Phylum: Charophyta   Family: Desmidiaceae

Cosmarium extensum (C.F.O. Nordstedt) F.A.C. Kouwets 2025

One of the larger taxa in the C. pseudopyramidatum complex. It should be compared with C. hostensiense although that species is slightly smaller and has a single pyrenoid per semicell whereas C. extensum can have 1, 2 or 3. The cell depicted below has two quite large pyrenoids per semicell.
Kouwets’ cell dimensions: L: 65-74 µm; B: 37-40 µm; Is: 13-17.5 µm; Th: 24-28 µm; L/B: 1.75-2.00.
My cell dimensions (2): L: 72-78 µm; B: 40-41 µm; Is: 16.5-18.5 µm; L/B: 1.80-1.91.
Due to earlier confusion with similar taxa, the distribution is unknown. Cells found to date have come from acidic oligotrophic waters.

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