Algae#Morphology

The singular {{lang|la|alga}} is the Latin word for 'seaweed' and retains that meaning in English.{{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}} The etymology is obscure. Although some speculate that it is related to Latin {{lang|la|algēre}}, 'be cold',{{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 }} no reason is known to associate seaweed with temperature. A more likely source is {{lang|la|alliga}}, 'binding, entwining'.{{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}}

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 {{lang|he|פוך}} ({{lang|he-Latn|pūk}}), 'paint' (if not that word itself), a cosmetic eye-shadow used by the ancient Egyptians and other inhabitants of the eastern Mediterranean. It could be any color: black, red, green, or blue.{{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}}

The study of algae is most commonly called phycology ({{Etymology|gre|phykos|seaweed}}); the term algology is falling out of use.{{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}}

File:Gephyrocapsa oceanica color.jpg of the unicellular coccolithophore Gephyrocapsa oceanica]]

The algae are a heterogeneous group of mostly photosynthetic organisms that produce oxygen and lack the reproductive features and structural complexity of land plants. This concept includes the cyanobacteria, which are prokaryotes, and all photosynthetic protists, which are eukaryotes. They contain chlorophyll a as their primary photosynthetic pigment, and generally inhabit aquatic environments.{{cite book |last=Lee |first=Robert Edward |date=2008 |title=Phycology |url=https://archive.org/details/phycology00leer_0 |url-access=registration |publisher=Cambridge University Press |isbn=9780521367448 }}

However, there are many exceptions to this definition. Many non-photosynthetic protists are included in the study of algae, such as the heterotrophic relatives of euglenophytes or the numerous species of colorless algae that have lost their chlorophyll during evolution (e.g., Prototheca). Some exceptional species of algae tolerate dry terrestrial habitats, such as soil, rocks, or caves hidden from light sources, although they still need enough moisture to become active.

File:Kelp-forest-Monterey.jpg exhibit at the Monterey Bay Aquarium: A three-dimensional, multicellular thallus]]

A range of algal morphologies is exhibited, and convergence of features in unrelated groups is common. The only groups to exhibit three-dimensional multicellular thalli are the reds and browns, and some chlorophytes.{{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 }} Apical growth is constrained to subsets of these groups: the florideophyte reds, various browns, and the charophytes. 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 horsetails occur at the nodes. Conceptacles are another polyphyletic trait; they appear in the coralline algae and the Hildenbrandiales, as well as the browns.

Most of the simpler algae are unicellular flagellates or amoeboids, 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 lifecycle of a species, are

• 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,{{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}}—some of which may reach 50 m in length (kelps){{cite book |last=Thomas |first=D. N. |title=Seaweeds |date=2002 |publisher=The Natural History Museum |location=London |isbn=978-0-565-09175-0}}—the red algae,{{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}} and the green algae.{{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}} 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 Embryophytes.

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 corals and kelps, and they are usually less than 15 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:{{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 }}

• 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.
• 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.
• Turfs have been defined as short algae, but this has been used to describe height ranges from less than 0.5 cm to more than 10 cm. In some regions, the descriptions approached heights which might be described as canopies (20 to 30 cm).

Many algae, particularly species of the Characeae,{{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}} have served as model experimental organisms to understand the mechanisms of the water permeability of membranes, osmoregulation, salt tolerance, cytoplasmic streaming, and the generation of action potentials. Plant hormones are found not only in higher plants, but in algae, too.{{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|bibcode=2007RuJPP..54..163T |s2cid=27373543 }}

{{details|Conceptacle}}

Rhodophyta, Chlorophyta, and Heterokontophyta, the three main algal divisions, have life cycles which show considerable variation and complexity. In general, an asexual phase exists where the seaweed's cells are diploid, a sexual phase where the cells are haploid, followed by fusion of the male and female gametes. 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.{{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 |bibcode=2014PlJ....79..632F }} 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.{{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}} However, sexual reproduction is more costly than asexual reproduction.{{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 |bibcode=2009ANat..174S...1O |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}} Meiosis has been shown to occur in many different species of algae.{{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}}

The most recent estimate (as of January 2024) documents 50,605 living and 10,556 fossil algal species, according to the online database AlgaeBase.{{efn|name=chlorarachniophytes|Chlorarachniophytes were ommitted from the 2024 AlgaeBase species report. The numbers shown here for the order Chlorarachniales were obtained from the 13th edition of Syllabus der Pflanzenfamilien (2015), where it contains 8 genera and 14 species total.{{cite book|editor-first1=Wolfgang|editor-last1=Frey|title=Syllabus of Plant Families: A. Engler's Syllabus der Pflanzenfamilien. Part 2/1: Photoautotrophic eukaryotic Algae: Glaucocystophyta, Cryptophyta, Dinophyta/Dinozoa, Haptophyta, Heterokontophyta/Ochrophyta, Chlorarachniophyta/Cercozoa, Euglenophyta/Euglenozoa, Chlorophyta, Streptophyta p.p.|chapter=Division Chlorarachniophyta D.J.Hibberd & R.E.Norris / Cercozoa Cavalier-Smith|first1=Hiroshi|last1=Kawai|first2=Takeshi|last2=Nakayama|date=2015|publisher=Gebr. Borntraeger Verlagsbuchhandlung|location=Stuttgart|isbn=978-3-443-01083-6|url=https://www.borntraeger-cramer.com/9783443010836}} The two remaining chlorarachniophyte genera, Minorisa and Rhabdamoeba, have one species each.{{cite journal|last1=del Campo|first1=Javier|last2=Not|first2=Fabrice|last3=Forn|first3=Irene|last4=Sieracki|first4=Michael E|last5=Massana|first5=Ramon|title=Taming the smallest predators of the oceans|journal=The ISME Journal|volume=7|issue=2|date=1 February 2013|issn=1751-7362|pmid=22810060|pmc=3554395|doi=10.1038/ismej.2012.85|doi-access=free|pages=351–358|bibcode=2013ISMEJ...7..351D |url=https://www.nature.com/articles/ismej201285.pdf|access-date=20 May 2025}}{{cite journal|last1=Shiratori|first1=Takashi|last2=Ishida|first2=Ken-ichiro|title=Rhabdamoeba marina is a heterotrophic relative of chlorarachnid algae|journal=Journal of Eukaryotic Microbiology|volume=71|issue=2|date=8 November 2023|issn=1066-5234|doi=10.1111/jeu.13010|doi-access=free|page=|pmid=37941507 }}}} They are classified into 15 phyla or divisions. Some phyla are not photosynthetic, namely Picozoa and Rhodelphidia, but they are included in the database due to their close relationship with red algae.Guiry, M.D. & Guiry, G.M. 2025. AlgaeBase. World-wide electronic publication, University of Galway. https://www.algaebase.org; searched on 4 June 2025.

