Editing Green algae

{{Short description|Paraphyletic group of eukaryotes}}
{{For|an explanation of its other names|Viridiplantae|Plantae}}
{{For|the bacteria commonly known as blue-green algae that can produce toxic blooms in lakes and other waters|Cyanobacteria}}
{{Paraphyletic group
| name = Green algae
| image = <imagemap>
File:Green_algae.png|250px
rect   0 0 1000 750 [[Picocystophyceae]]
rect   0 750 1000 1500 [[Trebouxiophyceae]]
rect   0 1500 1000 2250 [[Klebsormidiophyceae]]
rect   0 2250 1000 3000 [[Zygnematophyceae]]
rect   1000 0 2000 750 [[Ulvophyceae]]
rect   1000 750 2000 1500 [[Chlorophyceae]]
rect   1000 1500 2000 2250 [[Charophyceae]]
rect   1000 2250 2000 3000 [[Desmid]]
</imagemap>
| image_caption = Green algal diversity. From top left corner: ''Picocystis'' ([[Picocystophyceae]]), ''[[Acetabularia]]'' ([[Ulvophyceae]]), ''[[Botryococcus]]'' ([[Trebouxiophyceae]]), ''[[Volvox]]'' ([[Chlorophyceae]]), ''[[Klebsormidium]]'' ([[Klebsormidiophyceae]]), ''[[Chara (alga)|Chara]]'' ([[Charophyceae]]), ''[[Spirogyra]]'' and ''[[Micrasterias]]'' ([[Zygnematophyceae]])
| auto = yes
| parent = Viridiplantae
| includes =
* [[Prasinodermophyta]]
* [[Chlorophyta]]
* [[Mesostigmatophyceae]]
* [[Klebsormidiophyceae]]
* [[Chlorokybophyceae]]
* [[Coleochaetophyceae]]
* [[Charophyceae]]
* [[Zygnematophyceae]]
| excludes =
* [[Embryophyta]]
}}

The '''green algae''' ({{singular}}: '''green alga''') are a group of [[chlorophyll]]-containing [[autotroph]]ic [[eukaryote]]s consisting of the phylum [[Prasinodermophyta]] and its unnamed [[sister group]] that contains the [[Chlorophyta]] and [[Charophyta]]/[[Streptophyta]]. The [[land plant]]s ([[Embryophyte]]s) have emerged deep within the charophytes as a sister of the [[Zygnematophyceae]].<ref name="HuanLiu-2020" /><ref name="Delwiche-2011" /><ref name="Palmer-2004">{{cite journal |vauthors=Palmer JD, Soltis DE, Chase MW |date=October 2004 |title=The plant tree of life: an overview and some points of view |journal=American Journal of Botany |volume=91 |issue=10 |pages=1437–45 |doi=10.3732/ajb.91.10.1437 |pmid=21652302 |doi-access=free }}</ref> Since the realization that the Embryophytes emerged within the green algae, some authors are starting to include them.<ref name="Delwiche-2011">{{cite journal |vauthors=Delwiche CF, Timme RE |title=Plants |journal=Current Biology |volume=21 |issue=11 |pages=R417–22 |date=June 2011 |pmid=21640897 |doi=10.1016/j.cub.2011.04.021 |doi-access=free }}</ref><ref>{{Cite web |url=http://www.life.umd.edu/labs/delwiche/Charophyte.html|title=Charophycean Green Algae Home Page |website=www.life.umd.edu |access-date=2018-02-24 }}</ref><ref name="Ruhfel-2014" /><ref>{{cite journal |vauthors=Delwiche CF, Cooper ED |title=The Evolutionary Origin of a Terrestrial Flora |language=en |journal=Current Biology |volume=25 |issue=19 |pages=R899–910 |date=October 2015 |pmid=26439353 |doi=10.1016/j.cub.2015.08.029 |doi-access=free }}</ref><ref>{{cite journal |vauthors=Parfrey LW, Lahr DJ, Knoll AH, Katz LA |title=Estimating the timing of early eukaryotic diversification with multigene molecular clocks |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=108 |issue=33 |pages=13624–9 |date=August 2011 |pmid=21810989 |pmc=3158185 |doi=10.1073/pnas.1110633108 |bibcode=2011PNAS..10813624P |doi-access=free }}</ref>{{Excessive citations inline|date=November 2021}} The completed [[clade]] that includes both green algae and embryophytes is [[monophyletic]] and is referred to as the clade [[Viridiplantae]] and as the kingdom [[Plant]]ae. The green algae include unicellular and colonial [[flagellate]]s, most with two [[flagellum|flagella]] per cell, as well as various colonial, coccoid (spherical), and filamentous forms, and macroscopic, multicellular [[seaweed]]s. There are about 22,000 species of green algae,<ref>{{cite journal | vauthors = Guiry MD | title = How many species of algae are there? | journal = Journal of Phycology | volume = 48 | issue = 5 | pages = 1057–63 | date = October 2012 | pmid = 27011267 | doi = 10.1111/j.1529-8817.2012.01222.x | s2cid = 30911529 }}</ref> many of which live most of their lives as single cells, while other species form [[coenobium (morphology)|coenobia]] (colonies), long filaments, or highly differentiated macroscopic seaweeds.

