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]
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]
Phylogeny
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.
^ abcSym, 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.
^ abcdLeliaert F, Verbruggen H, Zechman FW (September 2011). "Into the deep: new discoveries at the base of the green plant phylogeny". BioEssays. 33 (9): 683–92. doi:10.1002/bies.201100035. PMID21744372. S2CID40459076.
^Guillou L, Eikrem W, Chrétiennot-Dinet MJ, Le Gall F, Massana R, Romari K, Pedrós-Alió C, Vaulot D (June 2004). "Diversity of picoplanktonic prasinophytes assessed by direct nuclear SSU rDNA sequencing of environmental samples and novel isolates retrieved from oceanic and coastal marine ecosystems". Protist. 155 (2): 193–214. doi:10.1078/143446104774199592. PMID15305796. S2CID15859454.
^Worden AZ, Nolan JK, Palenik B (2004). "Assessing the dynamics and ecology of marine picophytoplankton: The importance of the eukaryotic component". Limnology and Oceanography. 49 (1): 168–179. Bibcode:2004LimOc..49..168W. doi:10.4319/lo.2004.49.1.0168.
^Bellec L, Grimsley N, Derelle E, Moreau H, Desdevises Y (April 2010). "Abundance, spatial distribution and genetic diversity of Ostreococcus tauri viruses in two different environments". Environmental Microbiology Reports. 2 (2): 313–21. doi:10.1111/j.1758-2229.2010.00138.x. PMID23766083.
^Lewis LA, McCourt RM (October 2004). "Green algae and the origin of land plants". American Journal of Botany. 91 (10): 1535–56. doi:10.3732/ajb.91.10.1535. PMID21652308.
^Marin B (September 2012). "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". Protist. 163 (5): 778–805. doi:10.1016/j.protis.2011.11.004. PMID22192529.