The various algal phyla can be differentiated according to several biological traits. They have distinct morphologies, photosynthetic pigmentation, storage products, cell wall composition, and mechanisms of carbon concentration.{{cite book|chapter=Chapter 2. The Roles of Algae in Biochemistry|title=Algae|first1=Linda E.|last1=Graham|first2=James M.|last2=Graham|first3=Lee W.|last3=Wilcox|first4=Martha E.|last4=Cook|publisher=LJLM Press|date=2022|edition=4th|isbn=978-0-9863935-4-9}} Some phyla have unique cellular structures.{{cite book|chapter=Chapter 1. Introduction to the Algae|title=Algae|first1=Linda E.|last1=Graham|first2=James M.|last2=Graham|first3=Lee W.|last3=Wilcox|first4=Martha E.|last4=Cook|publisher=LJLM Press|date=2022|edition=4th|isbn=978-0-9863935-4-9}}

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|caption2=Macro- and microscopic photographs of Nostoc, the most common genus of cyanobacteria.{{Cite journal |last1=Fidor |first1=Anna |last2=Konkel |first2=Robert |last3=Mazur-Marzec |first3=Hanna |date=29 September 2019 |title=Bioactive Peptides Produced by Cyanobacteria of the Genus Nostoc: A Review |journal=Marine Drugs |volume=17 |issue=10 |pages=561 |doi=10.3390/md17100561 |pmid=31569531 |pmc=6835634 |issn=1660-3397|doi-access=free }}

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Among prokaryotes, five major groups of bacteria have evolved the ability to photosynthesize, including heliobacteria, green sulfur and nonsulfur bacteria and proteobacteria.{{cite journal|last=Gupta|first=Radhey S.|title=Evolutionary relationships among photosynthetic bacteria|journal=Photosynthesis Research|volume=76|date=2003|issue=1–3 |doi=10.1023/A:1024999314839|pages=173–183|pmid=16228576 |bibcode=2003PhoRe..76..173G |url=http://link.springer.com/10.1023/A:1024999314839}} However, the only lineage where oxygenic photosynthesis has evolved is in the cyanobacteria,{{cite book|first1=Richard W.|last1=Castenholz|chapter=Oxygenic Photosynthetic Bacteria|doi=10.1002/9781118960608.cbm00020|date=14 September 2015|title=Bergey's Manual of Systematics of Archaea and Bacteria|page=1 |publisher=John Wiley & Sons, Inc., in association with Bergey's Manual Trust|isbn=9781118960608|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/9781118960608.cbm00020}} named for their blue-green (cyan) coloration and often known as blue-green algae.{{cite book|chapter=Chapter 6. Cyanobacteria|title=Algae|first1=Linda E.|last1=Graham|first2=James M.|last2=Graham|first3=Lee W.|last3=Wilcox|first4=Martha E.|last4=Cook|publisher=LJLM Press|date=2022|edition=4th|isbn=978-0-9863935-4-9}} They are classified as the phylum Cyanobacteriota or Cyanophyta. However, this phylum also includes two classes of non-photosynthetic bacteria: Melainabacteria{{cite journal|last1=Matheus Carnevali|first1=Paula B.|last2=Schulz|first2=Frederik|last3=Castelle|first3=Cindy J.|last4=Kantor|first4=Rose S.|last5=Shih|first5=Patrick M.|last6=Sharon|first6=Itai|last7=Santini|first7=Joanne M.|last8=Olm|first8=Matthew R.|last9=Amano|first9=Yuki|last10=Thomas|first10=Brian C.|last11=Anantharaman|first11=Karthik|last12=Burstein|first12=David|last13=Becraft|first13=Eric D.|last14=Stepanauskas|first14=Ramunas|last15=Woyke|first15=Tanja|last16=Banfield|first16=Jillian F.|title=Hydrogen-based metabolism as an ancestral trait in lineages sibling to the Cyanobacteria|journal=Nature Communications|volume=10|issue=1|date=28 January 2019|issn=2041-1723|pmid=30692531|pmc=6349859|doi=10.1038/s41467-018-08246-y|doi-access=free|url=https://www.nature.com/articles/s41467-018-08246-y.pdf|access-date=21 May 2025|page=463|bibcode=2019NatCo..10..463M }} (also called Vampirovibrionia or Vampirovibrionophyceae){{cite journal|last1=Strunecký|first1=Otakar|last2=Ivanova|first2=Anna Pavlovna|last3=Mareš|first3=Jan|title=An updated classification of cyanobacterial orders and families based on phylogenomic and polyphasic analysis|journal=Journal of Phycology|volume=59|issue=1|date=2023|issn=0022-3646|doi=10.1111/jpy.13304|pages=12–51|pmid=36443823 |bibcode=2023JPcgy..59...12S |url=https://onlinelibrary.wiley.com/doi/10.1111/jpy.13304}} and Sericytochromatia (also known as Blackallbacteria).{{cite journal|last1=Pinevich|first1=Alexander|last2=Averina|first2=Svetlana|date=2021|title=New life for old discovery: amazing story about how bacterial predation on Chlorella resolved a paradox of dark cyanobacteria an gave the key to early history of oxygenic photosynthesis and aerobic respiration|journal=Protistology|volume=15|issue=3|pages=107–126|doi=10.21685/1680-0826-2021-15-3-2|doi-access=free}} A third class contains the photosynthetic ones, known as Cyanophyceae (also called Cyanobacteriia{{cite journal|last1=Soo|first1=Rochelle M.|last2=Hemp|first2=James|last3=Hugenholtz|first3=Philip|title=Evolution of photosynthesis and aerobic respiration in the cyanobacteria|journal=Free Radical Biology and Medicine|volume=140|date=2019|doi=10.1016/j.freeradbiomed.2019.03.029|pages=200–205|pmid=30930297 |url=https://linkinghub.elsevier.com/retrieve/pii/S0891584918323001}} or Oxyphotobacteria).{{cite journal|last1=Soo|first1=Rochelle M.|last2=Hemp|first2=James|last3=Parks|first3=Donovan H.|last4=Fischer|first4=Woodward W.|last5=Hugenholtz|first5=Philip|title=On the origins of oxygenic photosynthesis and aerobic respiration in Cyanobacteria|journal=Science|volume=355|issue=6332|date=31 March 2017|issn=0036-8075|doi=10.1126/science.aal3794|doi-access=free|pages=1436–1440|bibcode=2017Sci...355.1436S }}

