriculture Organization (FAO) of the United Nations |location=Rome |isbn=978-92-5-104958-7|chapter=9, Other Uses of Seaweeds |url-status=live |archive-url= https://web.archive.org/web/20081228115716/http://www.fao.org/docrep/006/y4765e/y4765e0c.htm#TopOfPage |archive-date=28 December 2008}}</ref> Aquatic and microscopic species are cultured in clear tanks or ponds and are either harvested or used to treat effluents pumped through the ponds. [[Algaculture]] on a large scale is an important type of [[aquaculture]] in some places. [[Maerl]] is commonly used as a soil conditioner.

===Nutrition===
{{See also|Edible seaweed|Algae powder}}
[[File:Dulse.JPG|thumb|Dulse, a type of edible seaweed]]
Naturally growing seaweeds are an important source of food, especially in Asia, leading some to label them as [[superfood]]s.<ref>{{cite journal |last1=Jung |first1=Frederich |last2=Kruger-Genge |first2=Anne |last3=Kupper |first3=J.-H. |last4=Waldeck |first4=P |title=Spirulina platensis, a super food? |journal=ResearchGate |date=April 2019 |volume=5 |page=43 |url=https://www.researchgate.net/publication/332594307 |access-date=21 December 2020 |language=en}}</ref> They provide many vitamins including: A, [[Thiamine|B<sub>1</sub>]], [[Riboflavin|B<sub>2</sub>]], [[Vitamin B6|B<sub>6</sub>]], [[Niacin (nutrient)|niacin]], and [[Vitamin C|C]], and are rich in [[iodine]], [[potassium]], iron, [[magnesium]], and [[calcium]].<ref>{{cite book |last=Simoons |first=Frederick J. |title=Food in China: A Cultural and Historical Inquiry |date=1991 |publisher=CRC Press |isbn=978-0-936923-29-1 |pages=179–190|chapter=6, Seaweeds and Other Algae}}</ref> In addition, commercially cultivated microalgae, including both algae and cyanobacteria, are marketed as nutritional supplements, such as [[Spirulina (dietary supplement)|spirulina]],<ref>{{cite web |url= http://www.siu.edu/~ebl/leaflets/algae.htm |work=Ethnobotanical Leaflets |title=Modern Uses of Cultivated Algae |publisher=Southern Illinois University Carbondale |first=Steve L. |last=Morton |access-date=26 December 2008 |archive-url= https://web.archive.org/web/20081223081614/http://www.siu.edu/~ebl/leaflets/algae.htm |archive-date=23 December 2008 |url-status=dead }}</ref> ''[[Chlorella]]'' and the vitamin-C supplement from ''[[Dunaliella]]'', high in [[beta-carotene]].

Algae are national foods of many nations: China consumes more than 70 species, including ''[[fat choy (vegetable)|fat choy]]'', a cyanobacterium considered a vegetable; Japan, over 20 species such as ''[[nori]]'' and ''[[aonori]]'';<ref>{{cite book |last1=Mondragón |first1=Jennifer |last2=Mondragón |first2=Jeff |title=Seaweeds of the Pacific Coast |date=2003 |publisher=Sea Challengers Publications |location=Monterey, California |isbn=978-0-930118-29-7}}</ref> Ireland, [[dulse]]; [[Chile]], [[cochayuyo]].<ref>{{cite web |url= http://www.algaebase.org/speciesdetail.lasso?species_id=11752&sk=0&from=results&-session=abv3:51909EC30802716127sVj3EDC9C7 |publisher=AlgaeBase |title=Durvillaea antarctica (Chamisso) Hariot}}</ref> [[Porphyra umbilicalis|Laver]] is used to make [[laverbread]] in [[Wales]], where it is known as {{lang|cy|bara lawr}}. In [[Korea]], [[green laver]] is used to make {{lang|ko-Latn|[[Gim (Korean food)|gim]]}}. It is also used along the west coast of North America from California to [[British Columbia]], in Hawaii and by the [[Māori people|Māori]] of [[New Zealand]]. [[Sea lettuce]] and [[Alaria esculenta|badderlocks]] are salad ingredients in [[Scotland]], Ireland, [[Greenland]], and [[Iceland]]. Algae is being considered a potential solution for world hunger problem.<ref>{{Cite web |url= https://www.weforum.org/agenda/2017/10/how-marine-algae-could-help-feed-the-world |title=How marine algae could help feed the world |work=World Economic Forum |date=25 October 2017 |access-date=21 June 2018}}</ref><ref>{{Cite web |url= https://www.weforum.org/agenda/2017/12/why-algae-and-seaweed-could-be-part-of-solving-the-global-hunger-crisis |title=One solution to global hunger could be at the bottom of the ocean |work=World Economic Forum |date=15 December 2017 |access-date=21 June 2018}}</ref><ref>{{Cite news |url= https://science.howstuffworks.com/innovation/edible-innovations/algae-food-future.htm |title=Algae: Pond Scum or Food of the Future? |date=12 June 2018 |work=HowStuffWorks |access-date=21 June 2018}}</ref>

