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Competition for electrons favors N2O reduction in denitrifying Bradyrhizobium isolates

The legume-rhizobium symbiosis accounts for the major part of the biological N2 fixation in agricultural systems. Despite their lower need for synthetic nitrogen fertilizers, legume-cropped fields are responsible for substantial N2O emissions. Several economically important legumes fix N2 through symbiosis with bacteria belonging to the genus Bradyrhizobium. Many bradyrhizobia are also denitrifiers, and inoculation of legumes with N2O-reducing strains has been suggested to mitigate N2O emissions. Here, we analyzed the phylogeny and denitrification capacity of Bradyrhizobium strains, most of them isolated from peanut nodules. All performed at least partial denitrification, but only 37% were complete denitrifiers. This group shared a common phenotype with a strong preference for N2O- over NO3−-reduction. We tested if this could be due to low quantities of NO3− reductase (periplasmic Nap) but found that Nap was more abundant than N2O reductase (Nos). This corroborates a recently proposed mechanism that the electron pathway from quinols to Nos via the cytochrome bc1 complex outcompetes that to Nap via the membrane-bound NapC. We propose that this applies to all organisms which, like many bradyrhizobia, carry Nos and Nap but lack membrane-bond NO3− reductase (Nar). Supporting this, Paracoccus denitrificans, which has Nap and Nar, reduced N2O and NO3− simultaneously.