As bacteria, their cells lack membrane-bound organelles, with the exception of thylakoids. Like other algae, cyanobacteria have chlorophyll a as their primary photosynthetic pigment. Their accessory pigments include phycobilins (phycoerythrobilin and phycocyanobilin), carotenoids and, in some cases, b, d, or f chlorophylls, generally distributed in phycobilisomes found in the surface of thylakoids. They display a variety of body forms, such as single cells, colonies, and unbranched or branched filaments. Their cells are commonly covered in a sheath of mucilage, and they also have a typical gram-negative bacterial cell wall composed largely of peptidoglycan. They have various storage particles, including cyanophycin as aminoacid and nitrogen reserves, "cyanophycean starch" (similar to plant amylose) for carbohydrates, and lipid droplets. Their Rubisco enzymes are concentrated in carboxysomes. They occupy a diverse array of aquatic and terrestrial habitats, including extreme environments from hot springs to polar glaciers. Some are subterranean, living via hydrogen-based lithoautotrophy instead of photosynthesis.

Three lineages of cyanobacteria, Prochloraceae, Prochlorothrix and Prochlorococcus, independently evolved to have chlorophylls a and b instead of phycobilisomes. Due to their different pigmentation, they were historically grouped in a separate division, Prochlorophyta, as this is the typical pigmentation seen in green algae (e.g., chlorophytes). Eventually, this classification became obsolete, as it is a polyphyletic grouping.{{cite journal|last1=Sciuto|first1=Katia|last2=Moro|first2=Isabella|title=Cyanobacteria: the bright and dark sides of a charming group|journal=Biodiversity and Conservation|volume=24|issue=4|date=2015|issn=0960-3115|doi=10.1007/s10531-015-0898-4|pages=711–738|bibcode=2015BiCon..24..711S |url=http://link.springer.com/10.1007/s10531-015-0898-4}}{{cite journal|last1=Singh|first1=K.|date=2021|title=Salient features of Protochlorophyta|journal=Innovative Research Thoughts|volume=7|issue=4|pages=70–77|url=https://irt.shodhsagar.com/index.php/j/article/view/1068/1057}}

Cyanobacteria are included as algae by most phycological sources and by the International Code of Nomenclature for algae, fungi, and plants,{{Cite book|editor-last1=Turland|editor-first1=Nicholas J.|editor-last2=Wiersema|editor-first2=John H.|editor-last3=Barrie|editor-first3=Fred R.|editor-last4=Greuter|editor-first4=Werner|editor-last5=Hawksworth|editor-first5=David L.|editor-last6=Herendeen|editor-first6=Patrick S.|editor-last7=Knapp|editor-first7=Sandra|editor-last8=Kusber|editor-first8=Wolf-Henning|editor-last9=Li|editor-first9=De-Zhu|editor-last10=Marhold|editor-first10=Karol|editor-last11=May|editor-first11=Tom W.|editor-last12=McNeill|editor-first12=John|editor-last13=Monro|editor-first13=Anna M.|editor-last14=Prado|editor-first14=Jefferson|editor-last15=Price|editor-first15=Michelle J.|editor-last16=Smith|editor-first16=Gideon F.|date=2018|title=International Code of Nomenclature for algae, fungi, and plants (Shenzhen Code) adopted by the Nineteenth International Botanical Congress Shenzhen, China, July 2017|series=Regnum Vegetabile|volume=159|location=Glashütten|publisher=Koeltz Botanical Books|doi=10.12705/Code.2018|doi-access=free|hdl=10141/622572 |isbn=978-3-946583-16-5 |url=http://www.iapt-taxon.org/nomen/main.php|access-date=2025-05-21|quote=The provisions of this Code apply to all organisms traditionally treated as algae, fungi, or plants, whether fossil or non-fossil, including blue-green algae (Cyanobacteria)|quote-page=Preamble, paragraph 8|no-pp=yes}} although a few authors exclude them from the definition of algae and reserve the term for eukaryotes only.{{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 |url=https://archive.org/details/introductiontobo0000nabo/page/386/mode/2up |url-access=registration}}{{cite dictionary |editor-last=Allaby |editor-first=M. |date=1992 |title=The Concise Dictionary of Botany |publisher=Oxford University Press |entry=Alga |url=https://archive.org/details/conciseoxforddic00mich/page/14/mode/2up |url-access=registration}}

