Chlorophyta

Phylum of green algae

Chlorophyta Temporal range: 1000–0 Ma^[1] Pha. Proterozoic Archean Had.

"Siphoneae" from Ernst Haeckel's Kunstformen der Natur, 1904
Scientific classification
Clade:	Diaphoretickes
Clade:	CAM
Clade:	Archaeplastida
(unranked):	Viridiplantae
Division:	Chlorophyta Reichenbach, 1828, emend. Pascher, 1914, emend. Lewis & McCourt, 2004^[2]^[3]^[4]
Classes^[5]
Chloropicophyceae †Chuariophyceae Mamiellophyceae Nephroselmidophyceae Picocystophyceae Pyramimonadophyceae Tetraphytina Chlorodendrophyceae Pedinophyceae UTC clade Ulvophyceae Trebouxiophyceae Chlorophyceae
Diversity
7,934 species (6,851 living, 1,083 fossil)^[6]
Synonyms
Chlorophycophyta Papenfuss 1946^[7] Chlorophycota Chlorophytina Chlorophyllophyceae Isokontae Stephanokontae

Chlorophyta is a taxon of green algae informally called chlorophytes.^[8]

Systematics

Taxonomic history

The first mention of Chlorophyta belongs to German botanist Heinrich Gottlieb Ludwig Reichenbach in his 1828 work Conspectus regni vegetabilis. Under this name, he grouped all algae, mosses ('musci') and ferns ('filices'), as well as some seed plants (Zamia and Cycas).^[9] This usage did not gain popularity. In 1914, Bohemian botanist Adolf Pascher modified the name to encompass exclusively green algae, that is, algae which contain chlorophylls a and b and store starch in their chloroplasts.^[10] Pascher established a scheme where Chlorophyta was composed of two groups: Chlorophyceae, which included algae now known as Chlorophyta, and Conjugatae, which are now known as Zygnematales and belong to the Streptophyta clade from which land plants evolved.^[3]^[11]

During the 20th century, many different classification schemes for the Chlorophyta arose. The Smith system, published in 1938 by American botanist Gilbert Morgan Smith, distinguished two classes: Chlorophyceae, which contained all green algae (unicellular and multicellular) that did not grow through an apical cell; and Charophyceae, which contained only multicellular green algae that grew via an apical cell and had special sterile envelopes to protect the sex organs.^[12]

With the advent of electron microscopy studies, botanists published various classification proposals based on finer cellular structures and phenomena, such as mitosis, cytokinesis, cytoskeleton, flagella and cell wall polysaccharides.^[13]^[14] British botanist Frank Eric Round [nl] proposed in 1971 a scheme which distinguishes Chlorophyta from other green algal divisions Charophyta, Prasinophyta and Euglenophyta. He included four classes of chlorophytes: Zygnemaphyceae, Oedogoniophyceae, Chlorophyceae and Bryopsidophyceae.^[15] Other proposals retained the Chlorophyta as containing all green algae, and varied from one another in the number of classes. For example, the 1984 proposal by Mattox & Stewart included five classes,^[13] while the 1985 proposal by Bold & Wynne included only two,^[16] and the 1995 proposal by Christiaan van den Hoek and coauthors included up to eleven classes.^[10]

The modern usage of the name 'Chlorophyta' was established in 2004, when phycologists Lewis & McCourt firmly separated the chlorophytes from the streptophytes on the basis of molecular phylogenetics. All green algae that were more closely related to land plants than to chlorophytes were grouped as a paraphyletic division Charophyta.^[11]

Within the green algae, the most basal lineages were grouped under the informal name of "prasinophytes", and they were all believed to belong to the Chlorophyta clade.^[11] However, in 2020 a study recovered a new clade and division known as Prasinodermophyta, which contains two prasinophyte lineages previously considered chlorophytes.^[17] Below is a cladogram representing the current state of green algal classification:^[18]^[17]^[19]^[20]

