Phylum: Charophyta   Family: Desmidiaceae

Cosmarium citriforme F.A.C. Kouwets 2025

Van Westen (2024: 328) as Cosmarium granatum var. messikommeri 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. Even with the recent slitting of the C. granatum. complex, this species has a notable size-range.
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: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
Van Westen, M.C. (2024).. Sieralgen in Drenthe.