We examined nitrate-dependent Fe~(2+) oxidation mediated by anaerobic ammonium oxidation (anammox) bacteria. Enrichment cultures of “ Candidatus Brocadia sinica” anaerobically oxidized Fe~(2+) and reduced NO_(3)~(?) 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 NH_(4)~(+) mg protein~(?1) min~(?1)). A ~(15)N tracer experiment demonstrated that coupling of nitrate-dependent Fe~(2+) oxidation and the anammox reaction was responsible for producing nitrogen gas from NO_(3)~(?) by “ Ca . Brocadia sinica.” The activities of nitrate-dependent Fe~(2+) 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 NO_(3)~(?) ± SD of “ Ca . Brocadia sinica” was determined to be 51 ± 21 μM. Nitrate-dependent Fe~(2+) oxidation was further demonstrated by another anammox bacterium, “ Candidatus Scalindua sp.,” whose rates of Fe~(2+) oxidation and NO_(3)~(?) 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 Fe~(2+) oxidation and the anammox reaction decreased the molar ratios of consumed NO_(2)~(?) to consumed NH_(4)~(+) (ΔNO_(2)~(?)/ΔNH_(4)~(+)) and produced NO_(3)~(?) to consumed NH_(4)~(+) (ΔNO_(3)~(?)/ΔNH_(4)~(+)). These reactions are preferable to the application of anammox processes for wastewater treatment.
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