A few other organisms rely on green algae to conduct [[photosynthesis]] for them. The [[chloroplast]]s in [[dinoflagellate]]s of the genus ''[[Lepidodinium]]'', [[euglenid]]s and [[chlorarachniophyte]]s were acquired from [[phagocytosis|ingested]] [[endosymbiont]] green algae,<ref>{{cite journal | doi=10.1038/s41598-017-18805-w | title=Plastid phylogenomics with broad taxon sampling further elucidates the distinct evolutionary origins and timing of secondary green plastids | year=2018 | last1=Jackson | first1=Christopher | last2=Knoll | first2=Andrew H. | last3=Chan | first3=Cheong Xin | last4=Verbruggen | first4=Heroen | journal=Scientific Reports | volume=8 | issue=1 | page=1523 | pmid=29367699 | pmc=5784168 | bibcode=2018NatSR...8.1523J }}</ref> and in the latter retain a [[nucleomorph]] (vestigial nucleus). Green algae are also found symbiotically in the ciliate ''[[Paramecium]]'', and in ''[[Hydra viridissima]]'' and in [[flatworm]]s. Some species of green algae, particularly of genera ''[[Trebouxia]]'' of the class ''[[Trebouxiophyceae]]'' and ''[[Trentepohlia (alga)|Trentepohlia]]'' (class [[Ulvophyceae]]), can be found in symbiotic associations with [[fungi]] to form [[lichen]]s. In general the fungal species that partner in lichens cannot live on their own, while the algal species is often found living in nature without the fungus. ''Trentepohlia'' is a filamentous green alga that can live independently on humid soil, rocks or tree bark or form the photosymbiont in lichens of the family [[Graphidaceae]]. Also the macroalga ''[[Prasiola calophylla]]'' (Trebouxiophyceae) is terrestrial,<ref>{{Cite journal |pmc = 5474099 |year = 2017 |last1 = Holzinger |first1 = A. |title = The terrestrial green macroalga ''Prasiola calophylla'' (Trebouxiophyceae, Chlorophyta): Ecophysiological performance under water-limiting conditions |journal = Protoplasma |volume = 254 |issue = 4 |pages = 1755–1767 |last2 = Herburger |first2 = K. |last3 = Blaas |first3 = K. |last4 = Lewis |first4 = L. A. |last5 = Karsten |first5 = U. |pmid = 28066876 |doi = 10.1007/s00709-016-1068-6 }}</ref> and
''[[Prasiola crispa]]'', which live in the [[supralittoral zone]], is terrestrial and can in the Antarctic form large carpets on humid soil, especially near bird colonies.<ref>{{Cite journal |doi=10.3389/fmolb.2017.00089 |pmid=29359133 |pmc=5766667 |title=De novo Assembly and Annotation of the Antarctic Alga Prasiola crispa Transcriptome |journal=Frontiers in Molecular Biosciences |volume=4 |pages=89 |year=2018 |last1=Carvalho |first1=Evelise L. |last2=MacIel |first2=Lucas F. |last3=MacEdo |first3=Pablo E. |last4=Dezordi |first4=Filipe Z. |last5=Abreu |first5=Maria E. T. |last6=Victória |first6=Filipe de Carvalho |last7=Pereira |first7=Antônio B. |last8=Boldo |first8=Juliano T. |last9=Wallau |first9=Gabriel da Luz |last10=Pinto |first10=Paulo M. |doi-access=free }}</ref>

