Fig. 3.

NH₂OH oxidation by recombinant Nitrososphaera viennensis NirK. The recombinant protein was incubated at 30°C and pH 7.5 in 1.8-mL vials (volume of the headspace: 1.5 mL), with i) 0.5 mM ¹⁵NH₂OH, ii) ¹⁵NH₂OH and ¹⁴NO₂⁻ (each 0.5 mM), or iii) ¹⁴NH₂OH and ¹⁵NO₂⁻. The production of N₂, NO, and N₂O in the headspace was examined by gas chromatography mass spectrometry (GC/MS). NH₃ and NO₂⁻ concentrations were also measured; however, they were not detectable during the incubation. During a 2-h incubation, i) 63±35 (mean±SD), ii) 149±1, and iii) 120±1 nmol N of NH₂OH were consumed in the liquid phase, resulting in 75–137% of the ¹⁵N-labeled nitrogen mass balance in the vials. Error bars represent the SD derived from triplicate incubations, and the graph bars represent the mean values. NH₂OH oxidation was not found in the vials without the addition of the recombinant protein.