Effect of Oxygen on the Fixation of Nitrogen by Azotobacter

Abstract

RECENT studies on the fixation of molecular nitrogen by both anaerobic¹ and aerobic² heterotrophic nitrogen-fixing bacteria indicate that ammonia is the first stable compound formed. This raises the question whether the necessary hydrogen is derived from water or from the metabolite. The latter source seems the more likely, especially since the union of metabolic hydrogen and molecular nitrogen to form ammonia could yield a considerable amount of free energy. If this were so, members of the anaerobic genus Clostridium would be expected to fix nitrogen more efficiently than the aerobe Azotobacter, which passes its metabolic hydrogen to oxygen. Results obtained in this laboratory show that Clostridium is, in fact, capable of more efficient nitrogen fixation than Azotobacter ³. Again, if metabolic hydrogen is involved in the reduction of molecular nitrogen then, with a nitrogen-fixing aerobe, oxygen must compete with nitrogen for this hydrogen and so depress fixation. Conflicting results have been reported for the effect of partial pressure of oxygen on nitrogen fixation by Azotobacter ⁴.