==Cellular structure==
Green algae have chloroplasts that contain [[chlorophyll a|chlorophyll ''a'']] and [[chlorophyll b|''b'']], giving them a bright green colour, as well as the accessory pigments [[beta carotene]] (red-orange) and [[xanthophyll]]s (yellow) in stacked [[thylakoid]]s.<ref name="Burrows 1991">Burrows 1991. ''Seaweeds of the British Isles.''  Volume '''2''' Natural History Museum, London. {{ISBN|0-565-00981-8}}</ref><ref name="vandenHoek-1995">{{cite book | vauthors = van den Hoek C, Mann DG, Jahns HM | year = 1995 | url = https://books.google.com/books?id=xuUoiFesSHMC | title = Algae An introduction to phycology | publisher = Cambridge University Press | location = Cambridge | isbn = 978-0-521-30419-1 }}</ref> The [[cell wall]]s of green algae usually contain [[cellulose]], and they store carbohydrate in the form of [[starch]].<ref name="Judd-2002">{{cite book |vauthors=Judd WS, Campbell CS, Kellogg EA, Stevens PF, Donoghue MJ | publisher = Sinauer Associates Inc. | location = Sunderland Mass. | date = 2002 | title = Plant systematics, a phylogenetic approach. | isbn = 978-0-87893-403-4 | pages = [https://archive.org/details/plantsystematics0002unse/page/156 156] | url = https://archive.org/details/plantsystematics0002unse/page/156 }}</ref>

All green algae have [[mitochondrion|mitochondria]] with flat [[cristae]]. When present, paired [[flagellum|flagella]] are used to move the cell. They are anchored by a cross-shaped system of [[microtubule]]s and fibrous strands. Flagella are only present in the motile male gametes of [[charophytes]]<ref>{{Cite journal |doi = 10.1016/0041-008x(92)90204-6|pmid = 1539170|title = Letter to the editor|journal = Toxicology and Applied Pharmacology|volume = 112|issue = 2|pages = 331–332|year = 1992| vauthors = Roberts DW }}</ref> bryophytes, pteridophytes, cycads and ''[[Ginkgo]]'', but are absent from the gametes of [[Pinophyta]] and [[Angiosperm|flowering plants]].

Members of the class [[Chlorophyceae]] undergo closed mitosis in the most common form of cell division among the green algae, which occurs via a [[phycoplast]].<ref name="Pickett-Heaps-1976">{{cite journal |vauthors=Pickett-Heaps J | year = 1976 | title = Cell division in eucaryotic algae | journal = BioScience | volume = 26 | issue = 7| pages = 445–450 | doi=10.2307/1297481| jstor = 1297481 }}</ref> By contrast, [[charophyte]] green algae and land plants (embryophytes) undergo open [[mitosis]] without [[centriole]]s. Instead, a 'raft' of microtubules, the [[phragmoplast]], is formed from the [[mitotic spindle]] and cell division involves the use of this [[phragmoplast]] in the production of a [[cell plate]].<ref name="P">P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): ''Biology of Plants'', 7th Edition, W.H. Freeman and Company Publishers, New York, {{ISBN|0-7167-1007-2}}</ref>