Viridiplantae

Prasinodermophyta

	Prasinodermophyceae

	Palmophyllophyceae

Chlorophyta

	Pyramimonadophyceae

	Mamiellophyceae

Nephroselmidophyceae

Pseudoscourfieldiales

core clade

UTC

	Chlorophyceae

	Ulvophyceae

Pedinophyceae

Streptophyta

	Mesostigmatophyceae

	Chlorokybophyceae

Klebsormidiophyceae

Charophyceae

Coleochaetophyceae

	Zygnematophyceae

	Embryophyta (land plants)

"charophytes"

"prasinophytes"

Classification

Representatives of all living classes of chlorophytes

Tetraselmis suecica (Chlorodendrophyceae)

Volvox aureus (Chlorophyceae)

Chloropicon sieburthii (Chloropicophyceae)

Micromonas pusilla (Mamiellophyceae)

Nephroselmis olivacea (Nephroselmidophyceae)

Mantoniella tinhauana (Pedinophyceae)

Picocystis salinarum (Picocystophyceae)

Pyramimonas longicauda (Pyramimonadophyceae)

Coccomyxa polymorpha (Trebouxiophyceae)

Ulva lactuca
(Ulvophyceae)

Currently eleven chlorophyte classes are accepted, here presented in alphabetical order with some of their characteristics and biodiversity:

Chlorodendrophyceae (60 species, 15 extinct):^[6] unicellular flagellates (monadoids) surrounded by an outer cell covering or theca of organic extracellular scales composed of proteins and ketosugars. Some of these scales make up hair-like structures. Capable of asexual reproduction through cell division inside the theca. No sexual reproduction has been described. Each cell contains a single chloroplast and exhibits two flagella. Present in marine and freshwater habitats.^[21]^[22]^[23]

Chlorophyceae (3,974 species):^[6] either unicellular monadoids (flagellated) or coccoids (without flagella) living solitary or in varied colonial forms (including coenobial), or multicellular filamentous (branch-like) thalli that may be ramified, or foliose (leaf-like) thalli. Cells are surrounded by a crystalline covering composed of glycoproteins abundant in glycine and hydroxyproline, as well as pectins, arabinogalactan proteins, and extensin. They exhibit a haplontic life cycle with isogamy, anisogamy or oogamy. They are capable of asexual reproduction through flagellated zoospores, aplanospores, or autospores. Each cell contains a single chloroplast, a variable number of pyrenoids (including lack thereof), and from one to hundreds of flagella without mastigonemes. Present in marine, freshwater and terrestrial habitats.^[24]^[25]^[26]^[27]

Chloropicophyceae (8 species):^[6] unicellular solitary coccoids. Cells are surrounded by a multi-layered cell wall. No sexual or asexual reproduction has been described. Each cell contains a single chloroplast with astaxanthin and loroxanthin, and lacks pyrenoids or flagella. They are exclusively marine.^[18]

Chuariophyceae (3 extinct species): exclusively fossil group containing carbonaceous megafossils found in Ediacaran rocks, such as Tawuia.^[6]^[28]

Mamiellophyceae (25 species):^[6] unicellular solitary monadoids. Cells are naked or covered by one or two layers of flat scales, mainly with spiderweb-like or reticulate ornamentation. Each cell contains one or rarely two chloroplasts, almost always with prasinoxanthin; two equal or unequal flagella, or just one flagellum, or lacking any flagella. If flagella are present, they can be either smooth or covered in scales in the same manner as the cells. Present in marine and freshwater habitats.^[29]^[22]

Nephroselmidophyceae (29 species):^[6] unicellular monadoids. Cells are covered by scales. They are capable of sexual reproduction through hologamy (fusion of entire cells), and of asexual reproduction through binary fission. Each cell contains a single cloroplast, a pyrenoid, and two flagella covered by scales. Present in marine and freshwater habitats.^[30]^[31]^[22]

Pedinophyceae (24 species):^[6] unicellular asymmetrical monadoids that undergo a coccoid palmelloid phase covered by mucilage. Cells lack extracellular scales, but in rare cases are covered on the posterior side by a theca. Each cell contains a single chloroplast, a pyrenoid, and a single flagellum usually covered in mastigonemes. Present in marine, freshwater and terrestrial habitats.^[32]^[22]^[33]

