Bradyrhizobium ottawaense efficiently reduces nitrous oxide through high nosZ gene expression.

NO is an important greenhouse gas influencing global warming, and agricultural land is the predominant (anthropogenic) source of NO emissions. Here, we report the high NO-reducing activity of Bradyrhizobium ottawaense, suggesting the potential for efficiently mitigating NO emission from agricultural lands. Among the 15 B. ottawaense isolates examined, the NO-reducing activities of most (13) strains were approximately five-fold higher than that of Bradyrhizobium diazoefficiens USDA110 under anaerobic conditions. This robust NO-reducing activity of B. ottawaense was confirmed by NO reductase (NosZ) protein levels and by mitigation of NO 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, suggesting the high NO-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 NO-respiring conditions, which may contribute to the high nosZ expression. Our study indicates the potential of B. ottawaense for effective NO reduction and unique regulation of nos gene expression towards the high performance of NO mitigation in the soil.