Phylum: Charophyta   Family: Desmidiaceae

Cosmarium sp. (17) 

Cells small, about as long as broad; sinus open, with a rounded apex and widening outwards. Semicells broadly rounded trapeziform, basal angles rounded and merging with the lower half of the sides, which are straight to slightly convex, lateral angles broadly rounded, upper half of the sides concave, apical angles rounded, apex quite broad and retuse. Cell wall smooth and with a yellowish colour (often affecting just one semicell), cell wall noticeably thickened at the lateral and apical angles. Apical view elliptic with a hint of tumidity, lateral view subcircular with two pairs of cell-wall thickening visible. Chloroplast axile with a single pyrenoid.
Cell dimensions: L: 9.9-11.4 µm; B: 9.8-11.4 µm; Is: 5.4-6.1 µm; Th: 6.7-6.9 µm; L/B: 0.92-1.07.
The collection was made by Paul Smith (VC110 botanical recorder) from a small unnamed lochan. His comments: “Squeeze of aquatic Sphagnum, moss (non-Sphagnum), dead bracken from in water, and decaying Potamogeton polygonifolius.” Circum-neutral water (pH 6.9).
This is superficially similar to Cosmarium tinctum var. subretusum Messikommer (1942: 145, pl. 5: 5) but that taxon doesn’t have wall-thickening and enjoys an acidic habitat. Another cell that is superficially similar is C. succisum var. alaskanum Croasdale (1956: 55, pl. 10:10). This has only slightly larger proportions, has lateral and apical angle wall-thickening and a pH of 7.0-8.0. However, it has rather more angularity at the angles than the cell under discussion. This led me to a proposed new species: C. pseudosuccisum Kouwets (in preparation). No images have been seen but the description is remarkably similar to the taxon in question. It is, however, slightly smaller.

References: 
Croasdale, H.T. (1956). Freshwater algae of Alaska I. Some desmids from the Interior. Part 2: Actinotaenium, Micrasterias and Cosmarium.
Kouwets, F.A.C. (in manuscript). European Flora of the Desmid Genus Cosmarium.
Messikommer, E. (1942). Beitrag zur Kenntnis der Algenflora und Algenvegetation des Hochgebirges um Davos.