Phylum: Charophyta   Family: Desmidiaceae

Cosmarium subcostatiforme F.A.C. Kouwets 2025

There are many similar crenate forms with a central granulate protuberance, but these central swellings are often disguised in live cells by the chloroplast. See Kouwets (2025) for detailed analysis and similar species.
Cell dimensions: L: 22-27 µm, B: 20.7-23 µm, Is: 7.5-8.3 µm; 12.8-14 µm, L/B: 1.10-18.
A reasonably common species found in the tychoplankton of circumneutral waters.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.