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Desmid Algae - immortal cells, or dual aged?

Desmids are a type of algae. The Desmidiales. It reminds me of a song - but that aside, these present us, with our narrow restricted view of life forms and the way that things should be, with another interesting conundrum.

Desmids can be recognised by the fact that these single celled organisms are generally formed of two symmetrical halves, connected in the middle.

Each half is known as a semicell, and each semicell contains a very large chloroplast by which it photosynthesises. The two semicells are joined in the middle by a very narrow isthmus where the cell's nucleus is to be found. Because the chloroplasts in the semicells are so large, the general appearance of a desmid is bright green. They stand out quite prominently when seen under the microscope. Indeed, they colour the waters of ponds and puddles, water tanks, drinking trucks, and lakes.

Desmids photosynthesise and are a lovely bright green. They are also beautiful shapes with a high degree of symmetry. They used to be regarded as part of the plant kingdom. And yet some of them are capable of movement, by either the use of cilia, hair like projections that enable a sort of swimming motion; or by flagellae, one or two long whips by which they ‘whip’ their way through the water. Hardly the behaviour of a typical plant.

A montage of various desmids from Three Lakes in West Cork. Note the two parts to each of them. The eye shaped ones are the same as the triangles, just seen from a different angle. Spot the diatom that sneaked into the picture.

When the desmid wishes to reproduce, the two semicells split, each taking a small replicated nucleus with it. So one desmid formed of two semicells joined by a nucleus, becomes two separated single semicells each with a small nucleus. Once separated, the second semicell grows anew.

What this means is that each desmid is made up of two semicells - of different ages. So how do we determine the age of the single celled desmids? One half is older than the other, and yet a semicell is not a complete desmid. Or is it?

And if reproduction continues in this fashion, one semicell continues on down through the generations. Does that make it immortal? Well, longlived anyway, until it dies.

Brook, A. J. 1958. Desmids from the plankton of some Irish loughs. Proc.R.Irish Acad.,B.59, 71-91.

West, G. S., and Carter. N. 1923 British Desmidiaceae. V. London, Royal Society.

West, W., and West, G. S. 1905 British Desmidiaceae. II. London, Royal Society.

West, W., and West, G. S. 1906 A comparative study of the plankton of some Irish Lakes. Trans. Royal Irish Acad., 33, B,77.

West, W., and West, G. S. 1908 British Desmidiaceae. III. London, Royal Society.