The anaerobic oxidation of ammonium

From recent research it has become clear that at least two different possib ilities for anaerobic ammonium oxidation exist in nature. 'Aerobic' ammoniu m oxidizers like Nitrosomonas eutropha were observed to reduce nitrite or n itrogen dioxide with hydroxylamine or ammonium as electron donor under anox ic conditions. The maximum rate for anaerobic ammonium oxidation was about 2 nmol NH4+ min(-1) (mg protein)(-1) using nitrogen dioxide as electron acc eptor. This reaction, which may involve NO as an intermediate, is thought t o generate energy sufficient for survival under anoxic conditions, but not for growth. A novel obligately anaerobic ammonium oxidation (Anammox) proce ss was recently discovered in a denitrifying pilot plant reactor. From this system, a highly enriched microbial community with one dominating peculiar autotrophic organism was obtained. With nitrite as electron acceptor a max imum specific oxidation rate of 55 nmol NH4+ min(-1) (mg protein)(-1) was d etermined. Although this reaction is 25-fold faster than in Nitrosomonas, i t allowed growth at a rate of only 0.003 h(-1) (doubling time 11 days). N-1 5 labeling studies showed that hydroxylamine and hydrazine were important i ntermediates in this new process. A novel type of hydroxylamine oxidoreduct ase containing an unusual P-468 cytochrome has been purified from the Anamm ox culture. Microsensor studies have shown that at the oxic/anoxic interfac e of many ecosystems nitrite and ammonia occur in the absence of oxygen. In addition, the number of reports on unaccounted high nitrogen losses in was tewater treatment is gradually increasing, indicating that anaerobic ammoni um oxidation may be more widespread than previously assumed. The recently d eveloped nitrification systems in which oxidation of nitrite to nitrate is prevented form an ideal partner for the Anammox process. The combination of these partial nitrification and Anammox processes remains a challenge for future application in the removal of ammonium from wastewater with high amm onium concentrations. (C) 1999 Federation of European Microbiological Socie ties. Published by Elsevier Science B.V. All rights reserved.