home Introduction to the Desmids
Desmids are unicellular micro-organisms belonging to the green algal families of Mesotaeniaceae and Desmidiaceae. They occur in standing freshwaters. Although among the microbes ranking as real giants, with the unaided eye even the largest representatives are hardly to be seen. So, for studying them a microscope is indispensable. Only under the microscope (magnification 40 to 400x) it appears how aesthetically appealing most of the species are. The bright-green cells show a remarkable symmetry. Actually, each cell consists of two half cells being mirror images of one another. In the connecting bridge between the half-cells the cellular nucleus is situated, but usually this is only to be seen by means of specific dyes.


Cell of Micrasterias rotata with its nucleus situated in the connecting bridge between the half-cells

 

 


Detail of the cell centre. Within the transparent nucleus a denser structured, globular nucleolus is located, in its turn including a globular nucleolinus

Both images © Jan van Arkel

The shape of the half-cells (semicells) is most various: ranging from more or less globular to disc- or spindle-like. Not seldom the semicell body itself is indented, or provided with processes.Cell wall may be smooth but often it shows a distinct pattern of granules, tubercles or spines. A rich cell wall ornamentation, whether or not in combination with deep semicellular incisions or long processes, use to render those algae an appealing, often elegant appearance - giving rise to the Dutch name of ‘sieralgen’ (literally ‘ornamental algae’).

Micrasterias crux-melitensis Micrasterias americana Micrasterias rotata
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Cosmarium quasillus Euastrum verrucosum
Remarkably, the appealing, aesthetical cell shape of desmids very well fits the environment in which they use to occur. Desmids are hardly or not to be expected in polluted waters which, in summer time, grow turbid by mass development of other unicellular algal groups. Reason for that absence is (among other things) that, under nutrient-rich conditions, desmids cannot grow as fast as other algae, so lose in competition. In contrast to that, desmids have to be looked for in clear waters, preferably in those with a luxurious, submerged vegetation of aquatics. Most promising are also small pits and moss-covered shallows in fens and bogs. In general it seems to hold that the more delicate and diversified the structure of (semi)aquatic plant vegetation, the richer and more interesting the desmid flora connected to it. So, desmidiologists are recommended to visit (aquatic) sites of natural beauty!
moorland pool
with Eriophorum
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example of
rich desmidflora

Due to the dependence on clear, relatively nutrient-poor waters, in large parts of the world desmids belong to the seriously threatened groups of organisms. As most of the species only occur at highly specific combinations of habitat variables, they may be considered excellent indicator organisms. As a consequense of their high environmental demands, they are very useful in monitoring conservation value in (semi)aquatic habitats.