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.