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.
Image © Jan van Arkel
Detail of the cell centre. Within the transparent nucleus a denser structured, globular nucleus is located, in its turn including a globose, vacuole-like structure (nucleolinus?).
Image © Jan van Arkel
Cell of Micrasterias rotata with its nucleus situated in the connecting bridge between the half-cells.
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).
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!
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.