Phylum: Charophyta   Family: Desmidiaceae

Cosmarium minutum G.B. Delponte 1877

These small cells are morphologically quite variable and have a considerable size-range, given their size. This is emphasised by Kouwets (2025) who provides 28 illustrations.
Cell dimensions: L. 17.3-23.5 µm; B. 12.8-18 µm; Is. 3.7-7.6 µm; Th. c.11-14 µm; L/B. 1.30-1.36.
Found in loch-margins, small moorland pools and abandoned peat-cuttings. These small cells would seem to be genuinely scarce in the Outer Hebrides but may have been misidentified in the past.

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.