STUDIES ON THE OXIDATION OF AMMONIA BY NITROSOMONAS

Abstract

Free-energy calculations for pH7 showed that the oxidation of NH3 hydroxylamine is endergonic and that of hydroxylamine to nitrite and hydrazine to N are exergonic. The oxidation of NH3 requires the expenditure of energy. The anaerobic dehydrogenation of hydrazine to N by extracts of the autotrophic nitrifying Nitrosomonas, in the presence of methylene-blue as electron acceptor, was less rapid than the anaerobic dehydrogenation of hydroxylamine to NO. The inhibition by hydrazine of the dehydrogenation of hydroxylamine is due to substrate competition. Whole cells in air did not produce nitrite from hydrazine. They produced nitrite from low concentrations of hydroxylamine more rapidly than from equimolar concentrations of NH3; this result is consistent if hydroxylamine is an intermediate of the oxidation of NH3. The production of nitrite from hydroxylamine by whole cells was slightly inhibited by hydrazine, but the production of nitrite from NH3 was greatly inhibited and small amounts of hydroxylamine were formed. The dehydrogenation of hydroxylamine may supply energy required for the oxidation of NH3. Hydroxylamine appeared because the energy production was replaced by that of the dehydrogenation of hydrazine. The oxidation of hydroxylamine by whole cells was not inhibited by thiourea, but micromolar concentrations of the metal-binding agent markedly inhibited the oxidation of NH3 to hydroxylamine, suggesting that the oxidation of NH3 involved Cu. A possible mechanism for the activation of NH3 is suggested.