Nitrous Oxide (N2O) Reduction by Denitrifying Bacteria: Relating Kinetics and Gene Expression

Nitrous oxide is a potent greenhouse gas that can be formed in wastewater treatment, and there is increasing interest in its fate in treatment processes. In this research, kinetic parameters and denitrification-gene expression levels were determined for Paracoccus pantotrophus, a common denitrifying bacterium. Batch tests were carried out with acetate as an electron donor and nitrate (NO3−), nitrite (NO2 −), or nitrous oxide (N2O) as electron acceptors. Tests also addressed concurrent growth on NO3− + N2O and NO2 − + N2O. The maximum specific growth rate μ for N2O was lower than that for NO3− or NO2 −. However, the maximum specific utilization rate q for N2O was 4.4 gN gCOD−1 d−1, compared to only 0.89 gN gCOD−1 d−1 for NO2 − reduction to N2O and 2.5 gN gCOD−1 d−1 for NO3− reduction to NO2 −. This suggests there is significant capacity for P. pantotrophus to reduce N2O, even during reduction of NO3− or NO2 −. When NO3− or NO2 − were grown in combination with N2O, the overall growth rates were higher than on N2O alone, but lower than on NO3− or NO2 − alone, suggesting concurrent reduction. The q for acetate also was higher for the combinations than for the individual acceptors. Gene expression results were consistent with the above, showing the highest levels of expression of the nitrous oxide reductase (N2OR) for growth on either N2O alone or N2O + NO3− or NO2 −. These results suggest that N2O not only can serve as a sole acceptor for P. pantotrophus, but that external N2O can be reduced concurrently with NO3− or NO2 −. Thus, denitrifying bacteria with kinetics similar to P. pantotrophus may serve as significant sinks for N2O formed in wastewater treatment processes.