Phylum: Charophyta   Family: Closteriaceae

Closterium acerosum  J. Ralfs 1848

First described by Ralfs (1848: 164, pl. 27: 2 a-b, d-e) and illustrated by Jenner. A long, slender cell with little arc. Ruzicka (1977: 157) gives rather small dimensions for the apices of 5-7 µm (replicated by Brook & Williamson (2010: 255)).
Cell dimensions: L: 300-730 µm; B: 40-67 µm; Ap: 13-15 µm; L/B: 10-12.
Tolerant of varying acidity, but more likely found in slightly alkaline waters. Rarely 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. (2023 second edition). Desmids of the Lowlands.
Ralfs, J. (1848). The British Desmidieae.
Ruzicka, J. (1977). Die Desmidiaceen Mitteleuropas, Volume 1, Part 1.