==Origins==
Photosynthetic eukaryotes originated following a primary [[endosymbiotic theory|endosymbiotic]] event, where a heterotrophic eukaryotic cell engulfed a photosynthetic [[cyanobacteria|cyanobacterium]]-like prokaryote that became stably integrated and eventually evolved into a membrane-bound [[organelle]]: the [[plastid]].<ref name="Keeling-2010">{{cite journal | vauthors = Keeling PJ | title = The endosymbiotic origin, diversification and fate of plastids | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 365 | issue = 1541 | pages = 729–48 | date = March 2010 | pmid = 20124341 | pmc = 2817223 | doi = 10.1098/rstb.2009.0103 }}</ref> This primary endosymbiosis event gave rise to three [[autotroph]]ic clades with primary plastids: the [[Viridiplantae|(green) plants]] (with [[chloroplast]]s) the [[red algae]] (with rhodoplasts) and the [[glaucophyte]]s (with muroplasts).<ref name="DeClerck-2012">{{cite book | vauthors = De Clerck O, Bogaert KA, Leliaert F |volume=64|year=2012|pages=55–86|issn=0065-2296|doi=10.1016/B978-0-12-391499-6.00002-5|series=Advances in Botanical Research|isbn=9780123914996| title = Genomic Insights into the Biology of Algae | chapter = Diversity and Evolution of Algae |chapter-url=https://books.google.com/books?id=lE6r5q5op94C&pg=PA55 }}</ref>

==Evolution and classification==
[[File:Intertidal greenalgae.jpg|thumb|right|A growth of the green seaweed ''[[Sea lettuce|Ulva]]'' on rock substratum at the ocean shore; some green seaweeds like ''Ulva'' are quick to utilize inorganic [[nutrient]]s from [[land runoff]], and thus can be indicators of [[nutrient pollution]]]]
Green algae are often classified with their embryophyte descendants in the green plant [[clade]] [[Viridiplantae]] (or [[Chlorobionta]]). Viridiplantae, together with red algae and [[glaucophyte]] algae, form the supergroup Primoplantae, also known as [[Archaeplastida]] or Plantae ''sensu lato''. The ancestral green alga was a unicellular flagellate.<ref name="Leliaert-2012"/>

