British distribution (genus): Widespread.
A genus of saccoderm desmids, the cells each a single unit rather than being composed of two semicells. In the classification followed here, the saccoderm "desmids" are included in the order Zygnematales rather than in the order Desmidiales (placoderm desmids).
The chloroplast is single, simple and bar-like, as can be seen in the photograph, where some cells are visible in side view. The number of pyrenoids is variable, and depends on the species. The photographed material has one to three pyrenoids, though some of these cells may be approaching cell-division. In nature, the cells are often set in mucilage.
Mesotaenium species especially form algal slimes on wet rocks and other damp, terrestrial substrates, though they can also occur in freshwater. One species, M. bergrenii is noted as a cause of purple-brown colours in permanent snowfields.
The photographs are of material in culture, 1998, species unnamed. Generally, Mesotaenium species tend to be narrowly ovoid or shortly cylidrical and the longer cells shown in the photograph are presumably approaching division, which would explain apparent disjunctions in the chloroplasts. Where the chloroplast can be seen in side view in the shorter cells, it is evidently single, which would confirm that this long-held teaching culture is indeed Mesotaenium. The related genus, Netrium, has two or more chloroplasts.