Eukaryotic algae contain chloroplasts 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 chloroplasts is different among separate lineages of algae, reflecting their acquisition during different endosymbiotic events. Many groups contain some members that are no longer photosynthetic. Some retain plastids, but not chloroplasts, while others have lost plastids entirely.{{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}}

These algae, grouped in the clade Archaeplastida (meaning 'ancient plastid'), have "primary chloroplasts", i.e. the chloroplasts are surrounded by two membranes and probably developed through a single endosymbiotic event with a cyanobacterium. The chloroplasts of red algae have chlorophylls a and c (often), and phycobilins, 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.{{fact|date=May 2025}}

There is also a minor group of algae with primary plastids of different origin than the chloroplasts of the archaeplastid algae. The photosynthetic plastid of three species of the genus Paulinella (Rhizaria – Cercozoa – Euglyphida), often referred to as a 'cyanelle', was originated in the endosymbiosis of a α-cyanobacterium (probably an ancestral member of Chroococcales).{{cite journal |last1=Gabr |first1=Arwa |last2=Grossman |first2=Arthur R. |last3=Bhattacharya |first3=Debashish |title=Paulinella, a model for understanding plastid primary endosymbiosis |journal=Journal of Phycology |date=August 2020 |volume=56 |issue=4 |pages=837–843 |doi=10.1111/jpy.13003 |pmid=32289879 |issn=1529-8817 |pmc=7734844|bibcode=2020JPcgy..56..837G }}{{cite journal |last1=Delaye |first1=Luis |last2=Valadez-Cano |first2=Cecilio |last3=Pérez-Zamorano |first3=Bernardo |title=How Really Ancient Is Paulinella Chromatophora? |journal=PLOS Currents |date=2016 |doi=10.1371/currents.tol.e68a099364bb1a1e129a17b4e06b0c6b |doi-access=free |pmid=28515968 |issn=2157-3999 |pmc=4866557}}

These algae appeared independently in various distantly related lineages after acquiring a chloroplast derived from another eukaryotic alga. Two lineages of secondary algae, chlorarachniophytes and euglenophytes have "green" chloroplasts containing chlorophylls a and b.{{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}} Their chloroplasts are surrounded by four and three membranes, respectively, and were probably retained from ingested green algae.{{fact|date=May 2025}}

• Chlorarachniophytes, which belong to the phylum Cercozoa, contain a small nucleomorph, which is a relict of the algae's nucleus.{{fact|date=May 2025}}

• Euglenophytes, 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.{{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}}

• 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.{{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 }}{{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|bibcode=2019NewPh.224..618P }}{{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}}

Other groups have "red" 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.{{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 }} In some of these groups, the chloroplast has four membranes, retaining a nucleomorph in cryptomonads, and they likely share a common pigmented ancestor, although other evidence casts doubt on whether the heterokonts, Haptophyta, and cryptomonads are in fact more closely related to each other than to other groups.{{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 }}{{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 }}

The typical dinoflagellate chloroplast has three membranes, but considerable diversity exists in chloroplasts within the group, and a number of endosymbiotic events apparently occurred. The Apicomplexa, a group of closely related parasites, also have plastids called apicoplasts, which are not photosynthetic. The Chromerida are the closest relatives of apicomplexans, and some have retained their chloroplasts.{{cite journal |title=A photosynthetic alveolate closely related to apicomplexan parasites |journal=Nature |volume=451 |issue=7181 |pages=959–963 |date=February 2008 |pmid=18288187 |doi=10.1038/nature06635 |author1=Moore RB |author2=Oborník M |author3=Janouskovec J |author4=Chrudimský T |author5=Vancová M |author6=Green DH |author7=Wright SW |author8=Davies NW |author9=Bolch CJ|display-authors=8 |last10=Heimann |first10=Kirsten |last11=Šlapeta |first11=Jan |last12=Hoegh-Guldberg |first12=Ove |last13=Logsdon |first13=John M. |last14=Carter |first14=Dee A. |bibcode=2008Natur.451..959M |s2cid=28005870 }} The three alveolate groups evolved from a common myzozoan ancestor that obtained chloroplasts.{{cite journal|first1=J.|last1=Janouškovec|first2=D.V.|last2=Tikhonenkov|first3=F.|last3=Burki|first4=A.T.|last4=Howe|first5=M.|last5=Kolísko|first6=A.P.|last6=Mylnikov|first7=P.J.|last7=Keeling|title=Factors mediating plastid dependency and the origins of parasitism in apicomplexans and their close relatives|journal=PNAS|volume=112|issue=33|pages=10200–10207|doi=10.1073/pnas.1423790112|date=2015|doi-access=free |pmc=4547307|bibcode=2015PNAS..11210200J }}

File:Gmelin - Historia Fucorum (Titelblatt).png's Historia Fucorum, dated 1768]]

Linnaeus, in Species Plantarum (1753),{{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}} the starting point for modern botanical nomenclature, recognized 14 genera of algae, of which only four are currently considered among algae.{{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 }} In 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.{{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}}{{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 }}

W. H. Harvey (1811–1866) and Lamouroux (1813){{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}} 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.{{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}}{{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}}.