The Viridiplantae diverged into two clades. The [[Chlorophyta]] include the early diverging [[Prasinophyceae|prasinophyte]] lineages and the core Chlorophyta, which contain the majority of described species of green algae. The [[Streptophyta]] include [[Charophyta|charophytes]] and land plants. Below is a consensus reconstruction of green algal relationships, mainly based on molecular data.<ref>{{cite journal | vauthors = Lewis LA, McCourt RM | title = Green algae and the origin of land plants | journal = American Journal of Botany | volume = 91 | issue = 10 | pages = 1535–56 | date = October 2004 | pmid = 21652308 | doi = 10.3732/ajb.91.10.1535 }}</ref><ref name="Leliaert-2012">{{cite journal |last1= Leliaert |first1=Frederik |last2=Smith |first2=David R. |last3=Moreau |first3=Hervé |last4=Herron |first4=Matthew D. |last5=Verbruggen |first5=Heroen |last6=Delwiche |first6=Charles F. |last7=De Clerck |first7=Olivier | name-list-style = vanc |title=Phylogeny and Molecular Evolution of the Green Algae | url = https://frederikleliaert.files.wordpress.com/2013/05/2012_leliaert_crps.pdf |s2cid-access=free |journal=Critical Reviews in Plant Sciences |volume=31 |year=2012 |pages=1–46 |doi=10.1080/07352689.2011.615705 |s2cid=17603352 |url-status=live |archive-url= https://web.archive.org/web/20230524164529/https://frederikleliaert.files.wordpress.com/2013/05/2012_leliaert_crps.pdf |archive-date= May 24, 2023 }}</ref><ref name="Marin-2012">{{cite journal | vauthors = Marin B | title = Nested in the Chlorellales or independent class? Phylogeny and classification of the Pedinophyceae (Viridiplantae) revealed by molecular phylogenetic analyses of complete nuclear and plastid-encoded rRNA operons | journal = Protist | volume = 163 | issue = 5 | pages = 778–805 | date = September 2012 | pmid = 22192529 | doi = 10.1016/j.protis.2011.11.004 }}</ref><ref name="Laurin-Lemay-2012">{{cite journal | vauthors = Laurin-Lemay S, Brinkmann H, Philippe H | title = Origin of land plants revisited in the light of sequence contamination and missing data | journal = Current Biology | volume = 22 | issue = 15 | pages = R593–4 | date = August 2012 | pmid = 22877776 | doi = 10.1016/j.cub.2012.06.013 | doi-access = free }}</ref><ref name="Ruhfel-2014">{{cite journal | vauthors = Ruhfel BR, Gitzendanner MA, Soltis PS, Soltis DE, Burleigh JG | title = From algae to angiosperms-inferring the phylogeny of green plants (Viridiplantae) from 360 plastid genomes | journal = BMC Evolutionary Biology | volume = 14 | pages = 23 | date = February 2014 | pmid = 24533922 | pmc = 3933183 | doi = 10.1186/1471-2148-14-23 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Leliaert F, Tronholm A, Lemieux C, Turmel M, DePriest MS, Bhattacharya D, Karol KG, Fredericq S, Zechman FW, Lopez-Bautista JM | title = Chloroplast phylogenomic analyses reveal the deepest-branching lineage of the Chlorophyta, Palmophyllophyceae class. nov | journal = Scientific Reports | volume = 6 | pages = 25367 | date = May 2016 | pmid = 27157793 | pmc = 4860620 | doi = 10.1038/srep25367 | bibcode = 2016NatSR...625367L }}</ref><ref>{{Cite book|url=https://books.google.com/books?id=S4LxB9MRdzMC&pg=PA158 |title=The Tree of Life: A Phylogenetic Classification|last1=Lecointre|first1= Guillaume |last2=Guyader|first2=Hervé Le | name-list-style = vanc |date=2006|publisher=Harvard University Press|isbn=9780674021839|language=en}}</ref><ref>{{cite journal | vauthors = Sánchez-Baracaldo P, Raven JA, Pisani D, Knoll AH | title = Early photosynthetic eukaryotes inhabited low-salinity habitats | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 114 | issue = 37 | pages = E7737–E7745 | date = September 2017 | pmid = 28808007 | doi = 10.1073/pnas.1620089114 | pmc=5603991| bibcode = 2017PNAS..114E7737S | doi-access = free }}</ref><ref name="Gitzendanner-2018">{{cite journal |vauthors=Gitzendanner MA, Soltis PS, Wong GK, Ruhfel BR, Soltis DE |date=March 2018 |title=Plastid phylogenomic analysis of green plants: A billion years of evolutionary history |journal=American Journal of Botany |volume=105 |issue=3 |pages=291–301 |doi=10.1002/ajb2.1048 |pmid=29603143 |doi-access=free}}</ref><ref name="Lemieux-2000">{{cite journal | vauthors = Lemieux C, Otis C, Turmel M | title = Ancestral chloroplast genome in Mesostigma viride reveals an early branch of green plant evolution | language = En | journal = Nature | volume = 403 | issue = 6770 | pages = 649–52 | date = February 2000 | pmid = 10688199 | doi = 10.1038/35001059 | bibcode = 2000Natur.403..649L | doi-access = free }}</ref><ref>{{Cite journal|last1=Riediger|first1=Matthias|last2=Hihara|first2=Yukako|last3=Hess |first3=Wolfgang R.|date= 2018-06-01 |title=From cyanobacteria and algae to land plants: The RpaB/Ycf27 regulatory network in transition|journal=Perspectives in Phycology|language=en |volume=5|issue=1|pages=13–25|doi=10.1127/pip/2018/0078|s2cid=90444313|issn=2198-011X}}</ref><ref>{{Citation|last1=Turmel|first1=Monique|title=Evolution of the Plastid Genome in Green Algae|date=2018|work=Advances in Botanical Research|pages=157–193|publisher=Elsevier|doi=10.1016/bs.abr.2017.11.010|isbn=9780128134573|last2=Lemieux|first2=Claude}}</ref><ref name="HuanLiu-2020">{{Cite journal |author1=Linzhou Li |author2=Sibo Wang |author3=Hongli Wang |author4=Sunil Kumar Sahu |author5=Birger Marin |author6=Haoyuan Li |author7=Yan Xu |author8=Hongping Liang |author9=Zhen Li |author10=Shifeng Chen |author11=Tanja Reder |date=22 June 2020 |title=The genome of Prasinoderma coloniale unveils the existence of a third phylum within green plants |journal=Nature Ecology & Evolution |volume=4 |issue=9 |pages=1220–1231 |doi=10.1038/s41559-020-1221-7 |pmc=7455551 |pmid=32572216 |doi-access=free |author22=Yves Van de Peer |author23=Michael Melkonian |author24=Huan Liu |author21=Xin Liu |author20=Xun Xu |author19=Gane Ka-Shu Wong |author17=Huanming Yang |author16=Hongli Du |author15=Barbara Melkonian |author14=Morten Petersen |author13=Sebastian Wittek |author12=Zehra Çebi |author18=Jian Wang}}</ref>{{Excessive citations inline|date=November 2021}}

