Phylum: Charophyta   Family: Desmidiacea

Cosmarium magnum (M. Raciborski) P.F.M. Coesel & K.J. Meesters 2023

Until recently this was a variety of Cosmarium cucumis. It is partly distinguished in having several chloroplasts, which are not always easy to distinguish, and a number of pyrenoids, which are clearly noted in the second image below. The cell-wall is relatively thick with numerous mucus pores.
Cell dimensions: L: 79-95 µm; B: 46-60 µm ; Is: 26-32 µm; Th: 35-46 µm; L/B: 1.6-1.8.
Infrequently recorded in mesotrophic, slightly acidic habitats such as loch-margins, moorland pools and abandoned peat-cuttings.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Kouwets, F.A.C. (in manuscript). European Flora of the Desmid Genus Cosmarium.
Van Westen, M.C. (2024). Sieralgen in Drenthe.