Denitrification in Rhizobium

Thirty-three strains of Rhizobium were examined for their reduction of nitrate under anoxic conditions. Three patterns of dissimilatory nitrate reduction were observed: (1) reduction to N2O and N2 (denitrification), (2) reduction to and subsequent accumulation of NO2- (nitrate respiration), (3) no reduction. Strains of R. japonicum and the cowpea miscellany displayed all three types, while strains of R. leguminosarum, R. phaseoli, and R. trifolii did not reduce nitrate by dissimilatory means. The production and subsequent metabolism of N2O was considerably different among the denitrifying strains: in some instances, N2O was a transient intermediate, while in others, it continued to accumulate during the incubation period.