Bradyrhizobium ottawaense efficiently reduces nitrous oxide through high nosZ gene expression

N₂O is an important greenhouse gas influencing global warming, and agricultural land is the predominant (anthropogenic) source of N₂O emissions. Here, we report the high N₂O-reducing activity of Bradyrhizobium ottawaense, suggesting the potential for efficiently mitigating N₂O emission from agricultural lands. Among the 15 B. ottawaense isolates examined, the N₂O-reducing activities of most (13) strains were approximately five-fold higher than that of Bradyrhizobium diazoefficiens USDA110^T under anaerobic conditions. This robust N₂O-reducing activity of B. ottawaense was confirmed by N₂O reductase (NosZ) protein levels and by mitigation of N₂O emitted by nodule decomposition in laboratory system. While the NosZ of B. ottawaense and B. diazoefficiens showed high homology, nosZ gene expression in B. ottawaense was over 150-fold higher than that in B. diazoefficiens USDA110^T, suggesting the high N₂O-reducing activity of B. ottawaense is achieved by high nos expression. Furthermore, we examined the nos operon transcription start sites and found that, unlike B. diazoefficiens, B. ottawaense has two transcription start sites under N₂O-respiring conditions, which may contribute to the high nosZ expression. Our study indicates the potential of B. ottawaense for effective N₂O reduction and unique regulation of nos gene expression towards the high performance of N₂O mitigation in the soil.