The prasinophytes are a group of unicellular green algae. Prasinophytes mainly include marine

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...

ic species, as well as some freshwater representatives.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. 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. Well studied genera include '' Ostreococcus'', considered to be the smallest (ca. 0.95 μm) free-living eukaryote, and ''

Micromonas ''Micromonas'' is a genus of green algae in the family '' Mamiellaceae''. Before the characterization in 2016 of a second species, ''Micromonas commoda'', ''Micromonas pusilla'' was considered to be the only species in the genus, which led to a ...

'', both of which are found in marine waters worldwide. Prasinophytes have simple cellular structures, containing a single chloroplast and a single mitochondrion. The

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...

s are relatively small compared to other eukaryotes (about 12 Mbp for '' Ostreococcus'' and 21 Mbp for ''

''). At least one species, the Antarctic form '' Pyramimonas gelidicola'', is capable of

phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...

and is therefore a

mixotroph A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comp ...

ic algae. Some authors treat the prasinophytes as a

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...

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.

Ecology

A study of photosynthetic gene-sequence diversity (''rbcL'') in the

Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United ...

indicated that Prasinophytes are particularly prevalent at the Subsurface Chlorophyll Maximum (SCM) and several different ecotypes of ''Ostreococcus'' have been detected in the environment. 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 and observed year-round in the coastal North Pacific Ocean. RCC141 was considered low-light, because it was isolated from the lower

euphotic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...

. 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. ''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. Prasinophytes are subject to infection by large double-stranded DNA viruses belonging to the genus ''

Prasinovirus ''Prasinovirus'' is a genus of large double-stranded DNA viruses, in the family ''Phycodnaviridae'' that infect phytoplankton in the '' Prasinophyceae''. There are two species in this genus, including ''Micromonas pusilla virus SP1'', which inf ...

'' in the family ''

Phycodnaviridae ''Phycodnaviridae'' is a family of large (100–560 kb) double-stranded DNA viruses that infect marine or freshwater eukaryotic algae. Viruses within this family have a similar morphology, with an icosahedral capsid (polyhedron with 20 f ...

'', as well as a

Reovirus ''Reoviridae'' is a family of double-stranded RNA viruses. Member viruses have a wide host range, including vertebrates, invertebrates, plants, protists and fungi. They lack lipid envelopes and package their segmented genome within multi-layered ...

. It has been estimated that from 2 to 10% of the ''Micromonas pusilla'' population is lysed per day by viruses.

Phylogeny

Recent studies agree that the prasinophytes are not a natural group, being highly paraphyletic. Relationships among the groups making up the Chlorophyta are not fully resolved. The

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

produced by Leliaert et al. 2011 and some modification according to Silar 2016, Leliaert 2016 and Lopes dos Santos et al. 2017 is shown below. The blue shaded groups are or have traditionally been placed in the Prasinophyceae). The species '' Mesostigma viride'' has been shown to be a member of the

Streptophyta Streptophyta (), informally the streptophytes (, from the Greek ''strepto'' 'twisted', for the morphology of the sperm of some members), is a clade of plants. The composition of the clade varies considerably between authors, but the definition e ...

or basal Green algae. The others are member of the

Chlorophyta Chlorophyta or Prasinophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular author. In older classification systems, it refers to a ...

. As 2020 paper places the

Palmophyllophyceae The Palmophyllophyceae are a proposed basal Chlorophyte clade consisting of the Palmophyllales and Prasinococcales. References {{Taxonbar, from=Q26962824 Green algae classes ...

(prasinophyte clade VI) in a new phylum outside of the Chlorophyta and Streptophyta, the

Prasinodermophyta The Prasinodermophyta are a proposed basal Viridiplantae clade, as sister of another clade comprising the Chlorophyta and the Streptophyta. It consists of the Prasinodermophyceae and the Palmophyllophyceae. They were previously considered to be ...

References

External links

{{Taxonbar, from=Q134915 Green algae classes Historically recognized algae taxa