{{barlabel
|size=23
|at=11.5
|label=Green algae
|style=font-size:80%;line-height:80%
|cladogram={{clade
    |label1=[[Viridiplantae]]
    |1={{clade
        |label1=[[Prasinodermophyta]]
        |barbegin1=darkgreen
        |1={{clade
            |1=[[Palmophyllophyceae]] (prasinophyte clade VI)
            |2=[[Prasinoderma|Prasinodermophyceae]]
        }}
        |2={{clade
            |label1=[[Chlorophyta]]
            |bar1=darkgreen
            |1={{clade
                |1={{clade
                    |1={{clade
                        |1={{clade
                            |label1=core Chlorophyta
                            |1={{clade
                                |label1=[[Chlorophytina]]
                                |1={{clade
                                    |1={{clade
                                        |1={{clade
                                            |1=[[Ulvophyceae]]
                                            |2=[[Chlorophyceae]]
                                        }}
                                        |2=[[Trebouxiophyceae]]
                                    }}
                                    |2=[[Chlorodendrales|Chlorodendrophyceae]]
                                }}
                                |2=[[Pedinophyceae]]
                            }}
                            |2=[[Prasinophyceae|Prasinophytes]] Clade VIIA
                        }}
                        |2={{clade
                            |1=[[Prasinophyceae|Prasinophytes]] Clade VIIC
                            |2=[[Pycnococcaceae]]
                        }}
                    }}
                    |2=[[Nephroselmidophyceae]]
                }}
                |2={{clade
                    |1=[[Mamiellophyceae]]
                    |2=[[Pyramimonadales]]
                }}
            }}
            |label2=[[Streptophyta]]/
            |sublabel2=[[Charophyta]]
            |2={{clade
                |bar1=darkgreen
                |1={{clade
                    |1=[[Mesostigmatophyceae]]
                    |2={{clade
                        |1=''[[Spirotaenia]]''
                        |2=[[Chlorokybophyceae]]
                    }}
                }}
                |2={{clade
                    |bar1=darkgreen
                    |1=''[[Streptofilum]]''
                    |2={{clade
                        |bar1=darkgreen
                        |1=[[Klebsormidiales|Klebsormidiophyceae]]
                        |label2=[[Phragmoplastophyta]]
                        |2={{clade
                            |bar1=darkgreen
                            |1=[[Charophyceae]]
                            |2={{clade
                                |bar1=darkgreen
                                |1=[[Coleochaetales|Coleochaetophyceae]]
                                |2={{clade
                                    |barend1=darkgreen
                                    |1={{clade
                                        |1=[[Zygnematales|Zygnematophyceae]]
                                        |2=[[Mesotaeniaceae]] s.s.
                                    }}
                                    |2=[[Embryophyta]] (land plants)
                                }}
                            }}
                        }}
                    }}
                }}
            }}
        }}
    }}
}}
}}

The basal character of the Mesostigmatophyceae, Chlorokybophyceae and ''spirotaenia'' are only more conventionally basal Streptophytes.

