''Acanthosphaera'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

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 ...

, in the family

Chlorellaceae Chlorellaceae are a family of green algae in the order Chlorellales. About 250 species are currently accepted in the family. Members of the family are distributed worldwide and are common in a variety of freshwater, terrestrial and marine enviro ...

. It contains the sole species ''Acanthosphaera zachariasii'', although some sources list a second species, ''Acanthosphaera tenuissima''. It is widely distributed, in

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

habitats and exists as

phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...

. ''Acanthosphaera'' consists of solitary, free-living

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 cells. The cell is spherical, about 8–15 μm in diameter and uniformly covered in 20–40 spines. The spines are distinctly thickened at the base, and very narrow at the tips. The spines are organized in four whorls of 4, 8, 8, and 4 respectively. Cells contain a single

nucleus Nucleus (: nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucleu ...

and one parietal, cup-shaped

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 ...

, which has a single, prominent

pyrenoid Pyrenoids are sub-cellular phase-separated micro-compartments found in chloroplasts of many algae,Giordano, M., Beardall, J., & Raven, J. A. (2005). CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution. ''An ...

with a

starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diet ...

sheath. Asexual reproduction occurs by the formation of two, four or eight

zoospore A zoospore is a motile asexual spore that uses a flagellum for locomotion in aqueous or moist environments. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Certain zoospores are ...

s, released via the rupture of the parental

cell wall A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...

. Zoospores as having two flagella, are ovoid and often have two

stigmata Stigmata (, plural of , 'mark, spot, brand'), in Roman Catholicism, Catholicism, are bodily wounds, scars and pain which appear in locations corresponding to the Crucifixion of Jesus, crucifixion Five Holy Wounds, wounds of Jesus in Christian ...

and four

contractile vacuole A contractile vacuole (CV) is a sub-cellular structure (organelle) involved in osmoregulation. It is found predominantly in protists, including unicellular algae. It was previously known as pulsatile or pulsating vacuole. Overview The contrac ...

s. The original author Korshikov described the presence of 4-flagellate zoospores, but this has not been confirmed in subsequent reports. Sexual reproduction has not been observed in this genus.

References

Trebouxiophyceae genera Chlorellaceae Freshwater algae {{Green algae-stub