Phylum: Charophyta   Family: Desmidiaceae

Cosmarium joostenii F.A.C. Kouwets 2025

A small cell with semicells sub-semicircular, the sinuses closed for the most part, basal angle bluntly rounded, sides convex and converging, blending smoothly with the slightly convex apex. Chloroplast axile with one pyrenoid.
Cell dimensions (3): L: 13.9-15.6 µm, B: 18.1-19.2 µm, Is: 6.2-8.2 µm; Th: c.9.5 µm, L/B: 1.76-0.82.
An acidophile found in moorland pools and bogs. Its status in the Outer Hebrides is unclear, but as of 2025 it has rarely been recorded.

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