Phylum: Charophyta   Family: Desmidiaceae

Cosmarium tinctum  J. Ralfs 1848

A small, variable, species which has spawned several varieties in the past. Kouwets (2025) has provided 44 images to show this variability, and I refer you to his flora for more information.
Cell dimensions: L: 8.5-13.4 µm; B: 7.5-11.5 µm; Is: 5.1-6.6 µm; Th: c.5-7 µm; L/B: 1.09-1.26.
An acidophile of moorland pools and Sphagnum bogs, frequently recorded in the Outer Hebrides.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates