''Asterionella'' is a genus of pennate

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 include ...

diatom A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...

s. They are frequently found in star-shaped colonies of individuals.

Description

''Asterionella'' average cell size is 60–85 micrometers long and 2–4 micrometers wide. It forms colonies that often consist of eight cells, but can vary up to 20 cells. The cells in the colony are attached by the apex by extracellular matter. By the way the cells are attached to each other, the colonies often look like stars or spiralling chains.

Reproduction

The silica skeleton of a diatom cell (called the frustule) consists of an

epitheca ''Epitheca'' is a genus of dragonflies in the family Corduliidae The Corduliidae, also knowns as the emeralds, emerald dragonflies or green-eyed skimmers, is a family of dragonflies. These dragonflies are usually black or dark brown with area ...

and an

hypotheca Hypothec (; german: Hypothek, french: hypothèque, pl, hipoteka, from Lat. ''hypotheca'', from Gk. : hypothēkē), sometimes tacit hypothec, is a term used in civil law systems (e.g. law of entire Continental Europe except Gibraltar) or mixed ...

. The hypotheca is slightly smaller than the epitheca. During asexual reproduction both thecae form the epitheca of the new daughter cell and each daughter produces a new hypotheca. Therefore one daughter cell is always smaller than the original cell. In order to regain their original size, it is usually assumed diatoms have to reproduce sexually, although this has not yet been observed in A. formosa. However, there may also be other ways to rejuvenate.

Ecology

''Asterionella formosa'' is known to be susceptible to the

chytrid Chytridiomycota are a division of zoosporic organisms in the kingdom Fungi, informally known as chytrids. The name is derived from the Ancient Greek ('), meaning "little pot", describing the structure containing unreleased zoöspores. Chytrid ...

fungus ''Zygorhizidium planktonicum''. If a chytrid attaches to an ''Asterionella'' cell, they can, however, protect others of their kind by committing apoptosis, or cellular suicide, stopping the spread of the parasite. This is called a hypersensitive reaction.''The Existence of a Hypersensitive Reaction in the Planktonic Diatom Asterionella Formosa'', Cantor et al. 1978, New Phytologist. 82p187-206. ''Asterionella'' have no means of locomotion. It is most likely a combination of gravity and currents that distribute the organism.

Some species

'' A. ralfsii''
'' A. bleakeleyi''
'' A. formosa''
'' A. gracillima''
'' A. glacialis''
''

A. japonica A is the first letter of the Latin and English alphabet. A may also refer to: Science and technology Quantities and units * ''a'', a measure for the attraction between particles in the Van der Waals equation * A value, ''A'' value, a mea ...

''
'' A. kariana''

References

External links