Phylum: Charophyta   Family: Closteriaceae

Closterium calosporum V.B. Wittrock 1869

A small cell with an arc in the range of 100-120° with an end-pore. It needs to be compared with the similar C. parvulum in the vegetative state, although the zygospores are different.
Cell dimensions: L: 92-125 µm; B: 7.7-11.4 µm; Ap: 2.3-2.5 µm; Arc: 102-122°; L/B: 9-14.
The British distribution is rather vague: West & West (1904: 138) suggest it's a scarce species. They give the Outer Hebrides as one of the locations, but it's rarely recorded. Most recent finds in the Outer Hebrides have been with mosses on wet rocks. This might suggest a different ecology from the much commoner variety brasiliense.

References: 
Brook, A.J. & Williamson, D.B. (2010). A Monograph on some British Desmids.
Van Westen, M. (2024). Sieralgen in Drenthe.
West, W. & West, G.S. (1904). A Monograph of the British Desmidiaceae, Volume 1.