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

Heejoo Han,Sukhwan Yoon,Bongkeun Song,Min Joon Song

doi:10.3389/fmicb.2019.01203

Heejoo Han, Sukhwan Yoon + Show 2 more

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https://doi.org/10.3389/fmicb.2019.01203

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Abstract

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 (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). 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 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−/vessel. Monitoring of temporal changes in nirS1, nirS2, and nosZ transcription suggested that the nosZ/(nirS1+nirS2) ratios were indicative of whether N2O was produced or consumed at the time points where measurements were taken. The salt and alkaline stresses altered the N2O consumption kinetics of the resting D. aromatica cells with expressed nitrous oxide reductases. The N2O 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 N2O concentration was 0.1 mM. As the rates of N2O production from NO2− reduction was not significantly affected by the stresses (0.45–0.55 μmoles min−1 mg protein−1), the N2O consumption rate was lower than the N2O 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 N2O production and consumption was another cause of enhanced N2O emission observed under the salt and alkaline stress conditions. These findings suggest that canonical denitrifiers may become a significant N2O source when faced with abrupt environmental changes.

Highlights

Nitrous oxide (N2O), currently constituting 320 ppbv of the atmosphere, is a potent greenhouse gas with approximately 300 times higher global warming potential than carbon dioxide and the most influential ozone depletion agent (Ravishankara et al, 2009; Montzka et al, 2011)
The effects of salt and alkaline stresses on growth and N2O production of D. aromatica strain RCB were monitored in anaerobic batch cultures with 10 mM acetate and 5.0 mM NO3− provided as the sole electron donor and acceptor, respectively (Figure 1, Table 1, and Supplementary Figure S1)
At the NaCl concentration of 7.0 g L−1, a significant increase (p < 0.05) in N2O accumulation associated with severe growth inhibition was observed

Summary

Introduction

Nitrous oxide (N2O), currently constituting 320 ppbv of the atmosphere, is a potent greenhouse gas with approximately 300 times higher global warming potential than carbon dioxide and the most influential ozone depletion agent (Ravishankara et al, 2009; Montzka et al, 2011). The stepwise reduction of NO3− and NO2− to dinitrogen (N2) via NO and N2O, N2O may accumulate as a transient intermediate or the final product (Betlach and Tiedje, 1981; Philippot et al, 2011; Thomson et al, 2012; Lycus et al, 2017). Even in the canonical denitrifiers capable of reducing NO3−/NO2− to N2, trace amounts of N2O is often transiently detected during the course of denitrification, the magnitudes of accumulation vary even within closely related taxa (Betlach and Tiedje, 1981; Liu et al, 2013; Chang et al, 2018). E.g., acidic pH and low copper bioavailability, may amplify N2O production via several mechanisms of action including inhibition of Nos-type N2O reductase (NosZ) activity, reduced/delayed transcription of nosZ, and/or disrupted synthesis of functional NosZ (Bergaust et al, 2010; Liu et al, 2010; Felgate et al, 2012; Chang et al, 2018)

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