Enhanced Nitrous Oxide Production in Denitrifying Dechloromonas aromatica Strain RCB Under Salt or Alkaline Stress Conditions

Salinity and pH have direct and indirect impacts on the growth and metabolic activities of microorganisms. In this study, the effects of salt and alkaline stresses on the kinetic balance between nitrous oxide (N(2)O) production and consumption in the denitrification pathway of Dechloromonas aromatica strain RCB were examined. N(2)O accumulated transiently only in insignificant amounts at low salinity (≤0.5% NaCl) and circumneutral pH (7.0 and 7.5). As compared to these control conditions, incubation at 0.7% salinity resulted in substantially longer lag phase and slower growth rate, along with the increase in the amounts of transiently accumulated N(2)O (15.8 ± 2.8 μmoles N(2)O-N/vessel). Incubation at pH 8.0 severely inhibited growth and resulted in permanent accumulation of 29.9 ± 1.3 μmoles N(2)O-N/vessel from reduction of 151 ± 20 μmoles NO(3)(−)/vessel. Monitoring of temporal changes in nirS(1), nirS(2), and nosZ transcription suggested that the nosZ/(nirS(1)+nirS(2)) ratios were indicative of whether N(2)O was produced or consumed at the time points where measurements were taken. The salt and alkaline stresses altered the N(2)O consumption kinetics of the resting D. aromatica cells with expressed nitrous oxide reductases. The N(2)O consumption rates of the cells subjected to the salt and alkaline stress conditions were significantly reduced from 0.84 ± 0.007 μmoles min(−1) mg protein(−1) of the control to 0.27 ± 0.02 μmoles min(−1) mg protein(−1) and 0.31 ± 0.03 μmoles min(−1) mg protein(−1), respectively, when the initial dissolved N(2)O concentration was 0.1 mM. As the rates of N(2)O production from NO(2)(−) reduction was not significantly affected by the stresses (0.45–0.55 μmoles min(−1) mg protein(−1)), the N(2)O consumption rate was lower than the N(2)O production rate at the stress conditions, but not at the control condition. These results clearly indicate that the altered kinetics of expressed nitrous oxide reductase and the resultant disruption of kinetic balance between N(2)O production and consumption was another cause of enhanced N(2)O emission observed under the salt and alkaline stress conditions. These findings suggest that canonical denitrifiers may become a significant N(2)O source when faced with abrupt environmental changes.