Phylum: Charophyta   Family: Closteriaceae

Closterium acerosum  J. Ralfs 1848

A long, slender cell with little arc. The apices are usually a little recurved. It needs to be carefully compared with Closterium pseudolunula Borge 1909.
Cell dimensions (3): L: 439-704 µm; B: 4-64 µm; Ap: 7-13 µm; L/B: 10-12.
Tolerant of varying acidity, but more likely found in slightly alkaline waters. Rarely found in the Outer Hebrides.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Van Westen, M.C. (2024). Sieralgen in Drenthe.