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 protein1 min1, respectively (37°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 protein1 min1). A 15N 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°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 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 protein1 min1, respectively (20°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 (NO2/NH4) and produced NO3 to consumed NH4 (NO3/NH4). These reactions are preferable to the application of anammox processes for wastewater treatment.
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