Effect of nitrous oxide (NO) on the structure and function of nitrogen-oxide reducing microbial communities.

Nitrous oxide (NO) is a potent greenhouse gas that can be produced by nitrifying and denitrifying bacteria. Yet the effects of NO on microbial communities is not well understood. We used batch tests to explore the effects of NO on mixed denitrifying communities. Batch tests were carried out with acetate as the electron donor and with the following electron acceptors: nitrate (NO), nitrite (NO), NO, NO + NO, and NO + NO. Activated sludge from a municipal wastewater treatment plant was used as the inoculum. The bacteria grew readily with NO as the sole acceptor. When NO was provided along with NO or NO, it was used concurrently and resulted in higher growth rates than the same acceptors without added NO. The microbial communities resulting from NO addition were significantly different at the genus level from those with just NO or NO. Tests with NO 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 NO or NO, relative to all other tests. This suggests that specific NO reduction rates are high, and that NO alone selects for a low-diversity, fully denitrifying community. When NO is present with NO or NO, the microbial communities were more diverse and did not select exclusively for full denitrifiers. NO 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 NO appeared to increase the number of microbes carrying nirK, while fully denitrifying bacteria appear more likely to carry nirS. Lastly, all the taxa in NO and NO samples were predicted to harbor nosZ. This suggests the potential for reduced NO emissions in denitrifying systems.