Prasinophyceae

The Prasinophytes (incl. Tetraphytina) or chlorophyta are a class of unicellular green algae.[2] Prasinophytes mainly include marine planktonic species, as well as some freshwater representatives.[2][3] 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.[3] Well studied genera include Ostreococcus, considered to be the smallest (ca. 0.95 μm) free-living eukaryote,[4] 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[5][6] and 21 Mbp for Micromonas[7]).

Prasinophyceae
Pyramimonas sp.
Scientific classification
Kingdom:
Viridiplantae
Division:
Chlorophyta
Class:
Prasinophyceae

Orders[1]

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]

Ecology
Micromonas pusilla

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.[3][21][22][23] Relationships among the groups making up the Chlorophyta are not fully resolved. The cladogram produced by Leliaert et al. 2011[3] 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[2]). 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
Chlorophyta/
Palmophyllophyceae
 

Prasinococcales ("prasinophyte clade VI")

 
 

Palmophyllales

 
 
 
 
 

Pyramimonadophyceae ("prasinophyte clade I")

 
Mamiellophyceae
 

Monomastigales

 
 
 

Dolichomastigales

 
 

Mamiellales

 
 
("prasinophyte clade II")
 
 
Nephroselmidophyceae

Nephroselmidales ("prasinophyte clade III")

 
 
 

?Pycnococcaceae ("prasinophyte clade V")

 
 

Picocystophyceae ("prasinophyte clade VII C")

 
 
 

Chloropicophyceae ("prasinophyte clade VII A/B")

 
Tetraphytina
 

Pedinophyceae

 
Chlorophytina
 

Chlorodendrophyceae ("prasinophyte clade IV")

 
 
 

Trebouxiophyceae

 
 
 

Ulvophyceae

 
 

Chlorophyceae

 
 
 
 
 
 
 
 
 
Prasinophyta
Streptophyta
 
 

Chlorokybophyceae

 
 

Mesostigmatophyceae

 
 
 
 

Klebsormidiophyceae

 
 
 

Charophyceae

 
 
 

Zygnematophyceae

 
 

Coleochaetophyceae

 
 

Land plants

 
 
 
 
 
 

References
  1. Guiry, M.D. & Guiry, G.M. (2007). "Class: Prasinophyceae taxonomy browser". AlgaeBase version 4.2 Worldwide electronic publication, National University of Ireland, Galway. Retrieved 2007-09-23.
  2. 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.
  3. Leliaert 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. PMID 21744372.
  4. Courties C, Vaquer A, Troussellier M, Lautier J, Chrétiennot-Dinet MJ, Neveux J, et al. (1994). "Smallest eukaryotic organism". Nature. 370 (6487): 255. Bibcode:1994Natur.370..255C. doi:10.1038/370255a0.
  5. Derelle E, Ferraz C, Rombauts S, Rouzé P, Worden AZ, Robbens S, Partensky F, Degroeve S, Echeynié S, Cooke R, Saeys Y, Wuyts J, Jabbari K, Bowler C, Panaud O, Piégu B, Ball SG, Ral JP, Bouget FY, Piganeau G, De Baets B, Picard A, Delseny M, Demaille J, Van de Peer Y, Moreau H (August 2006). "Genome analysis of the smallest free-living eukaryote Ostreococcus tauri unveils many unique features". Proceedings of the National Academy of Sciences of the United States of America. 103 (31): 11647–52. Bibcode:2006PNAS..10311647D. doi:10.1073/pnas.0604795103. PMC 1544224. PMID 16868079.
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