Abstract

Mercury contamination of soils constitutes a serious environmental problem worldwide. Rhizobium–legume symbioses represent an interesting potential tool in soil bioremediation. Besides their nitrogen-fixing capacity, they have the advantage of integrating microorganisms that may influence metal bioavailability and legume plants that may act as phytoextractors or phytostabilizers. Herbaceous forage legumes present additional advantages in that they grow rapidly, producing a high biomass, and they are easy to harvest. Analysing legume endosymbiont tolerance is essential to successfully manage these symbiotic associations in metal-polluted areas, although little information is as yet available regarding Hg-tolerant rhizobia. The aim of this study was to isolate and characterize rhizobia from nodules of Medicago spp. and Trifolium spp. plants that grow in Hg-contaminated soils. Fifty-nine bacterial strains were genetically characterized and identified as belonging to the Ensifer medicae and Rhizobium leguminosarum bv. trifolii species. Rhizobial strains highly tolerant to Hg and strains displaying multiple metal tolerances were identified. The bacteria showed important phenotypic diversity in terms of salt and pH tolerance, phosphate solubilization and tolerance to different heavy metals. Significant correlations were found between metal tolerance responses. The influences of the site of origin, the host plant and bacterial species on metal tolerance were analysed.