Two popular forms of algae are used in cuisine:
* ''[[Chlorella]]'': This form of alga is found in freshwater and contains [[photosynthetic]] pigments in its [[chloroplast]]. It is high in [[iron]], [[zinc]], [[magnesium]], [[vitamin B2]] and [[Omega-3 fatty acid]]s.
Furthermore, it contains all nine of the essential [[amino acid]]s the body does not produce on its own<ref>{{cite journal |last1=Rani |first1=Komal |last2=Sandal |first2=Nidi |last3=Sahoo |first3=PK |title=A comprehensive review on chlorella- its composition, health benefits, market and regulatory scenario |journal=The Pharma Innovation Journal |date=2018 |volume=7 |issue=7 |page=585 |url=https://www.thepharmajournal.com/archives/2018/vol7issue7/PartJ/7-7-62-339.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.thepharmajournal.com/archives/2018/vol7issue7/PartJ/7-7-62-339.pdf |archive-date=2022-10-09 |url-status=live |access-date=21 December 2020}}</ref>
* ''[[Spirulina (genus)|Spirulina]]'': Known otherwise as a cyanobacterium (a [[prokaryote]] or a "blue-green alga")

The oils from some algae have high levels of [[unsaturated fatty acid]]s. For example, ''[[Parietochloris incisa]]'' is high in [[arachidonic acid]], where it reaches up to 47% of the triglyceride pool.<ref>{{cite journal |first1=C. |last1=Bigogno |first2=I. |last2=Khozin-Goldberg |first3=S. |last3=Boussiba |first4=A. |last4=Vonshak |first5=Z. |last5=Cohen |year=2002 |journal=Phytochemistry |volume=60 |pages=497–503 |doi=10.1016/S0031-9422(02)00100-0 |pmid=12052516 |title=Lipid and fatty acid composition of the green oleaginous alga Parietochloris incisa, the richest plant source of arachidonic acid |issue=5 |bibcode=2002PChem..60..497B |url= https://www.researchgate.net/publication/11320290 |url-status=live |archive-url= https://web.archive.org/web/20171001204617/https://www.researchgate.net/publication/11320290 |archive-date=1 October 2017}}</ref> Some varieties of algae favored by [[vegetarianism]] and [[veganism]] contain the long-chain, essential [[omega-3 fatty acid]]s, [[docosahexaenoic acid]] (DHA) and [[eicosapentaenoic acid]] (EPA). Fish oil contains the omega-3 fatty acids, but the original source is algae (microalgae in particular), which are eaten by marine life such as [[copepod]]s and are passed up the food chain.<ref>{{cite news |url= https://www.npr.org/templates/story/story.php?storyId=15823852 |work=Morning Edition |publisher=[[NPR]] |title=Getting Brain Food Straight from the Source |first=Allison |last=Aubrey |date=1 November 2007 |url-status=live |archive-url= https://web.archive.org/web/20071103025842/http://www.npr.org/templates/story/story.php?storyId=15823852 |archive-date=3 November 2007}}</ref> Algae have emerged in recent years as a popular source of omega-3 fatty acids for vegetarians who cannot get long-chain EPA and DHA from other vegetarian sources such as [[flaxseed oil]], which only contains the short-chain [[alpha-linolenic acid]] (ALA).