Picocystophyceae (1 species):^[6] unicellular coccoids, ovoid and trilobed in shape. Cells are surrounded by a multi-layered cell wall of poly-arabinose, manose, galactose and glucose. No sexual reproduction has been described. They are capable of asexual reproduction through autosporulation, resulting in two or rarely four daughter cells. Each cell contains a single bilobed chloroplast with diatoxanthin and monadoxanthin, without any pyrenoid or flagella. Present in saline lakes.^[34]^[18]^[22]

Pyramimonadophyceae (166 species, 59 extinct):^[6] unicellular monadoids or coccoids. Cells are covered by two or more layers of organic scales. No sexual reproduction has been described, but some cells with only one flagellum have been interpreted as potential gametes. Asexual reproduction has only been observed in the coccoid forms, via zoospores. Each cell contains a single chloroplast, a pyrenoid, and between 4 and 16 flagella. The flagella are covered in at least two layers of organic scales: a bottom layer of pentagonal scales organized in 24 rows, and a top layer of limuloid scales distributed in 11 rows. They are exclusively marine.^[22]^[35]

Trebouxiophyceae (926 species, 1 extinct):^[6] unicellular monadoids occasionally without flagella, or colonial, or ramified filamentous thalli, or living as the photobionts of lichen. Cells are covered by a cell wall of cellulose, algaenans, and β-galactofuranane. No sexual reproduction has been described with the exception of some observations of gamete fusion and presence of meiotic genes. They are capable of asexual reproduction through autospores or zoospores. Each cell contains a single chloroplast, a pyrenoid, and one or two pairs of smooth flagella. They are present in marine, freshwater and terrestrial habitats.^[24]^[36]^[4]^[37]

Ulvophyceae (2,695 species, 990 extinct):^[6] macroscopic thalli, either filamentous (which may be ramified) or foliose (composed of monostromatic or distromatic layers) or even compact tubular forms, generally multinucleate. Cells surrounded by a cell wall that may be calcified, composed of cellulose, β-manane, β-xilane, sulphated or piruvilated polysaccharides or sulphated ramnogalacturonanes, arabinogalactan proteins, and extensin. They exhibit a haplodiplontic life cycle where the alternating generations can be isomorphic or heteromorphic. They reproduce asexually via zoospores that may be covered in scales. Each cell contains a single chloroplast, and one or two pairs of flagella without mastigonemes but covered in scales. They are present in marine, freshwater and terrestrial habitats.^[24]^[4]^[38]

Ecology

Most species of Chlorophyta are aquatic, prevalent in both marine and freshwater environments. About 90% of all known species live in freshwater.^[39] However, some species have adapted to a wide range of terrestrial environments. For example, Chlamydomonas nivalis lives on summer alpine snowfields, and Trentepohlia species, live attached to rocks or woody parts of trees.^[40]^[25] Several species have adapted to specialised and extreme environments, such as deserts, arctic environments, hypersaline habitats, marine deep waters, deep-sea hydrothermal vents and habitats that experiences extreme changes in temperature, light and salinity.^[41]^[42]^[43]^[44] Some groups, such as the Trentepohliales are exclusively found on land.^[45] Several species of Chlorophyta live in symbiosis with a diverse range of eukaryotes, including fungi (to form lichens), ciliates, forams, cnidarians and molluscs.^[25] Some species of Chlorophyta are heterotrophic, either free-living or parasitic.^[46]^[47] Others are mixotrophic bacterivores through phagocytosis.^[48] Two common species of the heterotrophic green alga Prototheca are pathogenic and can cause the disease protothecosis in humans and animals.^[49]

With the exception of the three classes Ulvophyceae, Trebouxiophyceae and Chlorophyceae in the UTC clade, which show various degrees of multicellularity, all the Chlorophyta lineages are unicellular.^[50] Some members of the group form symbiotic relationships with protozoa, sponges, and cnidarians. Others form symbiotic relationships with fungi to form lichens, but the majority of species are free-living. Some conduct sexual reproduction, which is oogamous or isogamous. All members of the clade have motile flagellated swimming cells.^[51] Monostroma kuroshiense, an edible green alga cultivated worldwide and most expensive among green algae, belongs to this group.

