Enhanced nitrous oxide production in denitrifying Dechloromonas aromatica strain RCB under salt and alkaline conditions

Salinity and pH are important environmental parameters with direct and indirect impacts on the viability and metabolic activities of microorganisms. In this study, the effects of salt and alkaline stresses on the kinetic balance between nitrous oxide (N2O) production and consumption in the denitrification pathway of Dechloromonas aromatica strain RCB were examined. N2O accumulated transiently only in insignificant amounts at low salinity (≤0.5% NaCl) and circumneutral pH (7.0 and 7.5). 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 N2O (15.8±2.8 μmoles N2O-N/vessel). Incubation at pH 8.0 severely inhibited growth and resulted in permanent accumulation of 29.9±1.3 μmoles N2O-N/vessel from reduction of 151±20 μmoles NO3-. The transcription analyses observed decreased nosZ/(nirS1+nirS2) ratios coinciding with N2O accumulation. The N2O consumption rates of resting D. aromatica cells subjected to the salt and alkaline stress conditions were significantly lower than the rates of N2O production from NO2- reduction at N2O / NO2- concentration of 0.1 mM, but not at a higher concentration (1.0 mM). These results indicate that alteration in N2O consumption kinetics was another cause of enhanced N2O production observed under the stress conditions. The findings in this study suggest that canonical denitrifiers may become a significant N2O source when faced with abrupt environmental changes.