Kinetics of NH 3 ‐oxidation, NO‐turnover, N 2 O‐production and electron flow during oxygen depletion in model bacterial and archaeal ammonia oxidisers

Summary Ammonia oxidising bacteria (AOB) are thought to emit more nitrous oxide (N 2 O) than ammonia oxidising archaea (AOA), due to their higher N 2 O yield under oxic conditions and denitrification in response to oxygen (O 2 ) limitation. We determined the kinetics of growth and turnover of nitric oxide (NO) and N 2 O at low cell densities of Nitrosomonas europaea (AOB) and Nitrosopumilus maritimus (AOA) during gradual depletion of TAN (NH 3 + ) and O 2 . Half‐saturation constants for O 2 and TAN were similar to those determined by others, except for the half‐saturation constant for ammonium in N. maritimus (0.2 mM), which is orders of magnitudes higher than previously reported. For both strains, cell‐specific rates of NO turnover and N 2 O production reached maxima near O 2 half‐saturation constant concentration (2–10 μM O 2 ) and decreased to zero in response to complete O 2 ‐depletion. Modelling of the electron flow in N. europaea demonstrated low electron flow to denitrification (≤1.2% of the total electron flow), even at sub‐micromolar O 2 concentrations. The results corroborate current understanding of the role of NO in the metabolism of AOA and suggest that denitrification is inconsequential for the energy metabolism of AOB, but possibly important as a route for dissipation of electrons at high ammonium concentration.