Prasinophyte
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The prasinophytes are a group of unicellular green algae.[1] Prasinophytes mainly include marine planktonic species, as well as some freshwater representatives.[1][2] The prasinophytes are morphologically diverse, including flagellates with one to eight flagella and non-motile (coccoid) unicells, as well as dormant or cyst stage phycoma. The cells of many species are covered with organic body scales; others are naked.[2] Well studied genera include Ostreococcus, considered to be the smallest (ca. 0.95 μm) free-living eukaryote,[3] and Micromonas, both of which are found in marine waters worldwide. Prasinophytes have simple cellular structures, containing a single chloroplast and a single mitochondrion. The genomes are relatively small compared to other eukaryotes (about 12 Mbp for Ostreococcus[4][5] and 21 Mbp for Micromonas[6]). At least one species, the Antarctic form Pyramimonas gelidicola, is capable of phagocytosis and is therefore a mixotrophic algae.[7]
Some authors treat the prasinophytes as a polyphyletic grouping of green algae from different clades. As the Tetraphytina emerged in the Prasinophytes, recently authors include it, rendering it monophyletic, and equivalent to chlorophyta.[8][9]
Morphology
Ecology
A study of photosynthetic gene-sequence diversity (rbcL) in the Gulf of Mexico indicated that Prasinophytes are particularly prevalent at the Subsurface Chlorophyll Maximum (SCM)[10] and several different ecotypes of Ostreococcus have been detected in the environment.[11] These ecotypes were thought to be distinguished in the environment by their adaptation to light intensities. O. lucimarinus was isolated from a high-light environment[12] and observed year-round in the coastal North Pacific Ocean.[13] RCC141 was considered low-light, because it was isolated from the lower euphotic zone. These strains, or ecotypes, were later shown to live in different habitats (open-ocean or mesotrophic) and their distributions do not appear to be connected to light availability.[14] O. tauri was isolated from a coastal lagoon and appears to be light-polyvalent. Genetic data indicates that distinct molecular differences exist between the different ecotypes that have been detected.[15]
Prasinophytes are subject to infection by large double-stranded DNA viruses belonging to the genus Prasinovirus in the family Phycodnaviridae,[16][17][18] as well as a Reovirus.[19] It has been estimated that from 2 to 10% of the Micromonas pusilla population is lysed per day by viruses.[20]
Phylogeny
Recent studies agree that the prasinophytes are not a natural group, being highly paraphyletic.[2][21][22][23] Relationships among the groups making up the Chlorophyta are not fully resolved. The cladogram produced by Leliaert et al. 2011[2] and some modification according to Silar 2016,[24] Leliaert 2016[25] and Lopes dos Santos et al. 2017[26] is shown below. The blue shaded groups are or have traditionally been placed in the Prasinophyceae[1]). The species Mesostigma viride has been shown to be a member of a basal clade inside Streptophyta. The others are members of the Chlorophyta.
Script error: No such module "Clade". A 2020 study placed the Palmophyllophyceae (prasinophyte clade VI) in a new phylum outside of the Chlorophyta and Streptophyta, the Prasinodermophyta, as shown below.[27] However, a subsequent genomic study recovered Prasinodermophyta as sister to the revised Chlorophyta and questioned the need for the third division or phylum.[28]
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See also
References
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- ↑ a b c Sym, S. D. and Pienaar, R. N. 1993. The class Prasinophyceae. In Round, F. E. and Chapman, D. J. (eds) Progress in Phycological Research, Vol. 9. Biopress Ltd., Bristol, pp. 281-376.
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- ↑ Mixotrophy in the Antarctic phytoflagellate, Pyramimonas gelidicola (Chlorophyta: Prasinophyceae)
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External links
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