Phylum: Charophyta   Family: Desmidiacea

Cosmarium granatum var. messikommeri (H.T. Croasdale) P.F.M. Coesel & K.J. Meesters 2023

Van Westen (2024: 328) notes: “The main difference with other varieties of C. granatum is that the cell wall is concave just below the apex, which also gives the impression that the apex has been extended somewhat, although this is not equally visible in all cells. Croasdale found this taxon in a sample from the Canadian Arctic and referred to a publication by Messikommer who found a similar form in Switzerland. It seems therefore that this is an arctic alpine species.” As well as an Arctic-alpine distribution it is probably an Atlantic species in Britain. Interestingly, John & Williamson (2009) didn’t record it in Ireland. The size-range given below is greater than most authors, but the cells have the characteristics of the taxon in question. Some of the cells depicted below exhibit some distortion, which might suggest a stressed environment.
Cell dimensions: L: 33-45 µm; B: 22-30 µm; Is: 6.3-10.7 µm; Th: 17-18 µm; L/B: 1.39-1.51.
Found in mesotrophic, circumneutral waters and especially from ephemeral winter floods. Status in the Outer Hebrides unclear due to confusion with similar species, but at present it is infrequently recorded but locally common.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
John, D.M. & Williamson, D.B. (2009). A Practical Guide to the Desmids of the West of Ireland.
Kouwets, F.A.C. (in manuscript). European Flora of the Desmid Genus Cosmarium.
Van Westen, M.C. (2024). Sieralgen in Drenthe.