''Tetrastrum'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ( Embryophytes) have emerged deep in the Charophyte alg ...

(Chlorophyta). It is a common component of the

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

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does in ...

habitats, particularly eutrophic and alkaline waters. Although traditionally classified in the family

Scenedesmaceae Scenedesmaceae is a family of green algae in the order Sphaeropleales. ''Scenedesmus'' algae are commonly found in freshwater plankton. The former family Coelastraceae is considered a synonym of Scenedesmaceae. Genera Genera in this family ...

in the class Chlorophyceae, it is phylogenetically more closely related to the genus '' Oocystis'' of the family

Oocystaceae Oocystaceae is a family of green algae, in the order Chlorellales. The type genus is '' Oocystis''. List of genera , AlgaeBase accepted the following genera: *'' Amphikrikos'' Korshikov – 6 species *'' Catenocystis'' F.Hindák – 2 species * ...

(

Trebouxiophyceae The Trebouxiophyceae are a class of green algae, in the division Chlorophyta. Their circumscription within the green algae is not well established due to the need for more genetic studies at higher levels within the group. Genera without inter ...

History

The genus ''Tetrastrum'' has a tangled taxonomic history. The name was first coined in 1895 by Robert Chodat for the species '' Tetrastrum heteracanthum'', which was previously placed in the now-obsolete genus ''Staurogenia''. Later, the genus ''Cohniella'' was created by Ludwig Julius Bruno Schröder, containing the species ''Cohniella staurogeniiforme''. Later, Ernst Lemmermann merged the genus ''Cohniella'' into ''Tetrastrum''.

Description

''Tetrastrum'' consists of four-celled colonies, which are sometimes aggregated together to form compound colonies. A layer of

mucilage Mucilage is a thick, gluey substance produced by nearly all plants and some microorganisms. These microorganisms include protists which use it for their locomotion. The direction of their movement is always opposite to that of the secretion of ...

surrounding the colony is sometimes present. Cells are tightly joined in a flat plane, with or without a small space in the middle. Cells are ovoid, triangular, or trapezoidal, 2–11.5 μm long. Some species may have spines on the cells; these spines are up to 46 μm and show various morphologies, such as one long spine and one short, or multiple short spines. Cells contain one

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 Nucl ...

and one to four

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it ...

s, with or without

pyrenoid Pyrenoids are sub-cellular 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. Annu. Rev. Plant Bio ...

s. The cell wall appears smooth in light microscopy, but is covered with small granules which are visible in scanning electron microscopy. ''Tetrastrum'' reproduces asexually through the formation of autospores. Four autospores are formed, organized into the shape of the colony; these are released through a tear in the mother cell wall.

Identification

''Tetrastrum'' is similar to, and has been taxonomically confused with, other genera such as ''

Crucigenia ''Crucigenia'' is a genus of green algae in the class Trebouxiophyceae The Trebouxiophyceae are a class of green algae, in the division Chlorophyta. Their circumscription within the green algae is not well established due to the need for more ...

''. The main morphological difference between the two is the mode of

reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – " offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual o ...

. In ''Tetrastrum'', the daughter colonies are produced with the cells in the same orientation as the mother cells. In ''Crucigenia'', the daughter colonies are produced with cells rotated 45° relative to the mother cells' orientation. Additionally, ''Tetrastrum'' may produce spines on its cells, while ''Crucigenia'' never does. Some species once classified in the genus ''Tetrastrum'' have been reclassified into the genus '' Lemmermannia''. ''Lemmermannia'' has square colonies that lack spines, and can sometimes form compound colonies; in contrast, ''Tetrastrum'' has colonies with spines, or in the case of the spineless '' Tetrastrum glabrum'', has oval and not square colonies). Species of ''Tetrastrum'' are identified based on the size and shape of the cells, placement and length of the spines, and presence or absence of pyrenoids. Considerable variation exists within species, blurring species boundaries.

References

* * {{Taxonbar, from=Q7706754 Sphaeropleales genera Sphaeropleales Freshwater algae