At this time, microscopic algae were discovered and reported by a different group of workers (e.g., O. F. Müller and Ehrenberg) studying the Infusoria (microscopic organisms). Unlike macroalgae, which were clearly viewed as plants, microalgae were frequently considered animals because they are often motile. Even the nonmotile (coccoid) microalgae were sometimes merely seen as stages of the lifecycle of plants, macroalgae, or animals.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}}.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}}.

Although used as a taxonomic category in some pre-Darwinian classifications, e.g., Linnaeus (1753),{{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}} 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}} Lamouroux (1813), Harvey (1836), Horaninow (1843), Agassiz (1859), Wilson & Cassin (1864), in further classifications, the "algae" are seen as an artificial, polyphyletic group.{{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 }}

Throughout the 20th century, most classifications treated the following groups as divisions or classes of algae: cyanophytes, rhodophytes, chrysophytes, xanthophytes, bacillariophytes, phaeophytes, pyrrhophytes (cryptophytes and dinophytes), euglenophytes, and chlorophytes. Later, many new groups were discovered (e.g., Bolidophyceae), and others were splintered from older groups: charophytes and glaucophytes (from chlorophytes), many heterokontophytes (e.g., synurophytes from chrysophytes, or eustigmatophytes from xanthophytes), haptophytes (from chrysophytes), and chlorarachniophytes (from xanthophytes).{{Cite journal |date=1980 |title=Compte rendu du premier colloque de l'association des Diatomistes de Langue Française. Paris, 25 janvier 1980 |url=https://doi.org/10.5962/p.308988 |journal=Cryptogamie. Algologie |volume=1 |issue=1 |pages=67–74 |doi=10.5962/p.308988 |bibcode=1980CrypA...1...67. |issn=0181-1568}}

With the abandonment of plant-animal dichotomous classification, most groups of algae (sometimes all) were included in Protista, later also abandoned in favour of Eukaryota. However, as a legacy of the older plant life scheme, some groups that were also treated as protozoans in the past still have duplicated classifications (see ambiregnal protists).{{Cite journal |last=Corliss |first=J O |date=1995 |title=The ambiregnal protists and the codes of nomenclature: a brief review of the problem and of proposed solutions |url=http://www.biodiversitylibrary.org/part/6717 |journal=The Bulletin of Zoological Nomenclature |volume=52 |pages=11–17 |doi=10.5962/bhl.part.6717 |issn=0007-5167|doi-access=free }}

Some parasitic algae (e.g., the green algae Prototheca and Helicosporidium, parasites of metazoans, or Cephaleuros, parasites of plants) were originally classified as fungi, sporozoans, or protistans of incertae sedis,{{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}} 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.Round (1981). pp. 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 }}. Some filamentous bacteria (e.g., Beggiatoa) were originally seen as algae. Furthermore, groups like the apicomplexans are also parasites derived from ancestors that possessed plastids, but are not included in any group traditionally seen as algae.{{Cite book |last1=Grabda |first1=Jadwiga |title=Marine fish parasitology: an outline |last2=Grabda |first2=Jadwiga |date=1991 |publisher=VCH-Verl.-Ges |isbn=978-0-89573-823-3 |location=Weinheim}}{{Cite journal |last1=Smith |first1=David Roy |last2=Keeling |first2=Patrick J. |date=2016-09-08 |title=Protists and the Wild, Wild West of Gene Expression: New Frontiers, Lawlessness, and Misfits |url=https://www.annualreviews.org/doi/10.1146/annurev-micro-102215-095448 |journal=Annual Review of Microbiology |language=en |volume=70 |issue=1 |pages=161–178 |doi=10.1146/annurev-micro-102215-095448 |pmid=27359218 |issn=0066-4227}}

Prokaryotic algae, i.e., cyanobacteria, are the only group of organisms where oxygenic photosynthesis has evolved. The oldest undisputed fossil evidence of cyanobacteria is dated at 2100 million years ago,{{cite journal | vauthors = Schirrmeister BE, de Vos JM, Antonelli A, Bagheri HC | title = Evolution of multicellularity coincided with increased diversification of cyanobacteria and the Great Oxidation Event | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 110 | issue = 5 | pages = 1791–1796 | date = January 2013 | pmid = 23319632 | pmc = 3562814 | doi = 10.1073/pnas.1209927110 | doi-access = free | bibcode = 2013PNAS..110.1791S }} although stromatolites, associated with cyanobacterial biofilms, appear as early as 3500 million years ago in the fossil record.{{cite journal |last1=Baumgartner |first1=Raphael J. |last2=Van Kranendonk |first2=Martin J. |last3=Wacey |first3=David |last4=Fiorentini |first4=Marco L. |last5=Saunders |first5=Martin |last6=Caruso |first6=Stefano |last7=Pages |first7=Anais |last8=Homann |first8=Martin |last9=Guagliardo |first9=Paul |title=Nano−porous pyrite and organic matter in 3.5-billion-year-old stromatolites record primordial life |journal=Geology |date=November 2019 |volume=47 |issue=11 |pages=1039–1043 |doi=10.1130/G46365.1 |bibcode=2019Geo....47.1039B |url=https://archimer.ifremer.fr/doc/00637/74900/ }}

