Ammonia oxidising bacteria (AOB) are thought to emit more nitrous oxide (N2O) than ammonia oxidising archaea (AOA), due to their higher N2O yield under oxic conditions and denitrification in response to oxygen (O2) limitation. We determined the kinetics of growth and turnover of nitric oxide (NO) and N2O at low cell densities of Nitrosomonas europaea (AOB) and Nitrosopumilus maritimus (AOA) during gradual depletion of TAN (NH3 + NH4+) and O2. Half-saturation constants for O2 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 N2O production reached maxima near O2 half-saturation constant concentration (2-10 µM O2) and decreased to zero in response to complete O2-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 O2 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. This article is protected by copyright. All rights reserved.