The algae of this paraphyletic group "Charophyta" were previously included in Chlorophyta, so green algae and Chlorophyta in this definition were synonyms. As the green algae clades get further resolved, the embryophytes, which are a deep charophyte branch, are included in "[[algae]]", "green algae" and "[[Charophyta|Charophytes]]", or these terms are replaced by cladistic terminology such as [[Archaeplastida]], [[Plant]]ae/[[Viridiplantae]], and [[Streptophyta|streptophytes]], respectively.<ref>{{Cite book|title=Handbook of the Protists|last1=Cook|first1=Martha E.|last2=Graham|first2=Linda E. |chapter=Chlorokybophyceae, Klebsormidiophyceae, Coleochaetophyceae | name-list-style = vanc |date=2017|publisher=Springer International Publishing|isbn=9783319281476|editor-last=Archibald|editor-first=John M.|pages=185–204|doi=10.1007/978-3-319-28149-0_36|editor-last2=Simpson|editor-first2=Alastair G. B.|editor-last3=Slamovits|editor-first3=Claudio H.}}</ref>

==Reproduction==
[[File:Algaeconjugate.jpg|thumb|right|Green algae conjugating]]
Green algae are a group of photosynthetic, eukaryotic organisms that include species with haplobiontic and diplobiontic life cycles. The diplobiontic species, such as ''[[Sea lettuce|Ulva]]'', follow a reproductive cycle called [[alternation of generations]] in which two multicellular forms, haploid and diploid, alternate, and these may or may not be isomorphic (having the same morphology). In haplobiontic species only the haploid generation, the [[gametophyte]] is multicellular. The fertilized egg cell, the diploid [[zygote]], undergoes [[meiosis]], giving rise to haploid cells which will become new gametophytes. The diplobiontic forms, which evolved from haplobiontic ancestors, have both a multicellular haploid generation and a multicellular diploid generation. Here the zygote divides repeatedly by [[mitosis]] and grows into a multicellular diploid [[sporophyte]]. The sporophyte produces haploid spores by meiosis that germinate to produce a multicellular gametophyte. All [[Embryophyte|land plants]] have a diplobiontic common ancestor, and diplobiontic forms have also evolved independently within [[Ulvophyceae]] more than once (as has also occurred in the red and brown algae).<ref name="Trillo-2015">{{cite book |last1=Trillo |first1=Inaki Ruiz |last2=Nedelcu |first2=Aurora M |date=2015 |title=Evolutionary transitions to multicellular life: Principles and mechanisms |publisher=Springer |isbn=978-94-017-9642-2 }}</ref>

Diplobiontic green algae include isomorphic and heteromorphic forms. In isomorphic algae, the morphology is identical in the haploid and diploid generations. In heteromorphic algae, the morphology and size are different in the gametophyte and sporophyte.<ref name="Bessho-2009">{{cite journal |first1=Kazuhiro |last1=Bessho |first2=Yoh |last2=Iwasa |s2cid=46519857 |title=Heteromorphic and isomorphic alternations of generations in macroalgae as adaptations to a seasonal environment |journal=Evolutionary Ecology Research |date=2009 |volume=11 |pages=691–711 }}</ref>

Reproduction varies from fusion of identical cells ([[isogamy]]) to [[fertilisation|fertilization]] of a large non-motile cell by a smaller motile one ([[oogamy]]). However, these traits show some variation, most notably among the basal green algae called [[prasinophyte]]s.{{cn|date=April 2025}}

Haploid algal cells (containing only one copy of their DNA) can fuse with other haploid cells to form diploid zygotes. When filamentous algae do this, they form bridges between cells, and leave empty cell walls behind that can be easily distinguished under the light microscope. This process is called ''conjugation'' and occurs for example in ''[[Spirogyra]]''.{{cn|date=April 2025}}

