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'', considered to be the smallest (ca. 0.95 μm) 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'', 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 '' Ostreococcus'' and 21 Mbp for '' Micromonas''). At least one species, the Antarctic form '' Pyramimonas gelidicola'', 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 mixotrophic 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. 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'' 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. 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 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 (prasinophyte clade VI) in a new phylum outside of the Chlorophyta and Streptophyta, the Prasinodermophyta, 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