Regulation of the Emissions of the Greenhouse Gas Nitrous Oxide by the Soybean Endosymbiont Bradyrhizobium diazoefficiens .

The greenhouse gas nitrous oxide (N 2 O) has strong potential to drive climate change. Soils are a major source of N 2 O, with microbial nitrification and denitrification being the primary processes involved in such emissions. The soybean endosymbiont Bradyrhizobium diazoefficiens is a model microorganism to study denitrification, a process that depends on a set of reductases, encoded by the napEDABC , nirK , norCBQD , and nosRZDYFLX genes, which sequentially reduce nitrate (NO 3 - ) to nitrite (NO 2 - ), nitric oxide (NO), N 2 O, and dinitrogen (N 2 ). In this bacterium, the regulatory network and environmental cues governing the expression of denitrification genes rely on the FixK 2 and NnrR transcriptional regulators. To understand the role of FixK 2 and NnrR proteins in N 2 O turnover, we monitored real-time kinetics of NO 3 - , NO 2 - , NO, N 2 O, N 2 , and oxygen (O 2 ) in a fixK 2 and nnrR mutant using a robotized incubation system. We confirmed that FixK 2 and NnrR are regulatory determinants essential for NO 3 - respiration and N 2 O reduction. Furthermore, we demonstrated that N 2 O reduction by B. diazoefficiens is independent of canonical inducers of denitrification, such as the nitrogen oxide NO 3 - , and it is negatively affected by acidic and alkaline conditions. These findings advance the understanding of how specific environmental conditions and two single regulators modulate N 2 O turnover in B. diazoefficiens .