Competition for electrons favours N 2 O reduction in denitrifying Bradyrhizobium isolates

Bradyrhizobia are common members of soil microbiomes and known as N2 -fixing symbionts of economically important legumes. Many are also denitrifiers, which can act as sinks or sources for N2 O. Inoculation with compatible rhizobia is often needed for optimal N2 -fixation, but the choice of inoculant may have consequences for N2 O emission. Here, we determined the phylogeny and denitrification capacity of Bradyrhizobium strains, most of them isolated from peanut-nodules. Analyses of genomes and denitrification end-points showed that all were denitrifiers, but only ~1/3 could reduce N2 O. The N2 O-reducing isolates had strong preference for N2 O- over NO3 - -reduction. Such preference was also observed in a study of other bradyrhizobia and tentatively ascribed to competition between the electron pathways to Nap (periplasmic NO3 - reductase) and Nos (N2 O reductase). Another possible explanation is lower abundance of Nap than Nos. Here, proteomics revealed that Nap was instead more abundant than Nos, supporting the hypothesis that the electron pathway to Nos outcompetes that to Nap. In contrast, Paracoccus denitrificans, which has membrane-bond NO3 - reductase (Nar), reduced N2 O and NO3 - simultaneously. We propose that the control at the metabolic level, favouring N2 O reduction over NO3 - reduction, applies also to other denitrifiers carrying Nos and Nap but lacking Nar.