''Cosmarium'' is a large

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 ...

of desmids (

Desmidiaceae The Desmidiaceae are one of four family (biology), families of Charophyta, charophyte green algae in the order (biology), order Desmidiales (desmids).See the National Center for Biotechnology Information, NCBI]webpage on Desmidiaceae Data extrac ...

), a group of

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 ...

closely related to the land plants (

Embryophyta The embryophytes () are a clade of plants, also known as Embryophyta (Plantae ''sensu strictissimo'') () or land plants. They are the most familiar group of photoautotrophs that make up the vegetation on Earth's dry lands and wetlands. Embryophy ...

). Members of this genus are microscopic and found 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 around the world. The name ''Cosmarium'' comes from the Greek word κοσμάριον - ''kosmarion'', meaning a small ornament.

Description

''Cosmarium'', with few exceptions, consists of single cells and is thus microscopic. Cells are constricted in the middle, giving them a bilobed appearance;Guiry, M.D., John, D.M., Rindi, F. and McCarthy, Y.K. 2007. ''New Survey of Clare Island Volume 6: The Freshwater and Terrestrial Algae.'' Royal Irish Academy. the two halves are called semicells. Cells come in a variety of different shapes. The cell wall may be smooth or ornamented with spines, granules, scrobiculations (pits), or pores. Cells are generally flattened. Cells usually have one or two

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 ...

s filling the cell; each chloroplast contains

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 ...

s, typically one or two.

Identification

Species identification of ''Cosmarium'' depends on the shape of the cells and their ornamentation, and can be difficult. In addition, the genera ''

Actinotaenium ''Actinotaenium'' is a genus of green algae, specifically of the Desmidiaceae. It is a freshwater genus, typically found in the benthos of acidic, oligotrophic lakes or ''Sphagnum'' bogs. It has a cosmopolitan distribution. ''Actinotaenium'' cons ...

'' and ''

Euastrum ''Euastrum'' is a genus of green algae of the Desmidiaceae family.See the National Center for Biotechnology Information, NCBIbr>webpage on Euastrum Data extracted from the It lives in acidic waters. Description It is made of two, symmetrical ...

'' can be similar to various species of ''Cosmarium''. ''Actinotaenium'' is generally circular in cross section, and ''Euastrum'' usually (but not always) has a notch at the tip of each semicell.

Taxonomy

''Cosmarium'' is the largest genus of desmids, with over 1,000 species. It is an artificial genus: the defining characteristics of the genus are vague and arbitrary, and there are some species that are intermediate between several different genera. For this reason, it has been called a

wastebasket taxon Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used by some taxonomists to refer to a taxon that has the purpose of classifying organisms that do not fit anywhere else. They are typically defined by e ...

. It is also not

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

, as molecular phylogenetic studies have shown that the form of ''Cosmarium'' evolved independently many times over. Despite this, the genus is still in use, because there is no known way of classifying desmids in line with their evolutionary history.

Habitat

''Cosmarium'' and other desmid genera tend to prefer

oligotrophic An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments. Oligotrophs are characterized by slow growth, low rates o ...

, somewhat acidic water habitats, but some species exist in more eutrophic and/or basic habitats. Some species, which are small and mostly plain, form a community known as a "Cosmarietum"; these may be further associated with a community of ''

Closterium ''Closterium'' is a genus of desmid, a group of charophyte green algae.See the NCBIbr>webpage on Closterium Data extracted from the It is placed in the family Closteriaceae. Species of ''Closterium'' are a common component of freshwater microal ...

'' species forming a Cosmarietum-Closterietum association.

References

External links

* {{Taxonbar, from=Q309516 Desmidiaceae Charophyta genera