===Pollution control===
* Sewage can be treated with algae,<ref>{{cite web |title=Re-imagining algae |date=12 October 2016 |publisher=Australian Broadcasting Corporation |url= http://www.abc.net.au/radionational/programs/futuretense/re-imagining-algae/7926214-AU |access-date=26 January 2017 |url-status=live |archive-url= https://web.archive.org/web/20170202043210/http://www.abc.net.au/radionational/programs/futuretense/re-imagining-algae/7926214 |archive-date=2 February 2017}}</ref> reducing the use of large amounts of toxic chemicals that would otherwise be needed.
* Algae can be used to capture fertilizers in runoff from farms. When subsequently harvested, the enriched algae can be used as fertilizer.
* Aquaria and ponds can be filtered using algae, which absorb nutrients from the water in a device called an [[algae scrubber]], also known as an algae turf scrubber.<ref>{{cite web |url= http://www.reefbase.org/resource_center/publication/main.aspx?refid=10859 |title=Nutrient cycling in the Great Barrier Reef Aquarium – Proceedings of the 6th International Coral Reef Symposium, Australia |year=1988 |volume=2 |last1=Morrissey |first1=J. |last2=Jones |first2=M. S. |last3=Harriott |first3=V. |publisher=ReefBase |url-status=live |archive-url= https://web.archive.org/web/20150223045428/http://www.reefbase.org/resource_center/publication/main.aspx?refid=10859 |archive-date=23 February 2015}}</ref><ref>{{cite journal |url= http://www3.interscience.wiley.com/journal/120083425/abstract |archive-url= https://archive.today/20101001181747/http://www3.interscience.wiley.com/journal/120083425/abstract |url-status=dead |archive-date=1 October 2010 |title=Algal Response to Nutrient Enrichment in Forested Oligotrophic Stream |doi=10.1111/j.1529-8817.2008.00503.x |pmid=27041416 |volume=44 |issue=3 |journal=Journal of Phycology |pages=564–572 |year=2008 |last1=Veraart |first1=Annelies J. |last2=Romaní |first2=Anna M. |last3=Tornés |first3=Elisabet |last4=Sabater |first4=Sergi|bibcode= 2008JPcgy..44..564V |s2cid= 2040067 }}</ref>

[[Agricultural Research Service]] scientists found that 60–90% of nitrogen runoff and 70–100% of phosphorus runoff can be captured from [[manure effluents]] using a horizontal algae scrubber, also called an [[algal turf scrubber]] (ATS). Scientists developed the ATS, which consists of shallow, 100-foot raceways of nylon netting where algae colonies can form, and studied its efficacy for three years. They found that algae can readily be used to reduce the nutrient runoff from agricultural fields and increase the quality of water flowing into rivers, streams, and oceans. Researchers collected and dried the nutrient-rich algae from the ATS and studied its potential as an organic fertilizer. They found that cucumber and corn seedlings grew just as well using ATS organic fertilizer as they did with commercial fertilizers.<ref>{{cite web |url= http://www.ars.usda.gov/is/AR/archive/may10/algae0510.htm |title=Algae: A Mean, Green Cleaning Machine |publisher=USDA Agricultural Research Service |date=7 May 2010 |url-status=live |archive-url= https://web.archive.org/web/20101019142625/http://www.ars.usda.gov/is/AR/archive/may10/algae0510.htm |archive-date=19 October 2010}}</ref> Algae scrubbers, using bubbling upflow or vertical waterfall versions, are now also being used to filter aquaria and ponds.

