Acetylene reduction by Rhodospirillaceae from the Aswan High Dam Lake.

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 wereRhodocyclus gelatinosus andRhodomicrobium vannielii followed byRhodobacter capsulatus, Rhodopseudomonas viridis andRhodopseudomonas 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).