Evidence for the occurrence of an alternative nitrogenase system inAzospirillum brasilense

Azospirillum brasilense strain Cd, but not A. lipoferum, could grow and reduce acetylene to ethylene when grown on Mo-free Nfb medium with Fe. Purified Nfb medium lacking both Mo and Fe did not support growth of A. brasilense Cd. Supplementation of −Mo+Fe Nfb medium with vanadium or several other metal salts did not stimulate growth or acetylene reduction. In Mo-free Nfb semisolid medium containing Fe and W, the organism showed good pellicle growth and reduced acetylene to ethylene and ethane. Moreover, a significantly higher amount of H2 was produced in Mo-free medium than in Nfb containing Mo. The data suggest the presence of an alternate nitrogenase system 3 (Fe-nitrogenase) in A. brasilense Cd besides the normal Mo-nitrogenase.