SUMMARY: Initiation of growth of nitrogen-fixing Azotobacter species was prevented by efficient aeration but proceeded normally with gentle aeration; addition of CO to the air did not relieve inhibition. The ratio of oxygen solution rate to concentration of organisms determined whether growth would be inhibited or not. Populations growing in media containing fixed nitrogen (NH ) showed no unusual sensitivity to oxygen though inhibition could be induced at a value of 0.6 atm. Nitrogen-limited continuous cultures fixed about twice as much N/g. carbon source utilized at 0.03 atm. O than at the atmospheric value (0.2 atm.); even at relatively high cell concentrations growth was inhibited at 0.6 atm. O. Carbon- and phosphate-limited continuous cultures showed even more sensitivity to oxygen when fixing nitrogen but none when growing with NH ; excessive oxygen was lethal to phosphate-limited populations. These observations suggest that two mechanisms exist in the cell to protect the oxygen-sensitive components of nitrogenase from oxygen: augmented respiration to scavenge excess oxygen and a conformational state of nitrogenase that prevents damage by O.


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