Phylum: Charophyta Family: Desmidiaceae
Xanthidium variabile (Nordstedt) West & G.S.West 1900
An acidophile mainly from moorland pools and bogs, especially with Sphagnum. John, et al. (2011) note: "… widespread and often abundant in Sphagnum bogs throughout the British Isles although easily overlooked." It's proving to be rather scarce in the Outer Hebrides even though we have ideal habitat.
References:
John, D.M., Whitton, B.A. & Brook, A.J. (2011 second edition) The Freshwater Algal Flora of the British Isles
West, W. & West, G.S. (1912) A Monograph of the British Desmidiaceae, Volume 4
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands