Potential nitrogen-fixing freshwater photosynthetic bacteria of 32 isolates from the Aswan High Dam Lake were
investigated using the acetylene reduction technique to ascertain the magnitude of their contribution of fixed
nitrogen to the ecosystem. The superior nitrogen-fixing Rhodospirillaceae were Rhodocyclus gelatinosus and
Rhodomicrobium vannielii followed by Rhodobacter capsulatus, Rhodopseudomonas viridis and Rhodopseudomonas
palustris. High and moderate nitrogenase activities (534 to 1528 nmol ethylene produced/4 ml/h) were
found in 56% of the isolates, while the remainder showed low activities (76 to 477 nmol ethylene produced/4 ml/h).