===Polymers===
Various polymers can be created from algae, which can be especially useful in the creation of bioplastics. These include hybrid plastics, cellulose-based plastics, poly-lactic acid, and bio-polyethylene.<ref>{{Cite web |url= http://www.oilgae.com/non_fuel_products/biopolymers.html |title=Algae Biopolymers, Companies, Production, Market – Oilgae – Oil from Algae |work=oilgae.com |access-date=18 November 2017}}</ref> Several companies have begun to produce algae polymers commercially, including for use in flip-flops<ref>{{Cite news |url= https://www.zmescience.com/science/algae-flip-flop/ |title=Renewable flip flops: scientists produce the 'No. 1' footwear in the world from algae |date=9 October 2017 |work=ZME Science |access-date=18 November 2017}}</ref> and in surf boards.<ref>{{Cite web |url= https://www.energy.gov/eere/bioenergy/articles/world-s-first-algae-surfboard-makes-waves-san-diego |title=World's First Algae Surfboard Makes Waves in San Diego |work=Energy.gov |access-date=18 November 2017}}</ref>

===Bioremediation===
The alga ''[[Stichococcus bacillaris]]'' has been seen to colonize silicone resins used at archaeological sites; [[Biodegradation|biodegrading]] the synthetic substance.<ref>{{cite journal |title=Microorganisms Attack Synthetic Polymers in Items Representing Our Cultural Heritage |first1=Francesca |last1=Cappitelli |first2=Claudia |last2=Sorlini |journal=Applied and Environmental Microbiology |year=2008 |volume=74 |pmc=2227722 |issue=3 |pages=564–569 |doi=10.1128/AEM.01768-07 |pmid=18065627|bibcode=2008ApEnM..74..564C }}</ref>

===Pigments===
The natural [[pigment]]s ([[carotenoid]]s and [[chlorophyll]]s) produced by algae can be used as alternatives to chemical [[dye]]s and coloring agents.<ref>{{cite book |last1=Arad |first1=Shoshana |last2=Spharim |first2=Ishai |editor-last=Altman |editor-first=Arie |title=Agricultural Biotechnology |series=Books in Soils, Plants, and the Environment |volume=61 |date=1998 |publisher=CRC Press |isbn=978-0-8247-9439-2 |page=638 |contribution=Production of Valuable Products from Microalgae: An Emerging Agroindustry}}</ref>
The presence of some individual algal pigments, together with specific pigment concentration ratios, are taxon-specific: analysis of their concentrations with various analytical methods, particularly [[high-performance liquid chromatography]], can therefore offer deep insight into the taxonomic composition and relative abundance of natural algae populations in sea water samples.<ref>{{cite journal |first1=C. |last1=Rathbun |first2=A. |last2=Doyle |first3=T. |last3=Waterhouse |date=June 1994 |title=Measurement of Algal Chlorophylls and Carotenoids by HPLC |url= http://bats.bios.edu/methods/chapter13.pdf |journal=Joint Global Ocean Flux Study Protocols |volume=13 |pages=91–96 |url-status=dead |archive-url= https://web.archive.org/web/20160304064738/http://bats.bios.edu/methods/chapter13.pdf |archive-date=4 March 2016 |access-date=7 July 2014}}</ref><ref>{{cite journal |first1=M. |last1=Latasa |first2=R. |last2=Bidigare |year=1998 |title=A comparison of phytoplankton populations of the Arabian Sea during the Spring Intermonsoon and Southwest Monsoon of 1995 as described by HPLC-analyzed pigments |journal=Deep-Sea Research Part II |issue=10–11 |pages=2133–2170 |doi=10.1016/S0967-0645(98)00066-6 |bibcode=1998DSRII..45.2133L |volume=45}}</ref>

===Stabilizing substances===
{{Main|Carrageenan|Chondrus crispus}}
Carrageenan, from the red alga ''Chondrus crispus'', is used as a stabilizer in milk products.

