Bradyrhizobium ottawaense efficiently reduces nitrous oxide through high nosZ gene expression

Abstract

Abstract N2O is the major greenhouse gases 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 5-fold higher than that of Bradyrhizobium diazoefficiens USDA110T under anaerobic free-living conditions. This robust N2O-reducing activity of B. ottawaense was confirmed by N2O reductase (NosZ) protein levels and in the soybean rhizosphere after nodule decomposition. 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 USDA110T, 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 proposes the potential of B. ottawaense for effective N2O reduction and unique regulation of nos gene expression that contributes to the high performance of N2O mitigation in the soil.