Heavy metal toxicity in Rhizobium leguminosarum biovar viciae isolated from soils subjected to different sources of heavy-metal contamination: effects on protein expression

Heavy metals adversely influence microorganisms, affecting their growth, morphology and activities. Metals also can exert a selective pressure on the organisms, resulting in microbial populations with higher tolerance to metals. Given the importance of legumes in animal and human consumption and their use in maintaining soil fertility, some attention has been given to the effects that heavy metals exert on Rhizobium isolates. In this context, Rhizobium leguminosarum biovar viciae was isolated from areas with different heavy metal contents and their tolerances were compared. Alterations in the protein pool of Rhizobium populations were also evaluated. Physicochemical parameters were determined and heavy metal concentrations in soils were analysed by ICP-AES. Isolates were screened for their tolerance in YEM media supplemented with different heavy metals (Zn, Pb, Co, Cd, Ni, Cr). Proteins were extracted and separated by SDS-PAGE. EI₁ and EI₂ (engineering industries) soils presented the highest metal concentration, and were therefore the most polluted soils. Isolates showed different growth responses to heavy metals. C (control soil) and M (mines) isolates were less tolerant than EI₁, EI₂ and CI (chemical industries) isolates. Metals influenced their protein profiles, most of the alterations corresponding to decreases in polypeptide expression. However, in tolerant isolates these alterations corresponding basically to increases, as occurred in CI isolates. This work suggests that there is a relationship between Rhizobium's tolerance, heavy metal soil contamination and alterations in protein pool. As a result, the analysis of protein alterations seems to be a good indicator to estimate the level of stress imposed on Rhizobium populations submitted to heavy-metal contamination.