==Additional images==
<gallery widths="200" heights="150">
File:Algae bladder 4290.jpg|Algae bladder
</gallery>

==See also==
{{portal|Algae}}
<!---♦♦♦ Please keep the list in alphabetical order ♦♦♦--->
* [[AlgaeBase]]
* [[AlgaePARC]]
* [[Eutrophication]]
* [[Iron fertilization]]
* ''[[Marimo]]'' algae
* [[Microbiofuels]]
* [[Microphyte]]
* [[Photobioreactor]]
* [[Phycotechnology]]
* [[Plant|Plants]]
* [[Toxoid]] – anatoxin

==References==
{{Reflist}}

==Bibliography==
{{Refbegin|30em}}

===General===
* {{cite book |last=Chapman |first=V.J. |title=Seaweeds and their Uses |date=1950 |publisher=Methuen |location=London |isbn=978-0-412-15740-0}}
* {{cite book |last=Fritsch |first=F. E. |orig-year=1935 |date=1945 |title=The Structure and Reproduction of the Algae |volume=I & II |publisher=Cambridge University Press}}
* {{cite book |first1=C. |last1=van den Hoek |first2=D. G. |last2=Mann |first3=H. M. |last3=Jahns |date=1995 |title=Algae: An Introduction to Phycology |publisher=Cambridge University Press}}
* {{cite book|first=Ruth|last=Kassinger|title=Slime: How Algae Created Us, Plague Us, and Just Might Save Us|publisher=Mariner|year=2020}}
* {{cite book |last1=Lembi |first1=C. A. |last2=Waaland |first2=J.R. |title=Algae and Human Affairs |date=1988 |publisher=Cambridge University Press |isbn=978-0-521-32115-0}}
* {{cite book |last1=Mumford |first1=T. F. |last2=Miura |first2=A. |chapter=''Porphyra'' as food: cultivation and economic |editor-last=Lembi |editor-first=C. A. |editor2-last=Waaland |editor2-first=J. R. |title=Algae and Human Affairs |date=1988 |publisher=Cambridge University Press |isbn=978-0-521-32115-0 |pages=87–117}}.
* {{cite book |last=Round |first=F. E. |title=The Ecology of Algae |date=1981 |publisher=Cambridge University Press |location=London |isbn=978-0-521-22583-0}}
* {{cite book |last=Smith |first=G. M. |date=1938 |url= https://archive.org/details/cryptogamicbotan031880mbp |title=Cryptogamic Botany |volume=I |publisher=McGraw-Hill |location=New York}}
* {{cite book |title=Cottonii and Spinosum Cultivation Handbook |last=Ask |first=E.I |year=1990 |publisher=FMC BioPolymer Corporation.Philippines }}

===Regional===
====Britain and Ireland====
* {{cite book |last1=Brodie |first1=Juliet |last2=Burrows |first2=Elsie M. |last3=Chamberlain |first3=Yvonne M. |last4=Christensen |first4=Tyge |last5=Dixon |first5=Peter Stanley |last6=Fletcher |first6=R. L. |last7=Hommersand |first7=Max H. |last8=Irvine |first8=Linda M. |last9=Maggs |first9=Christine A. |title=Seaweeds of the British Isles: A Collaborative Project of the British Phycological Society and the British Museum (Natural History) |date=1977–2003 |publisher=British Museum of Natural History, HMSO / Intercept |location=London / Andover |isbn=978-0-565-00781-2}}
* {{cite book |last=Cullinane |first=John P. |date=1973 |title=Phycology of the South Coast of Ireland |location=Cork |publisher=Cork University Press}}
* {{cite book |last1=Hardy |first1=F. G. |last2=Aspinall |first2=R. J. |title=An Atlas of the Seaweeds of Northumberland and Durham |date=1988 |publisher=Northumberland Biological Records Centre |location=The Hancock Museum, University Newcastle upon Tyne |isbn=978-0-9509680-5-6}}
* {{cite book |last1=Hardy |first1=F. G. |last2=Guiry |first2=Michael D. |author2-link=Michael D. Guiry |last3=Arnold|first3=Henry R. |title=A Check-list and Atlas of the Seaweeds of Britain and Ireland |edition=Revised |date=2006 |publisher=British Phycological Society |location=London |isbn=978-3-906166-35-3}}
* {{cite book |last1=John |first1=D. M. |last2=Whitton |first2=B. A. |last3=Brook |first3=J. A. |title=The Freshwater Algal Flora of the British Isles |date=2002 |publisher=Cambridge University Press |location=Cambridge / New York |isbn=978-0-521-77051-4}}
* {{cite book |last1=Knight |first1=Margery |last2=Parke |first2=Mary W. |title=Manx Algae: An Algal Survey of the South End of the Isle of Man |date=1931 |location=Liverpool |publisher=University Press |series=Liverpool Marine Biology Committee Memoirs on Typical British Marine Plants & Animals |volume=XXX}}
* {{cite book |last=Morton |first=Osborne |date=1994 |title=Marine Algae of Northern Ireland |location=Belfast |publisher=Ulster Museum |isbn=978-0-900761-28-7}}
* {{cite journal |last=Morton |first=Osborne |title=The Marine Macroalgae of County Donegal, Ireland |journal=Bulletin of the Irish Biogeographical Society |volume=27 |pages=3–164 |date=1 December 2003}}