Eukaryotic 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 coccoid cyanobacteria got phagocytized by a unicellular heterotrophic eukaryote (a protist),{{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}} giving rise to double-membranous primary plastids. Such symbiogenic events (primary symbiogenesis) are believed to have occurred more than 1.5 billion years ago during the Calymmian period, early in Boring Billion, but it is difficult to track the key events because of so much time gap.{{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 }} Primary symbiogenesis gave rise to three divisions of archaeplastids, namely the Viridiplantae (green algae and later plants), 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). This process of serial cell "capture" and "enslavement" explains the diversity of photosynthetic eukaryotes. The oldest undisputed fossil evidence of eukaryotic algae is Bangiomorpha pubescens, a red alga found in rocks around 1047 million years old.{{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 |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}}

{{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 |volume=46 |issue=2 |pages=135–138 |doi=10.1130/G39829.1 |bibcode=2018Geo....46..135G }}

{{cladogram|title=Evolution of eukaryotic algae

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|label1={{background color|white|Diaphoretickes}}|1={{clade

|1={{clade

|1={{clade|label1={{background color|white|Archaeplastida}}|1={{clade

|1={{clade

|1={{nowrap|Viridiplantae}}

|2=Glaucophyta

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|2={{clade

|1={{clade

|1=Rhodophyta

|2=Rhodelphidia

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|2=Picozoa

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|2=Cryptista (includes Cryptophyta)

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|2=Haptista (includes Haptophyta)

|3={{clade

|1=Telonemia

|label2={{background color|white|SAR}}|2={{clade

|1=Stramenopiles (includes Ochrophyta)

|2=Alveolata (includes Myzozoa)

|3=Rhizaria (includes Chlorarachniophyta)

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|label2={{background color|white|Discoba}}|2=Euglenozoa (includes Euglenophyta)

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|2=Other eukaryotes

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|caption=Cladogram summarizing the occurrence of chloroplasts across eukaryotes, with each photosynthetic clade shown in bold (colored {{color|green|green}} or {{color|brown|red}} according to their plastid heritage).{{cite journal|last=Eliáš|first=Marek|title=Protist diversity: Novel groups enrich the algal tree of life|journal=Current Biology|volume=31|issue=11|date=2021|doi=10.1016/j.cub.2021.04.025|doi-access=free|pages=R733–R735|pmid=34102125 |bibcode=2021CBio...31.R733E |url=https://www.cell.com/article/S0960982221005388/pdf|access-date=13 May 2025}}

}}

Recent genomic and phylogenomic approaches have significantly clarified plastid genome evolution, the horizontal movement of endosymbiont genes to the "host" nuclear genome, and plastid spread throughout the eukaryotic tree of life. It is accepted that both euglenophytes and chlorarachniophytes obtained their chloroplasts from chlorophytes that became endosymbionts.{{cite book|last=Keeling|first=Patrick J.|title=Handbook of the Protists|chapter=Chlorarachniophytes|date=2017|isbn=978-3-319-28147-6|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer|doi=10.1007/978-3-319-28149-0_34|chapter-url=http://link.springer.com/10.1007/978-3-319-28149-0_34|volume=1|pages=765–781}} In particular, euglenophyte chloroplasts share the most resemblance with the genus Pyramimonas.{{cite journal|last1=Bicudo|first1=Carlos E. de M.|last2=Menezes|first2=Mariângela|title=Phylogeny and Classification of Euglenophyceae: A Brief Review|journal=Frontiers in Ecology and Evolution|volume=4|date=16 March 2016|issn=2296-701X|doi=10.3389/fevo.2016.00017|doi-access=free|page=17|bibcode=2016FrEEv...4...17B }}

However, there is still no clear order in which the secondary and tertiary endosymbioses occurred for the "chromist" lineages (ochrophytes, cryptophytes, haptophytes and myzozoans). Two main models have been proposed to explain the order, both of which agree that cryptophytes obtained their chloroplasts from red algae. One model, hypothesized in 2014 by John W. Stiller and coauthors,{{cite journal|last1=Stiller|first1=John W.|last2=Schreiber|first2=John|last3=Yue|first3=Jipei|last4=Guo|first4=Hui|last5=Ding|first5=Qin|last6=Huang|first6=Jinling|title=The evolution of photosynthesis in chromist algae through serial endosymbioses|journal=Nature Communications|volume=5|issue=1|date=10 December 2014|issn=2041-1723|pmid=25493338|pmc=4284659|doi=10.1038/ncomms6764|doi-access=free|url=https://www.nature.com/articles/ncomms6764.pdf|access-date=13 May 2025|pages=5764|bibcode=2014NatCo...5.5764S }} suggests that a cryptophyte became the plastid of ochrophytes, which in turn became the plastid of myzozoans and haptophytes. The other model, suggested by Andrzej Bodył and coauthors in 2009,{{cite journal|last1=Bodył|first1=Andrzej|last2=Stiller|first2=John W.|last3=Mackiewicz|first3=Paweł|title=Chromalveolate plastids: direct descent or multiple endosymbioses?|journal=Trends in Ecology & Evolution|volume=24|issue=3|date=2009|doi=10.1016/j.tree.2008.11.003|pages=119–121|pmid=19200617 |bibcode=2009TEcoE..24..119B |url=https://linkinghub.elsevier.com/retrieve/pii/S0169534709000251|access-date=13 May 2025}} describes that a cryptophyte became the plastid of both haptophytes and ochrophytes, and it is a haptophyte that became the plastid of myzozoans instead.{{cite journal|last1=Strassert|first1=Jürgen F. H.|last2=Irisarri|first2=Iker|last3=Williams|first3=Tom A.|last4=Burki|first4=Fabien|title=A molecular timescale for eukaryote evolution with implications for the origin of red algal-derived plastids|journal=Nature Communications|volume=12|issue=1|date=25 March 2021|issn=2041-1723|pmid=33767194|pmc=7994803|doi=10.1038/s41467-021-22044-z|doi-access=free|url=https://www.nature.com/articles/s41467-021-22044-z.pdf|access-date=13 May 2025|page=1879|bibcode=2021NatCo..12.1879S }}

