Phylum: Cyanobacteria   Family: Microcystaceae

Gloeocapsa atrata Kützing

Colonies variable in size depending on age and, maybe, habitat. Widespread on various substrates and not necessarily acidic

References: 
John, D.M., Whitton, B.A. & Brook, A.J. (2011) - The Freshwater Algal Flora of the British Isles