====Australia====
* {{cite book |last=Huisman |first=J. M. |title=Marine Plants of Australia |date=2000 |publisher=University of Western Australia Press |isbn=978-1-876268-33-6}}

====New Zealand====
* {{cite book |last1=Chapman |first1=Valentine Jackson |last2=Lindauer |first2=VW |last3=Aiken |first3=M. |last4=Dromgoole |first4=F. I. |title=The Marine algae of New Zealand |date=1970 |orig-year=1900, 1956, 1961, 1969 |location=London / Lehre, Germany |publisher=Linnean Society of London / Cramer}}

====Europe====
* {{cite book |last1=Cabioc'h |first1=Jacqueline |last2=Floc'h |first2=Jean-Yves |last3=Le Toquin |first3=Alain |last4=Boudouresque |first4=Charles-François |last5=Meinesz |first5=Alexandre |last6=Verlaque |first6=Marc |title=Guide des algues des mers d'Europe: Manche/Atlantique-Méditerranée |date=1992 |publisher=Delachaux et Niestlé |location=Lausanne, Suisse |language=fr |isbn=978-2-603-00848-5}}
* {{cite book |title=Les Algues de côtes françaises (manche et atlantique), notions fondamentales sur l'écologie, la biologie et la systématique des algues marines |first=Paulette |last=Gayral |language=fr |location=Paris |publisher=Doin, Deren et Cie |date=1966}}
* {{cite book |last1=Guiry |first1=Michael. D. |author1-link=Michael D. Guiry |last2=Blunden |first2=G. |title=Seaweed Resources in Europe: Uses and Potential |date=1991 |publisher=John Wiley & Sons |isbn=978-0-471-92947-5}}
* {{cite book |last=Míguez Rodríguez |first=Luís |date=1998 |title=Algas mariñas de Galicia: Bioloxía, gastronomía, industria |language=gl |publisher=Edicións Xerais de Galicia |location=Vigo |isbn=978-84-8302-263-4}}
* {{cite book |last=Otero |first=J. |title=Guía das macroalgas de Galicia |date=2002 |publisher=Baía Edicións |location=A Coruña |language=gl |isbn=978-84-89803-22-0}}
* {{cite book |last1=Bárbara |first1=I. |last2=Cremades |first2=J. |date=1993 |title=Guía de las algas del litoral gallego |language=es |publisher=Concello da Coruña – Casa das Ciencias |location=A Coruña}}

====Arctic====
* {{cite book |last=Kjellman |first=Frans Reinhold |date=1883 |title=The algae of the Arctic Sea: A survey of the species, together with an exposition of the general characters and the development of the flora |location=Stockholm |publisher=Kungl. Svenska vetenskapsakademiens handlingar |volume=20 |issue=5 |pages=1–350}}

====Greenland====
* {{cite book |last=Lund |first=Søren Jensen |date=1959 |title=The Marine Algae of East Greenland |location=Kövenhavn |publisher=C.A. Reitzel |id=9584734}}

====Faroe Islands====
* {{cite book |first=Frederik |last=Børgesen |contribution=Marine Algae |pages=339–532 |editor-last=Warming |editor-first=Eugene |title=Botany of the Faröes Based Upon Danish Investigations, Part II |location=Copenhagen |publisher=Det nordiske Forlag |orig-year=1903 |date=1970}}.

