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 gasat 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 thehighest 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.