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°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 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 μM. 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°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+ (ΔNO2−/ΔNH4+) and produced NO3− to consumed NH4+ (ΔNO3−/ΔNH4+). These reactions are preferable to the application of anammox processes for wastewater treatment.