====Canary Islands====
* {{cite book |first=Frederik |last=Børgesen |title=Marine Algae from the Canary Islands |date=1936 |orig-year=1925, 1926, 1927, 1929, 1930 |location=Copenhagen |publisher=Bianco Lunos}}

====Morocco====
* {{cite book |title=Algues de la côte atlantique marocaine |first=Paulette |last=Gayral |date=1958 |publisher=Rabat [Société des sciences naturelles et physiques du Maroc] |location=Casablanca |language=fr}}

====South Africa====
* {{cite book |last1=Stegenga |first1=H. |last2=Bolton |first2=J. J. |last3=Anderson |first3=R. J. |title=Seaweeds of the South African West Coast |date=1997 |publisher=Bolus Herbarium, University of Cape Town |isbn=978-0-7992-1793-3}}

====North America====
* {{cite book |last1=Abbott |first1=I. A. |last2=Hollenberg |first2=G. J. |title=Marine Algae of California |date=1976 |publisher=Stanford University Press |location=California |isbn=978-0-8047-0867-8}}
* {{cite book |last=Greeson |first=Phillip E. |date=1982 |title=An annotated key to the identification of commonly occurring and dominant genera of Algae observed in the Phytoplankton of the United States |publisher=US Department of the Interior, Geological Survey |location=Washington DC |url= https://archive.org/details/annotatedkeytoid00gree |access-date=19 December 2008}}
* {{cite book |last=Taylor |first=William Randolph |date=1969 |orig-year=1937, 1957, 1962 |title=Marine Algae of the Northeastern Coast of North America |publisher=University of Michigan Press |location=Ann Arbor |isbn=978-0-472-04904-2}}
* {{cite book |last1=Wehr |first1=J. D. |last2=Sheath |first2=R. G. |title=Freshwater Algae of North America: Ecology and Classification |date=2003 |publisher=Academic Press |isbn=978-0-12-741550-5}}
{{Refend}}

==External links==
{{Commons category|Algae}}
{{Wikispecies|Algae}}
* {{cite web |url= http://www.algaebase.org |title=AlgaeBase |first1=Michael |last1=Guiry |author1-link=Michael D. Guiry |first2=Wendy |last2=Guiry}} – a database of all algal names including images, nomenclature, taxonomy, distribution, bibliography, uses, extracts
* {{cite web |url= http://ccdb.ucsd.edu/sand/main?stype=lite&keyword=algae&event=display&Submit=Go&start=1 |title=Algae – Cell Centered Database |work=CCDb.UCSD.edu |publisher=University of California |location=San Diego}}
* {{cite web |first1=Don |last1=Anderson |first2=Bruce |last2=Keafer |first3=Judy |last3=Kleindinst |first4=Katie |last4=Shaughnessy |first5=Katherine |last5=Joyce |first6=Danielle |last6=Fino |first7=Adam |last7=Shepherd |title=Harmful Algae |year=2007 |publisher=US National Office for Harmful Algal Blooms |url= http://www.whoi.edu/redtide/page.do?pid=14779 |access-date=19 December 2008 |archive-url= https://web.archive.org/web/20081205151336/http://www.whoi.edu/redtide/page.do?pid=14779 |archive-date=5 December 2008 |url-status=live}}
* {{cite web |title=About Algae |work=NMH.ac.uk|publisher=Natural History Museum, United Kingdom |url= http://www.nhm.ac.uk/research-curation/scientific-resources/biodiversity/uk-biodiversity/algaevision/about-algae/index.html }}

{{Botany}}
{{Protist structures}}

{{Authority control}}

[[Category:Algae| ]]
[[Category:Endosymbiotic events]]
[[Category:Polyphyletic groups]]
[[Category:Common names of organisms]]