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Chlorophyceae
	Light micrograph of Pediastrum sp.
Scientific classification
Clade:	Viridiplantae
Division:	Chlorophyta
Subphylum:	Chlorophytina
Class:	Chlorophyceae; Wille in Warming, 1884
Orders
	See text.

The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology.^[2] They are usually green due to the dominance of pigments chlorophyll a and chlorophyll b. The chloroplast may be discoid, plate-like, reticulate, cup-shaped, spiral- or ribbon-shaped in different species. Most of the members have one or more storage bodies called pyrenoids located in the chloroplast. Pyrenoids contain protein besides starch. Some green algae may store food in the form of oil droplets. They usually have a cell wall made up of an inner layer of cellulose and outer layer of pectose.

General characteristics

Depending on the species, Chlorophyceae can grow unicellular (e.g. Chlamydomonas), colonial (e.g. Volvox), filamentous (e.g. Ulothrix), or multicellular.^{[example needed]} They are usually green due to the presence of chlorophyll a and chlorophyll b; they can also contain the pigment beta-carotene. There are two clades of Chlorophyceae as defined by the arrangement of their flagella, called CW and DO. Members of the CW clade have flagella that are displaced in a "clockwise" (CW, 1–7 o'clock) direction e.g. Chlamydomonadales. Members of the DO clade have flagella that are "directly opposed" (DO, 12–6 o'clock) e.g. Sphaeropleales.^[3]

The chloroplast may be discoid, cup-shaped (e.g. Chlamydomonas), spiral or ribbon shaped.^{[example needed]} Most chlorophytes have one or more storage bodies called pyrenoids (central proteinaceous body covered with a starch sheath) that are localised around the chloroplast. Some algae may also store food in the form of oil droplets. The inner cell wall layer is made of cellulose and the outer layer of pectose.

Reproduction

Chlorophyceae can reproduce both asexually and sexually. Vegetative reproduction usually takes place by fragmentation. Asexual reproduction is by flagellated zoospores. And haplospore, perennation (akinate and palmella stage). Asexual reproduction by mitospore absent in spyrogyra. Also by aplanospores, hypnospores, Palmella stage, etc.

Sexual reproduction shows considerable variation in the type and formation of sex cells and it may be isogamous e.g. Chlamydomonas, Ulothrix, anisogamous e.g. Chlamydomonas, Eudorina or Oogamous e.g. Chlamydomonas, Volvox. Chlamydomonas has all three types of sexual reproduction.

They share many similarities with the higher plants, including the presence of asymmetrical flagellated cells, the breakdown of the nuclear envelope at mitosis, and the presence of phytochromes, flavonoids, and the chemical precursors to the cuticle.^[4]

The sole method of reproduction in Chlorella is asexual and azoosporic. The content of the cell divides into 2,4 (B), 8(C) sometimes daughter protoplasts. Each daughter protoplast rounds off to form a non-motile spore. These autospores (spores having the same distinctive shape as the parent cell) are liberated by the rupture of the parent cell wall (D). On release each autospore grows to become a new individual. The presence of sulphur in the culture medium is considered essential for cell division. It takes place even in the dark with sulphur alone as the source material but under light conditions nitrogen also required in addition. Pearsall and Loose (1937)^[5] reported the occurrence of motile cells in Chlorella. Bendix (1964)^[6] also observed that Chlorella produces motile cells which might be gametes. These observations have an important bearing on the concept of the life cycle of Chlorella, which at present is considered to be strictly asexual in character.

Asexual reproduction in Chlorella ellipsoides has been studied in detail and the following four phases have been observed during the asexual reproduction.

Growth phase - During this phase the cells grow in size by utilizing the photosynthetic products.
Ripening phase - In this phase the cells mature and prepare themselves for division.
Post ripening phase - During this phase, each mature cell divides twice either in dark or in light. The cells formed in dark are known as dark to light phase, cells again grow in size.
Division phase - During this phase the parent cell wall ruptures and unicells are released.

Orders

As of May 2023^[update], AlgaeBase accepted the following orders in the class Chlorophyceae:

Chaetopeltidales C.J.O'Kelly, Shin Watanabe, & G.L.Floyd – 16 species
Chaetophorales Wille – 225 species
Chlamydomonadales F.E.Fritsch (also known as Volvocales) – 1793 species
Oedogoniales Heering – 792 species
Sphaeropleales Luerssen – 941 species

Along with these genera, AlgaeBase recognizes several taxa that are incertae sedis (i.e. unplaced to an order):

Dangeardinellaceae Ettl - 1 species

Other orders that have been recognized include:

Dunaliellales – Dunaliella and Dunaliellaceae are placed in Chlamydomonadales by AlgaeBase^[7]
Chlorococcales – Chlorococcum and Chlorococcaceae are placed in Chlamydomonadales by AlgaeBase^[8]
Microsporales – Microspora and Microsporaceae are placed in Sphaeropleales by AlgaeBase^[9]
Tetrasporales – Tetraspora and Tetrasporaceae are placed in Chlamydomonadales by AlgaeBase^[10]

In older classifications, the term Chlorophyceae is sometimes used to apply to all the green algae except the Charales, and the internal division is considerably different.^{[citation needed]}

Diversity of Chlorophyceae

Chlamydomonas globosa 400x
Volvox aureus
Pediastrum duplex

References

^ Warming, E., 1884. Haandbog i den systematiske botanik. Anden gjennemsete udgave. 2nd ed. Kjøbenhavn, 434 pp. German translation (1890) of the 2nd Danish edition available at archive.org: [1]. English translation (1895) of the 3rd Danish edition (1892) available at archive.org: [2].
^ Štenclová, Lenka, Karolina Fučíková, Jan Kaštovský, and Marie Pažoutová (December 2017). "Molecular and morphological delimitation and generic classification of the family Oocystaceae (Trebouxiophyceae, Chlorophyta)". Journal of Phycology. 53 (6): 1263–1282. Bibcode:2017JPcgy..53.1263S. doi:10.1111/jpy.12581. PMID 28833138. S2CID 21278460 – via PubMed.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Lewis, Louise A. and Richard M. McCourt (2004-10-01). "Green algae and the origin of land plants". American Journal of Botany. 91 (10): 1535–1556. doi:10.3732/ajb.91.10.1535. PMID 21652308 – via onlinelibrary.wiley.com.
^ Raven, Evert and Eichhorn. The Biology of Plants 7th edition, pg. 335. W. H. Freeman and Company, New York, 2005.
^ Pearsall WH, Loose L (1937). "The Growth of Chlorella Vulgaris in Pure Culture". Proc Roy Soc B. 121 (824): 451–501. doi:10.1098/rspb.1936.0075. Retrieved 2024-08-13.
^ Bendix S (1964-01-22). "Phenotypic Variability in Certain Chlorella pyrenoidosa Strains". Phycologia. 4 (2): 84–92. Bibcode:1964Phyco...4...84B. doi:10.2216/i0031-8884-4-2-84.1.
^ Guiry, M.D.; Guiry, G.M. "Dunaliella". AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. Retrieved 2022-02-25.
^ Guiry, M.D.; Guiry, G.M. "Chlorococcum". AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. Retrieved 2022-02-25.
^ Guiry, M.D.; Guiry, G.M. "Microspora". AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. Retrieved 2022-02-25.
^ Guiry, M.D.; Guiry, G.M. "Tetraspora". AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. Retrieved 2022-02-25.