Effect of nitrous oxide (N2O) on the structure and function of nitrogen-oxide reducing microbial communities

Nitrous oxide (N₂O) is a potent greenhouse gas that can be produced by nitrifying and denitrifying bacteria. Yet the effects of N₂O on microbial communities is not well understood. We used batch tests to explore the effects of N₂O on mixed denitrifying communities. Batch tests were carried out with acetate as the electron donor and with the following electron acceptors: nitrate (NO₃^-), nitrite (NO₂^-), N₂O, NO₃^- + N₂O, and NO₂^- + N₂O. Activated sludge from a municipal wastewater treatment plant was used as the inoculum. The bacteria grew readily with N₂O as the sole acceptor. When N₂O was provided along with NO₃^- or NO₂^-, it was used concurrently and resulted in higher growth rates than the same acceptors without added N₂O. The microbial communities resulting from N₂O addition were significantly different at the genus level from those with just NO₃^- or NO₂^-. Tests with N₂O as the sole added acceptor revealed a reduced diversity. Analysis of inferred gene content using PICRUSt2 indicated a greater abundance of genera with a complete denitrification pathway when growing on N₂O or NO₂^-, relative to all other tests. This suggests that specific N₂O reduction rates are high, and that N₂O alone selects for a low-diversity, fully denitrifying community. When N₂O is present with NO₂^- or NO₃^-, the microbial communities were more diverse and did not select exclusively for full denitrifiers. N₂O alone appears to select for a "generalist" community with full denitrification pathways and lower diversity. In terms of denitrification genes, the combination of acceptors with N₂O appeared to increase the number of microbes carrying nirK, while fully denitrifying bacteria appear more likely to carry nirS. Lastly, all the taxa in NO₂^- and N₂O samples were predicted to harbor nosZ. This suggests the potential for reduced N₂O emissions in denitrifying systems.

Kinetics of nitrous oxide (N2O) formation and reduction by Paracoccus pantotrophus

Nitrous oxide (N₂O) is a powerful greenhouse gas emitted from wastewater treatment, as well as natural systems, as a result of biological nitrification and denitrification. While denitrifying bacteria can be a significant source of N₂O, they can also reduce N₂O to N₂. More information on the kinetics of N₂O formation and reduction by denitrifying bacteria is needed to predict and quantify their impact on N₂O emissions. In this study, kinetic parameters were determined for Paracoccus pantotrophus, a common denitrifying bacterium. Parameters included the maximum specific reduction rates, [Formula: see text], growth rates, [Formula: see text], and yields, Y, for reduction of NO₃^- (nitrate) to nitrite (NO₂^-), NO₂^- to N₂O, and N₂O to N₂, with acetate as the electron donor. The [Formula: see text] values were 2.9 gN gCOD^-1 d^-1 for NO₃^- to NO₂^-, 1.4 gN gCOD^-1 d^-1 for NO₂^- to N₂O, and 5.3 gN gCOD^-1 d^-1 for N₂O to N₂. The [Formula: see text] values were 2.7, 0.93, and 1.5 d^-1, respectively. When N₂O and NO₃^- were added concurrently, the apparent (extant) kinetics, [Formula: see text], assuming reduction to N₂, were 6.3 gCOD gCOD^-1 d^-1, compared to 5.4 gCOD gCOD^-1 d^-1 for NO₃^- as the sole added acceptor. The [Formula: see text] was 1.6 d^-1, compared to 2.5 d^-1 for NO₃^- alone. These results suggest that NO₃^- and N₂O were reduced concurrently. Based on this research, denitrifying bacteria like P. pantotrophus may serve as a significant sink for N₂O. With careful design and operation, treatment plants can use denitrifying bacteria to minimize N₂O emissions.