Phylum: Charophyta   Family: Desmidiaceae

Cosmarium hostensiense J. Štastný & F.A.C. Kouwets 2012

A quite larger taxon similar to C. extensum but a little smaller yet worth comparison. However, the latter species can support up to three pyrenoids, whereas C. hostensiense has just the one.
Cell dimensions: L: 59-66 µm; B: 32-36 µm; Is: 15-17 µm; Th: c.25-26 µm; L/B: 1.80-1.98.
This is quite common in the Outer Hebrides, from acidic oligotrophic waters and also from moss in wet rocks.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.