Production of nitrous oxide by Nitrosomonas europaea: effects of acetylene, pH, and oxygen

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Abstract
Aerobic cell suspensions of N. europaea oxidized ammonium (NH4+) to nitrous oxide (N2O) and nitrite (NO2-) and exogenous NO2- in the presence or absence of (NH4+) did not stimulate N2O formation. Acetylene (C2H2) inhibited the production of NO2- and N2O from NH4+ but not from hydroxylamine (NH2OH). The total amount of N2O formed in air was proportional to the amount of NH4+ oxidized; the total N2O N formed as a percentage of NO2- N formed varied very little (0.05-0.15%) over the range of NH4+ concentrations examined (0.05-20.4 mM). Rates of production of N2O and NO2- showed similar response to pH over the range of 5.4-9.5, with maxima at pH 8.5. Anaerobically, 5 times more N2O was formed than under aerobic conditions. The highest rates of anaerobic N2O formation were observed in the presence of NH4+ and NO2 combined (2 and 1 mM, respectively) and C2H2 reduced this rate of N2O formation to that observed with 1 mM NO2- alone in the presence or absence of C2H2. The presence of the NO2- oxidizer Nitrobacter winogradskyi had no effect on the formation of N2O by N. europaea in liquid culture or in sterile soil. The presence of sterile soil as a suspending matrix increased by 10-fold the production of N2O and broadened the range of O2 concentrations under which relatively high rates of N2O production occurred. Maximum N2O production by N. europaea occurred at 0.75 kPa [kilopascals] O2 in liquid suspension and at 2.5 kPa O2 in sterile soil.