Evolution

In February 2020, the fossilized remains of a green alga, named Proterocladus antiquus were discovered in the northern province of Liaoning, China. At around a billion years old, it is believed to be one of the oldest examples of a multicellular chlorophyte. It is currently classified as a member of order Siphonocladales, class Ulvophyceae.^[1] In 2023, a study calculated the molecular age of green algae as calibrated by this fossil. The study estimated the origin of Chlorophyta within the Mesoproterozoic era, at around 2.04–1.23 billion years ago.^[20]

Usage

Model organisms

Among chlorophytes, a small group known as the volvocine green algae is being researched to understand the origins of cell differentiation and multicellularity. In particular, the unicellular flagellate Chlamydomonas reinhardtii and the colonial organism Volvox carteri are object of interest due to sharing homologous genes that in Volvox are directly involved in the development of two different cell types with full division of labor between swimming and reproduction, whereas in Chlamydomonas only one cell type exists that can function as a gamete. Other volvocine species, with intermediate characters between these two, are studied to further understand the transition towards the cellular division of labor, namely Gonium pectorale, Pandorina morum, Eudorina elegans and Pleodorina starrii.^[52]

Industrial uses

Chlorophyte microalgae are a valuable source of biofuel and various chemicals and products in industrial amounts, such as carotenoids, vitamins and unsaturated fatty acids. The genus Botryococcus is an efficient producer of hydrocarbons, which are converted into biodiesel. Various genera (Chlorella, Scenedesmus, Haematococcus, Dunaliella and Tetraselmis) are used as cellular factories of biomass, lipids and different vitamins for either human or animal consumption, and even for usage as pharmaceuticals. Some of their pigments are employed for cosmetics.^[53]

References

Citations

^ ^a ^b Tang et al. 2020.
^ Reichenbach 1828, p. 23.
^ ^a ^b Pascher 1914.
^ ^a ^b ^c Adl et al. 2019, p. 36.
^ Guiry 2024.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l Guiry 2024, p. 5.
^ Papenfuss 1955.
^ Rockwell et al. 2017.
^ Reichenbach 1828, p. 23–40.
^ ^a ^b van den Hoek, Mann & Jahns 1995.
^ ^a ^b ^c Lewis & McCourt 2004.
^ Smith 1938, p. 12.
^ ^a ^b Mattox & Stewart 1984.
^ Lobban & Wynne 1981, p. 88.
^ Round 1971.
^ Bold & Wynne 1985.
^ ^a ^b Li et al. 2020.
^ ^a ^b ^c Lopes dos Santos et al. 2017.
^ Gulbrandsen et al. 2021.
^ ^a ^b Yang et al. 2023.
^ Hori, Norris & Chihara 1986.
^ ^a ^b ^c ^d ^e ^f Adl et al. 2019, p. 37.
^ Graham et al. 2022, p. 17-2.
^ ^a ^b ^c Domozych et al. 2012.
^ ^a ^b ^c Leliaert et al. 2012.
^ Adl et al. 2019, p. 36–37.
^ Graham et al. 2022, p. 19-2–19-5.
^ Srivastava 2002.
^ Marin & Melkonian 2010.
^ Nakayama et al. 2007.
^ Yamaguchi et al. 2010.
^ Marin 2012.
^ Graham et al. 2022, p. 17-3.
^ Lewin et al. 2000.
^ Daugbjerg, Fassel & Moestrup 2020.
^ Fučíková, Pažoutová & Rindi 2015.
^ Graham et al. 2022, p. 17-4–17-11.
^ Graham et al. 2022, p. 18-2–18-24.
^ Lee 2018.
^ Graham et al. 2022.
^ Lewis & Lewis 2005.
^ De Wever et al. 2009.
^ Leliaert, Verbruggen & Zechman 2011.
^ Foflonker et al. 2016.
^ López-Bautista, Rindi & Guiry 2006.
^ Joubert & Rijkenberg 1971.
^ Nedelcu 2001.
^ Anderson, Charvet & Hansen 2018.
^ Tartar et al. 2002.
^ Umen 2014.
^ Kapraun 2007.
^ Nishii & Miller 2010.
^ Baudelet et al. 2017.

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