In 2024, a third model by Filip Pietluch and coauthors proposed that there were two independent endosymbioses with red algae: one that originated the cryptophyte plastids (as in the previous models), and subsequently the haptophyte plastids; and another that originated the ochrophyte plastids, where the myzozoans obtained theirs.{{cite journal |last1=Pietluch |first1=Filip |last2=Mackiewicz |first2=Paweł |last3=Ludwig |first3=Kacper |last4=Gagat |first4=Przemysław |title=A New Model and Dating for the Evolution of Complex Plastids of Red Alga Origin |journal=Genome Biology and Evolution |date=3 September 2024 |volume=16 |issue=9: evae192 |doi=10.1093/gbe/evae192 |pmid=39240751 |pmc=11413572}}

Fossils of isolated spores suggest land plants may have been around as long as 475 million years ago (mya) during the Late Cambrian/Early Ordovician period,{{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}}{{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}} from sessile shallow freshwater charophyte algae much like Chara,{{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}} which likely got stranded ashore when riverine/lacustrine water levels dropped during dry seasons.{{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}} These charophyte algae probably already developed filamentous thalli and holdfasts that superficially resembled plant stems and roots, and probably had an isomorphic alternation of generations. They perhaps evolved some 850 mya{{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 }} and might even be as early as 1 Gya during the late phase of the Boring Billion.{{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 }}

The distribution of algal species has been fairly well studied since the founding of phytogeography in the mid-19th century.{{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}} Algae spread mainly by the dispersal of spores analogously to the dispersal of cryptogamic plants by spores. Spores can be found in a variety of environments: fresh and marine waters, air, soil, and in or on other organisms. 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. 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. 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.Round (1981), p. 362.

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."Round (1981), p. 357. For example, Clathromorphum is an arctic genus and is not mapped far south of there.{{Where|date=January 2025}}Round (1981), p. 371. However, scientists regard the overall data as insufficient due to the "difficulties of undertaking such studies."Round (1981), p. 366.

File:Taiwan 2009 East Coast ShihTiPing Giant Stone Steps Algae FRD 6581.jpg 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).{{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}} Estimates vary widely. For example, according to one standard textbook,John (2002), p. 1. 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 ..."

Regional and group estimates have been made, as well:

• 5,000–5,500 species of red algae worldwide{{fact|date=May 2025}}
• "some 1,300 in Australian Seas"Huisman (2000), p. 25.
• 400 seaweed species for the western coastline of South Africa,Stegenga (1997). and 212 species from the coast of KwaZulu-Natal.{{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}} 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. These exclude phytoplankton and crustose corallines.
• 669 marine species from California (US)Abbott and Hollenberg (1976), p. 2.
• 642 in the check-list of Britain and IrelandHardy and Guiry (2006).

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.{{fact|date=May 2025}}

File:Phytoplankton Lake Chuzenji.jpg]]

Algae are prominent in bodies of water, common in terrestrial environments, and are found in unusual environments, such as on snow and 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}}.Round (1981), p. 176. 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.{{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 }} The same algae was found in the Italian Alps, after pink ice appeared on parts of the Presena glacier.{{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 }}

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 chains. In very high densities (algal blooms), these algae may discolor the water and outcompete, poison, or asphyxiate other life forms.{{fact|date=May 2025}}

Algae can be used as indicator organisms to monitor pollution in various aquatic systems.{{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}} 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. To detect these changes, algae can be sampled from the environment and maintained in laboratories with relative ease.

On the basis of their habitat, algae can be categorized as: aquatic (planktonic, benthic, marine, freshwater, lentic, lotic),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 }}. terrestrial, aerial (subaerial),{{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}} lithophytic, halophytic (or euryhaline), psammon, thermophilic, cryophilic, epibiont (epiphytic, epizoic), endosymbiont (endophytic, endozoic), parasitic, calcifilic or lichenic (phycobiont).Sharma, O. P. (1986). pp. 2–6, [https://books.google.com/books?id=hOa74Hm4zDIC&pg=PA2].

Some species of algae form 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.{{fact|date=May 2025}} Examples are:

{{Main|Lichen}}

File:Lichens near Clogher Head (stevefe).jpg

Lichens 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".{{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}} 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.{{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}} One or more{{Cite journal |last1=Tuovinen |first1=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 |pmid=30661799 |bibcode=2019CBio...29E.476T |issn=0960-9822}} 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.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)." 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.Brodo et al. (2001), p. 8.

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 (aerophytes) growing on exposed surfaces such as tree trunks and rocks and sometimes discoloring them.{{fact|date=May 2025}}

File:Coral Reef.jpg Coral reefs are accumulated from the calcareous exoskeletons of marine invertebrates of the order Scleractinia (stony corals). These animals 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 endosymbionts 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 corals) require endosymbiotic algae from the genus Symbiodinium to be in a healthy condition.{{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}} The loss of Symbiodinium from the host is known as coral bleaching, a condition which leads to the deterioration of a reef.

