Phylum: Charophyta   Family: Desmidiaceae

Cosmarium minutum G.B. Delponte 1877

These small cells would seem to be genuinely scarce in the Outer Hebrides.
Cell dimensions: L. 18-30 µm; B. 13-25 µm; Is. 4-7 µm; Th. 11-14 µm; L/B. 1.1-1.4.
Found in loch-margins, small moorland pools and abandoned peat-cuttings.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Delponte, G.B. (1877). Specimen Desmidiacearum subalpinarum.
Kouwets, F.A.C. (in manuscript). European Flora of the Desmid Genus Cosmarium.
Van Westen, M. (2024). Sieralgen in Drenthe.