Phylum: Charophyta   Family: Desmidiaceae

Xanthidium tenuissimum (W. Archer) W.B. Turner 1893

This tiny species is the smallest in the genus. It has a wide-open sinus, the upper lateral angles support a small spine, the apical angles support paired spines giving the apex an undulate appearance. There are also subapical spines best seen in apical view.
Single cell dimensions: L: 8.7 µm; B: 7.9 µm; Is: 5.2 µm; Sp: 2.1-2.6 µm; Th: 5.1 µm; overall LxB: 8.7 x 11.8.
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: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Van Westen, M. (2024). Sieralgen in Drenthe.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.