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 for N₂O was lower than that for NO₃^-or NO₂^-. However, the maximum specific utilization rate for N₂O was 4.4 gN gCOD^-1d^-1, compared to only 0.89 gN gCOD^-1d^-1for NO₂^-reduction to N₂O and 2.5 gN gCOD^-1d^-1for 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 NO₂were 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 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.