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Class of algae From Wikipedia, the free encyclopedia
The prasinophytes are a group of unicellular green algae.[3] Prasinophytes mainly include marine planktonic species, as well as some freshwater representatives.[3][4] The prasinophytes are morphologically diverse, including flagellates with one to eight flagella and non-motile (coccoid) unicells. The cells of many species are covered with organic body scales; others are naked.[4] Well studied genera include Ostreococcus, considered to be the smallest (ca. 0.95 μm) free-living eukaryote,[5] 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[6][7] and 21 Mbp for Micromonas[8]). At least one species, the Antarctic form Pyramimonas gelidicola, is capable of phagocytosis and is therefore a mixotrophic algae.[9]
| Prasinophyte Paraphyletic group of chlorophytes | |
|---|---|
| |
| Pyramimonas sp. | |
Scientific classification | |
| Clade: | Viridiplantae |
| Division: | Chlorophyta |
| Informal group: | Prasinophyte |
| Included classes[1][2] | |
| Excluded classes (i.e. chlorophytes not considered prasinophytes) | |
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.[10][11]
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)[12] and several different ecotypes of Ostreococcus have been detected in the environment.[13] 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[14] and observed year-round in the coastal North Pacific Ocean.[15] 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.[16] 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.[17]
Prasinophytes are subject to infection by large double-stranded DNA viruses belonging to the genus Prasinovirus in the family Phycodnaviridae,[18][19][20] as well as a Reovirus.[21] It has been estimated that from 2 to 10% of the Micromonas pusilla population is lysed per day by viruses.[22]
Recent studies agree that the prasinophytes are not a natural group, being highly paraphyletic.[4][23][24][25] Relationships among the groups making up the Chlorophyta are not fully resolved. The cladogram produced by Leliaert et al. 2011[4] and some modification according to Silar 2016,[26] Leliaert 2016[27] and Lopes dos Santos et al. 2017[1] is shown below. The blue shaded groups are or have traditionally been placed in the Prasinophyceae[3]). The species Mesostigma viride has been shown to be a member of the Streptophyta or basal Green algae. The others are member of the Chlorophyta.
| Green Algae |
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As 2020 paper places the Palmophyllophyceae (prasinophyte clade VI) in a new phylum outside of the Chlorophyta and Streptophyta, the Prasinodermophyta.[28]
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