Phylum: Charophyta   Family: Desmidiaceae

Xanthidium tenuissimum 

First described and illustrated by Archer (1864: 175, pl. 6: 50-55) as Arthrodesmus tenuissimus. It was later moved to its current genus by Turner (1893: 137). This tiny species is the smallest in the genus.
Cell dimensions: L: 7.6-9.3 µm; B: 7.9-11.5 µm; Is: 5-6 µm; Sp: 1.8-2.6 µm; overall LxB: 8-10 x 11.8-13.6.
West & West (1912: 117) note: “It is very rare, but when it does occur it is often in quantity and generally amongst submerged Sphagnum.” This is precisely the habitat where it has been found in the Outer Hebrides. It would seem to have a predominantly Arctic-alpine distribution.

References: 
Archer, W. (1864). Description of two new species of Cosmarium (Corda) of Penium (Bréb.) and of Arthrodesmus (Ehr.).
Turner, W.B. (1893). Algae aquae dulcis Indiae orientalis. The freshwater algae (principally Desmidieae) of East India.
Van Westen, M. (2024). Sieralgen in Drenthe.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.