Endosymbiontic 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.{{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}}

In classical Chinese, the word {{lang|zh|{{linktext|藻}}}} is used both for "algae" and (in the modest tradition of the 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".{{fact|date=May 2025}}

{{Excerpt|Algaculture}}

{{Excerpt|seaweed farming}}

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

File:Algae Harvester.jpg

{{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,{{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}}{{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 |bibcode=2013BB......6...67Y }} 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.{{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 }}

{{Details|Seaweed fertiliser}}

File:Inisheer landscape.jpg]]

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:{{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}}

{{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 ... 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 fertilizers, soil conditioners, and livestock feed.{{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}} 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.{{fact|date=May 2025}}

{{See also|Edible seaweed|Algae powder}}

File:Dulse.JPG

Algae are used as foods in many countries: China consumes more than 70 species, including fat choy, a cyanobacterium considered a vegetable; Japan, over 20 species such as nori and aonori;{{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}} Ireland, dulse; Chile, cochayuyo.{{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}} 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}}.{{Cite web |title=Laver Archives |url=https://kimchimari.com/ingredient/laver/ |access-date=2024-12-15 |website=Kimchimari |language=en-US}}

Three forms of algae used as food:{{fact|date=May 2025}}

• Chlorella: This form of alga is found in freshwater and contains photosynthetic pigments in its chloroplast.{{fact|date=May 2025}}
• Klamath AFA: A subspecies of Aphanizomenon flos-aquae found wild in many bodies of water worldwide but harvested only from Upper Klamath Lake, Oregon.{{fact|date=May 2025}}
• Spirulina: Known otherwise as a cyanobacterium (a prokaryote or a "blue-green alga"){{fact|date=May 2025}}

The oils from some algae have high levels of unsaturated fatty acids. Some varieties of algae favored by vegetarianism and veganism contain the long-chain, essential omega-3 fatty acids, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA).{{Cite web |date=2013-05-22 |title=Vegetarian EPA DHA and Essential Fats |url=http://vegetarian-dha-epa.co.uk/ |access-date=2024-12-15 |archive-url=https://web.archive.org/web/20130522103010/http://vegetarian-dha-epa.co.uk/ |archive-date=22 May 2013 }} 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 copepods and are passed up the food chain.

The natural pigments (carotenoids and chlorophylls) produced by algae can be used as alternatives to chemical dyes and coloring agents.{{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}}

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.{{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}}{{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}}

Carrageenan, from the red alga Chondrus crispus, is used as a stabilizer in milk products.{{fact|date=May 2025}}

Agar, a gelatinous substance derived from red algae, has a number of commercial uses.{{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}} It is a good medium on which to grow bacteria and fungi, as most microorganisms cannot digest agar.{{fact|date=May 2025}}

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 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.{{fact|date=May 2025}}

Between 100,000 and 170,000 wet tons of Macrocystis are harvested annually in New Mexico for alginate extraction and abalone feed.{{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}}{{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}}

• Sewage can be treated with algae,{{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}} 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.{{fact|date=May 2025}}
• 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.{{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}}{{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 }}

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.{{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}} Algae scrubbers, using bubbling upflow or vertical waterfall versions, are now also being used to filter aquaria and ponds.{{fact|date=May 2025}}

The alga Stichococcus bacillaris has been seen to colonize silicone resins used at archaeological sites; biodegrading the synthetic substance.{{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 }}

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.{{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}} Several companies have begun to produce algae polymers commercially, including for use in flip-flops{{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}} and in surf boards.{{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}} Even algae is also used to prepare various polymeric resins suitable for coating applications.Chandrashekhar K Patil, Harishchandra D Jirimali, Jayasinh S Paradeshi, Bhushan L Chaudhari, Prakash K Alagi, Sung Chul Hong, Vikas V Gite, Synthesis of biobased polyols using algae oil for multifunctional polyurethane coatings, Volume 6 Issue 4, December 2018, pp. 165-177, https://doi.org/10.1680/jgrma.18.00046CK Patil, HD Jirimali, JS Paradeshi, BL Chaudhari, VV Gite, Functional antimicrobial and anticorrosive polyurethane composite coatings from algae oil and silver doped egg shell hydroxyapatite for sustainable development, Progress in Organic Coatings 128, 127-136, https://doi.org/10.1016/j.porgcoat.2018.11.002Chandrashekhar K Patil, Harishchandra D Jirimali, Jayasinh S Paradeshi, Bhushan L Chaudhari, Prakash K Alagi, Pramod P Mahulikar, Sung Chul Hong, Vikas V Gite, Chemical transformation of renewable algae oil to polyetheramide polyols for polyurethane coatings, Progress in Organic Coatings 151, 106084, https://doi.org/10.1016/j.porgcoat.2020.106084

File:Algae bladder 4290.jpg|Algae bladder

{{portal|Algae}}

• AlgaeBase
• AlgaePARC
• Eutrophication
• Iron fertilization
• Marimo algae
• Microbiofuels
• Microphyte
• Photobioreactor
• Phycotechnology
• Plants
• Toxoid – anatoxin

{{notelist}}

{{Reflist}}

{{Refbegin|30em}}

• {{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 }}

• {{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}}

• {{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}}

• {{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}}

• {{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}}

• {{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}}

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

• {{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}}.

• {{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}}

• {{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}}

• {{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}}

• {{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}}

{{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:Endosymbiotic events

Category:Polyphyletic groups

Category:Common names of organisms