Phylum: Charophyta   Family: Desmidiaceae

Actinotaenium minutissimum (C.F.O. Nordstedt) E. Teiling 1954

Nordstedt (1873: 46, pl. 1: 21) first described the species as Penium minutissimum, later moved by Teiling (1954: 408) to Actinotaenium. Cells almost cylindrical, slightly tumid with a hint of a sinus. This species is very similar to A. mooreanum but the cells of this latter species are slightly larger and taper more rapidly towards the apices. Most reports give A. mooreanum as tumid (close to barrel-shaped); a useful distinction. However, A. minutissimum can exhibit tumidity to a lesser extent. Both species can exhibit one or two pyrenoids per cell.
Teiling (l.c.), gives size-ranges for these species: A. minutissimum: 12-19 x 9-11. A. mooreanum: 17-26 x 11-15. Note there is little overlap in length and none in breadth. Ružicka (1981), gives a greater range: A. minutissimum: (10-)12-18(-21) x (7-)8-13. L/B: 1.30-2-00 (-2.5?). A. mooreanum: 17-28(-30) x 9.5-15. L/B: 1.30-2.10.
A species of slightly acidic conditions in moorland pools and disused peat-cuttings. It’s reported as rare in Europe and would seem to have a north-west bias. The favoured habitat is common in the Outer Hebrides but is proving rare. However, it is easily overlooked and can be confused with Actinotaenium mooreanum.

References: 
Nordstedt, C.F.O. (1873) Bidrag till kännedomen om sydligare Norges Desmidiéer.
Ruzicka, J. (1981) Die Desmidiaceen Mitteleuropas, Volume 1, Part 2.
Teiling, E. (1954) Actinotaenium, genus Desmidiacearum resuscitatum.