Phylum: Charophyta   Family: Closteriaceae

Closterium macilentum L.A. de Brébisson 1856

First described and illustrated by Brébisson (1856: 153, pl. 2: 36). West & West (1904: 118) list a few sites, some after Roy, Bisset and Archer. Brook & Williamson Note: “This acidophilic species … although according to Förster (1982) it can also occur in the littoral of oligo-mesotrophic lakes (pH 5.2-7).” These same authors suggest this taxon may be of doubtful validity. Most authors also describe the cell-wall as smooth. Having now found the first specimen from the Outer Hebrides, I suggest it’s a very good species and has very fine striae (observed under high magnification), which do not reach the apices. It is also found in very slightly alkaline waters.
Cell dimensions: L: 264-720 µm; B: 10-20 µm; Ap: 4-8 µm; L/B: 24-40.
A very rare species, reported from acidic oligotrophic moorland bogs and pools, but also from circumneutral habitats.

References: 
Brébisson, L.A. de (1856) Liste des Desmidiées, observées en Basse-Normandie.
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.
West, W. & West, G.S. (1904) A Monograph of the British Desmidiaceae, Volume 1.