Theprasinophytes are a group of unicellulargreen algae.[4] Prasinophytes mainly include marineplanktonic species, as well as some freshwater representatives.[4][5] The prasinophytes are morphologically diverse, includingflagellates 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.[5] Well studied genera includeOstreococcus, considered to be the smallest (ca. 0.95μm) free-livingeukaryote,[6] andMicromonas, both of which are found in marine waters worldwide. Prasinophytes have simple cellular structures, containing a singlechloroplast and a singlemitochondrion. Thegenomes are relatively small compared to other eukaryotes (about 12 Mbp forOstreococcus[7][8] and 21 Mbp forMicromonas[9]).At least one species, the Antarctic formPyramimonas gelidicola, is capable ofphagocytosis and is therefore amixotrophic algae.[10]
Some authors treat the prasinophytes as apolyphyletic grouping of green algae from different clades. As theTetraphytina emerged in the Prasinophytes, recently authors include it, rendering it monophyletic, and equivalent to chlorophyta.[11][12]
A study of photosynthetic gene-sequence diversity (rbcL) in theGulf of Mexico indicated that Prasinophytes are particularly prevalent at theSubsurface Chlorophyll Maximum (SCM)[13] and several different ecotypes ofOstreococcus have been detected in the environment.[14] 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[15] and observed year-round in the coastal North Pacific Ocean.[16] RCC141 was considered low-light, because it was isolated from the lowereuphotic 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.[17]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.[18]
Prasinophytes are subject to infection by large double-stranded DNA viruses belonging to the genusPrasinovirus in the familyPhycodnaviridae,[19][20][21] as well as aReovirus.[22] It has been estimated that from 2 to 10% of theMicromonas pusilla population is lysed per day by viruses.[23]
Recent studies agree that the prasinophytes are not a natural group, being highlyparaphyletic.[5][24][25][26] Relationships among the groups making up the Chlorophyta are not fully resolved. Thecladogram produced by Leliaert et al. 2011[5] and some modification according to Silar 2016,[27] Leliaert 2016[28] and Lopes dos Santos et al. 2017[1] is shown below. Theblue shaded groups are or have traditionally been placed in the Prasinophyceae[4]). The speciesMesostigma viride has been shown to be a member of a basal clade insideStreptophyta. The others are members of theChlorophyta.
A 2020 study placed thePalmophyllophyceae (prasinophyte clade VI) in a new phylum outside of the Chlorophyta and Streptophyta, thePrasinodermophyta, as shown below.[29] However, a subsequent genomic study recovered Prasinodermophyta as sister to the revised Chlorophyta and questioned the need for the third division or phylum.[30]
^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.
^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.
^Rodríguez F, Derelle E, Guillou L, Le Gall F, Vaulot D, Moreau H (June 2005). "Ecotype diversity in the marine picoeukaryote Ostreococcus (Chlorophyta, Prasinophyceae)".Environmental Microbiology.7 (6):853–9.doi:10.1111/j.1462-2920.2005.00758.x.PMID15892704.
^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.
