METABOLIC PATHWAY OF ANAEROBIC AMMONIUM OXIDATION ON THE BASIS OF N-15 STUDIES IN A FLUIDIZED-BED REACTOR

A novel metabolic pathway for anaerobic ammonium oxidation with nitrit e as the electron acceptor has been elucidated using N-15-labelled nit rogen compounds. These experiments showed that ammonium was biological ly oxidized with hydroxylamine as the most probable electron acceptor. The hydroxylamine itself is most likely derived from nitrite. Batch e xperiments in which ammonium was oxidized with hydroxylamine transient ly accumulated hydrazine. The conversion of hydrazine to dinitrogen ga s is postulated as the reaction generating electron equivalents for th e reduction of nitrite to hydroxylamine. During the conversion of ammo nium, a small amount of nitrate was formed from some of the nitrite. T he addition of NH,OH to an operating fluidized bed system caused a sto ichiometric increase in the ammonium conversion rate (1 mmol l(-1) h(- 1)) and a decrease in the nitrate production rate (0.5 mmol l(-1) h(-1 )). 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 b acteria with the reducing equivalents necessary for CO2 fixation.