Nitrate-Dependent Ferrous Iron Oxidation by Anaerobic Ammonium Oxidation (Anammox) Bacteria

We examined nitrate-dependent Fe2+ oxidation mediated by anaerobic ammonium oxidation (anammox) bacteria. Enrichment cultures of "Candidatus Brocadia sinica" anaerobically oxidized Fe2+ and reduced NO3- to nitrogen gas at rates of 3.7 +/- 0.2 and 1.3 +/- 0.1 (mean +/- standard deviation [SD]) nmol mg protein(-1) min(-1), respectively (37 degrees C and pH 7.3). This nitrate reduction rate is an order of magnitude lower than the anammox activity of "Ca. Brocadia sinica" (10 to 75 nmol NH4+ mg protein(-1) min(-1)). A N-15 tracer experiment demonstrated that coupling of nitrate-dependent Fe2+ oxidation and the anammox reaction was responsible for producing nitrogen gas from NO3- by "Ca. Brocadia sinica." The activities of nitrate-dependent Fe2+ oxidation were dependent on temperature and pH, and the highest activities were seen at temperatures of 30 to 45 degrees C and pHs ranging from 5.9 to 9.8. The mean half-saturation constant for NO3- +/- SD of "Ca. Brocadia sinica" was determined to be 51 +/- 21 mu M. Nitrate-dependent Fe2+ oxidation was further demonstrated by another anammox bacterium, " Candidatus Scalindua sp.," whose rates of Fe2+ oxidation and NO3- reduction were 4.7 +/- 0.59 and 1.45 +/- 0.05 nmol mg protein(-1) min(-1), respectively (20 degrees C and pH 7.3). Co-occurrence of nitrate-dependent Fe2+ oxidation and the anammox reaction decreased the molar ratios of consumed NO2- to consumed NH4+ (Delta NO2-/Delta NH4+) and produced NO3- to consumed NH4+ (Delta NO3-/Delta NH4+). These reactions are preferable to the application of anammox processes for wastewater treatment.