Summary: A novel metabolic pathway for anaerobic ammonium oxidation with nitrite as the electron acceptor has been elucidated using N-Iabelled nitrogen compounds. These experiments showed that ammonium was biologically oxidized with hydroxylamine as the most probable electron acceptor. The hydroxylamine itself is most likely derived from nitrite. Batch experiments in which ammonium was oxidized with hydroxylamine transiently accumulated hydrazine. The conversion of hydrazine to dinitrogen gas is postulated as the reaction generating electron equivalents for the reduction of nitrite to hydroxylamine. During the conversion of ammonium, a small amount of nitrate was formed from some of the nitrite. The addition of NHOH to an operating fluidized bed system caused a stoichiometric increase in the ammonium conversion rate (1 mmol I h) and a decrease in the nitrate production rate (0.5 mmol I h). Addition of hydrazine also caused a decrease in nitrate production. On the basis of these findings, it is postulated that the oxidation of nitrite to nitrate could provide the anaerobic ammonium-oxidizing bacteria with the reducing equivalents necessary for CO fixation.


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