===Sex pheromone===
[[File:Freshly harvested algae 01.jpg|thumb|Freshly harvested algae]]
[[Sex pheromone]] production is likely a common feature of green algae, although only studied in detail in a few model organisms. ''[[Volvox]]'' is a genus of [[Chlorophyta|chlorophytes]]. Different species form spherical colonies of up to 50,000 cells. One well-studied species, ''[[Volvox carteri]]'' (2,000 – 6,000 cells) occupies temporary pools of water that tend to dry out in the heat of late summer. As their environment dries out, asexual ''V. carteri'' quickly die. However, they are able to escape death by switching, shortly before drying is complete, to the sexual phase of their life cycle that leads to production of dormant desiccation-resistant [[zygote]]s. Sexual development is initiated by a [[glycoprotein]] pheromone (Hallmann et al., 1998). This pheromone is one of the most potent known biological effector molecules. It can trigger sexual development at concentrations as low as 10<sup>−16</sup>M.<ref name="Hallmann-1998">{{cite journal | vauthors = Hallmann A, Godl K, Wenzl S, Sumper M | title = The highly efficient sex-inducing pheromone system of Volvox | journal = Trends in Microbiology | volume = 6 | issue = 5 | pages = 185–9 | date = May 1998 | pmid = 9614342 | doi = 10.1016/s0966-842x(98)01234-7 }}</ref> Kirk and Kirk<ref name="Kirk-1986">{{cite journal | vauthors = Kirk DL, Kirk MM | title = Heat shock elicits production of sexual inducer in Volvox | journal = Science | volume = 231 | issue = 4733 | pages = 51–4 | date = January 1986 | pmid = 3941891 | doi = 10.1126/science.3941891 | bibcode = 1986Sci...231...51K }}</ref> showed that sex-inducing pheromone production can be triggered experimentally in [[Somatic (biology)|somatic]] cells by [[heat shock]]. Thus heat shock may be a condition that ordinarily triggers sex-inducing pheromone in nature.<ref name="Hallmann-1998" />

The ''Closterium peracerosum-strigosum-littorale (C. psl)'' complex is a unicellular, isogamous [[Charophyceae|charophycean]] alga group that is the closest unicellular relative to land plants. [[Heterothallic]] strains of different [[mating type]] can conjugate to form [[zygospore]]s. Sex pheromones termed protoplast-release inducing proteins (glycopolypeptides) produced by mating-type (-) and mating-type (+) cells facilitate this process.<ref name="Sekimoto-1990">{{cite journal | vauthors = Sekimoto H, Satoh S, Fujii T | title = Biochemical and physiological properties of a protein inducing protoplast release during conjugation in theClosterium peracerosum-strigosum-littorale complex | journal = Planta | volume = 182 | issue = 3 | pages = 348–54 | date = October 1990 | pmid = 24197184 | doi = 10.1007/BF02411384 | s2cid = 1999634 }}</ref>

==Physiology==
The green algae, including the characean algae, have served as [[model experimental organism]]s to understand the mechanisms of the ionic and water permeability of membranes, [[osmoregulation]], [[turgor]] regulation, [[halophyte|salt tolerance]], [[cytoplasmic streaming]], and the generation of [[action potentials]].<ref>{{cite book|last=Tazawa|first=Masashi | name-list-style = vanc |year=2010|volume=72|pages=5–34|chapter-url=https://books.google.com/books?id=iMxH0-q42PkC&pg=PA31 |access-date=2012-07-10|doi=10.1007/978-3-642-13145-5_1|isbn=978-3-642-13145-5|title=Progress in Botany 72 |chapter=Sixty Years Research with Characean Cells: Fascinating Material for Plant Cell Biology }}</ref>

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

== External links ==
* [https://web.archive.org/web/20060921203727/http://home.arcor.de/stefan.wic/diplomarbeit.pdf#search=%22%22stefan%20wic%22%22/ Green algae and cyanobacteria in lichens]
* [https://web.archive.org/web/20070213103838/http://www.ucmp.berkeley.edu/greenalgae/greenalgae.html Green algae (UC Berkeley)]
* [https://web.archive.org/web/20070206081452/http://www.mbari.org/staff/conn/botany/flora/green.htm Monterey Bay green algae]

{{Plant classification}}
{{Authority control}}

[[Category:Green algae]]
[[Category:Paraphyletic groups]]