DENITRIFYING ABILITY OF 13 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 ox ide or dinitrogen. However, they failed to use nitrate as electron acc eptor 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 wa s dependent not only on low levels of oxygen but also on the presence of a high nitrate concentration in the medium. Denitrification activit y proceeded immediately once a threshold level of nitrite was accumula ted in the medium or in cells incubated with 1 mM nitrite. However, a lag period was required when cells were incubated with nitrate.