Denitrifying ability of thirteen Rhizobium meliloti strains

The denitrifying ability of thirteen strains of Rhizobium meliloti was tested. Most of the strains were able to reduce nitrate to nitrous oxide or dinitrogen. However, they failed to use nitrate as electron acceptor for ATP generation or growth at low oxygen tensions. Under micro-aerobic conditions, free-living cells of R. meliloti 102-F-51 strain exhibited a constitutive nitrate reductase activity independent of the presence of nitrate. On the other hand, nitrite reductase activity was dependent not only on low levels of oxygen but also on the presence of a high nitrate concentration in the medium. Denitrification activity proceeded immediately once a threshold level of nitrite was accumulated in the medium or in cells incubated with 1mM nitrite. However, a lag period was required when cells were incubated with nitrate.