Phylum: Charophyta   Family: Closteriaceae

Closterium acerosum  Ehrenberg ex Ralfs 1848

Tolerant of varying acidity, but more likely found in slightly alkaline waters. Infrequently found in the Outer Hebrides.

References: 
Brook, A.J. & Williamson, D.B. (2010) A Monograph on some British Desmids
Coesel, P.F.M. & Meesters, K.J. (2007) Mesotaeniaceae and Desmidiaceae of the European Lowlands
John, D.M., Whitton, B.A. & Brook, A.J. (2011) The Freshwater Algal Flora of the British Isles