The prasinophytes are a group of unicellular

green algae The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ...

. Prasinophytes mainly include marine

plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

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

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and the ...

s 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. Well studied genera include ''

Ostreococcus ''Ostreococcus'' is a genus of unicellular coccoid or spherically shaped green algae belonging to the class Mamiellophyceae. It includes prominent members of the global picoplankton community, which plays a central role in the oceanic carbon cyc ...

'', considered to be the smallest (ca. 0.95

μm The micrometre (Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a unit of length in the International System ...

) free-living

eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

, and ''

Micromonas ''Micromonas'' is a genus of green algae in the family '' Mamiellaceae''. ''Micromonas'' is a widespread prasinophyte alga that is very small in size, motile, and phototactic. Before characterization and naming of a second species, ''Micromonas ...

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

chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

and a single

mitochondrion A mitochondrion () is an organelle found in the cell (biology), cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosine tri ...

. The

genome 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 genes, other functional regions of the genome such as ...

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

'' and 21 Mbp for ''

''). At least one species, the Antarctic form ''

Pyramimonas gelidicola ''Pyramimonas'' is a genus of green algae in the order Pyramimonadales. Phototropic euglenids inherited their plastids from a close relative of ''Pyramimonas'' which was an endosymbiont An endosymbiont or endobiont is an organism that lives ...

'', is capable of

phagocytosis Phagocytosis () is the process by which a cell (biology), 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 ph ...

and is therefore a

mixotroph A mixotroph is an organism that uses a mix of different sources of energy and carbon, instead of having a single trophic mode, on the continuum from complete autotrophy to complete heterotrophy. It is estimated that mixotrophs comprise more than ...

ic algae. Some authors treat the prasinophytes as a

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

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.

Morphology

Ecology

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

Gulf of Mexico The Gulf of Mexico () is an oceanic basin and a marginal sea of the Atlantic Ocean, mostly surrounded by the North American continent. It is bounded on the northeast, north, and northwest by the Gulf Coast of the United States; on the southw ...

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 (or 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 ...

. 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''. The genus contains two species, including Micromonas pusilla virus SP1, which infects the c ...

'' 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 ''Sedoreoviridae'' (''sedo'' = smooth) is a family of the ''Reovirales'' order of viruses. Viruses in this family are distinguished by the absence of a turreted protein on the inner capsid to produce a smooth surface. Taxonomy The family contain ...

. 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 Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

. Relationships among the groups making up the Chlorophyta are not fully resolved. The

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

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 ''Mesostigma'' is a genus of unicellular biflagellate freshwater green algae, with a single species ''Mesostigma viride'', covered by an outer layer of basket‐like scales instead of a cell wall. It is the only known genus in the class Mesostig ...

viride'' has been shown to be a member of a basal clade inside

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 ...

. The others are members of the

Chlorophyta Chlorophyta is a division of green algae informally called chlorophytes. Description Chlorophytes are eukaryotic organisms composed of cells with a variety of coverings or walls, and usually a single green chloroplast in each cell. They are ...

. A 2020 study placed the

Palmophyllophyceae The Palmophyllophyceae are a proposed basal green plant clade consisting of the Palmophyllales and Prasinococcales. It belongs to Prasinodermophyta, a group that – according to Li et al.'s phylogenetic analyses – is basal to the rest of t ...

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

Prasinodermophyta The marine Prasinodermophyta are a proposed basal Viridiplantae clade, as sister of another clade comprising the Chlorophyta and the Streptophyta Streptophyta (), informally the streptophytes (, from the Greek ''strepto'' 'twisted', for the mo ...

, as shown below. However, a subsequent genomic study recovered Prasinodermophyta as sister to the revised Chlorophyta and questioned the need for the third division or phylum.

References

External links

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

Morphology

Ecology

Phylogeny

See also

References

External links