Phylum: Charophyta   Family: Desmidiaceae

Actinotaenium phymatosporum (C.F.O. Nordstedt) F.A.C. Kouwets & P.F.M. Coesel 1984

Cells cylindric-ellipsoid and very slightly constricted. Apices truncately rounded. Cell-wall delicately striate. Chloroplast asteroid with one pyrenoid. Very similar to A. spinospermum but broader, usually greater than 13 µm. The zygospore is rectangular, thick-walled with protracted corners. The central section of the longest side is convex, whereas on the shorter side it is concave.
Cell dimensions: L: 27-45 µm; B: 13-18 µm; L/B: 2.00-2.50.
Zygospore L: 36-42 µm.
It occurs in oligotrophic, weakly acidic waters and also found in moss on wet rocks. Not uncommon in suitable habitat.

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