Phylum: Charophyta   Family: Desmidiaceae

Cosmarium isthmium W. West 1890

There are other ‘dumbbell-shape’ desmids to compare it to, but these can usually be sorted by size, L/B ratio or the configuration of the granules.
Single cell dimensions: L: 48.4 µm, B: 27.9 µm, Is: 12.1 µm; L/B: 1.74.
The most likely habitat is oligo-mesotrophic lochs and lochans, also boggy pools. The single Outer Hebridean find was from the shallow, small, loch with aquatics. A very rare desmid with probably an Atlantic-subarctic distribution.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.
West, W. & West, G.S. (1908). A Monograph of the British Desmidiaceae, Volume 3.