Metal tolerance of Rhizobium meliloti isolated from heavy-metal contaminated soils

Abstract

Soil populations of rhizobia have been reported to resond to the presence of high concentrations of heavy metals by the acquisition of tolerance to specific metals. To examine this possibility, alfalfa plants ( Medicago sativa) were collected from soils containing low to very high concentrations of the metals. Cd and Zn. The source of the metals was a Zn smelter in operation for nearly 100 yr. Fifty isolates of Rhizobium meliloti were collected and purified from each soil. All isolates, regardless of their origin, were capable of growing on media containing very high concentrations of the heavy metals, Zn, Cu, Ni and Cd. For example, R. meliloti isolated from soil with extractable [10 mm Ca(NO 3) 2 Zn and Cd concentrations of 0.025 and 0.003 μg g −1 were tolerant of medium concentrations of 162 μg Zn g −1 and 22 μg Cd g −1, respectively. There was no correlation between extractable soil metal concentrations and the ability of the isolates to tolerate metal salts in their growth medium. In fact, the greatest number of metal-tolerant rhizobia were isolated from soil containing the lowest metal concentration. These results indicate that the intrinsic level of metal tolerance of R. meliloti is much higher than metal activities in soil, even highly contaminated soils. This intrinsic level of metal tolerance probably explains the lack of metal response by rhizobia collected from these soils.