Anaerobic Reduction of Nitrate to Nitrous Oxide Is Lower in Bradyrhizobium japonicum than in Bradyrhizobium diazoefficiens

When soil oxygen levels decrease, some bradyrhizobia use denitrification as an alternative form of respiration. Bradyrhizobium diazoefficiens (nos+) completely denitrifies nitrate (NO3-) to dinitrogen, whereas B. japonicum (nos-) is unable to reduce nitrous oxide to dinitrogen. We found that anaerobic growth with NO3- as the electron acceptor was significantly lower in B. japonicum than in B. diazoefficiens, and this was not explained by the absence of nos in B. japonicum. Our results indicate that the reason for the limited growth of B. japonicum is weak NO3- reduction due to impaired periplasmic nitrate reductase activity, which may rely on posttranscriptional events.