Metal tolerance of rhizobial strains isolated from nodules of herbaceous legumes (Medicago spp. and Trifolium spp.) growing in mercury-contaminated soils

• Published in: Applied soil ecology : a section of Agriculture, ecosystems & environment ecosystems & environment, 2012-10, Vol.61, p.49-59
• Main Authors: Nonnoi, Francesca, Chinnaswamy, Appunu, García de la Torre, Vanesa S., Coba de la Peña, Teodoro, Lucas, M. Mercedes, Pueyo, José J.
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; bacteria; bioavailability; Biological and medical sciences; biomass; Bioremediation; Chemical, physicochemical, biochemical and biological properties; Economic plant physiology; Ensifer; forage legumes; Fundamental and applied biological sciences. Psychology; host plants; Legumes; Medicago; Mercury; Metal tolerance; phosphates; Physics, chemistry, biochemistry and biology of agricultural and forest soils; Rhizobium; Rhizobium leguminosarum; salt tolerance; soil; Soil and water pollution; soil pollution; Soil science; solubilization; Symbiosis; Symbiosis (nodules, symbiotic nitrogen fixation, mycorrhiza...); Trifolium
• ISSN: 0929-1393; 1873-0272
• DOI: 10.1016/j.apsoil.2012.06.004

► We isolated 59 rhizobial strains from herbaceous legumes growing in Hg-polluted soils. ► Several strains exhibited extremely high tolerance to Hg. ► Strains displaying multiple metal tolerances were also identified. ► Positive and negative correlations were found between metal tolerance responses. ► Tolerant strains may be potentially useful to develop Hg bioremediation technologies. 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.