Physiological responses to cadmium, copper, lead, and zinc of Sinorhizobium sp. strains nodulating Medicago sativa grown in Tunisian mining soils
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The capacity of nodulating bacteria to survive in soil containing various heavy metal elements has been investigated with the aim of promoting the revegetation of mining sites with Medicago sp. Soil samples were collected from three different mining sites and one agricultural site at a location north of Tunisia. Heavy metal composition analysis showed that the soil samples were contaminated with different concentrations of cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn). The forage plant Medicago sativa was able to grow normally and to develop effective nodules in these contaminated soils. Sinorhizobium sp. strains nodulating Medicago sativa plants grown in these mining soil samples were isolated and characterized. The isolated strains were able to grow in soils containing up to 2.5 mM Zn, 0.3 mM Cd, 1 mM Cu and 2 mM Pb. The bioaccumulation was tested for two contrasting strains for each metal. For Cd, Pb, and Zn, strain S532 (tolerant strain) adsorbed lower amounts of metals than sensitive strain S112. For Cu, tolerant strain S412 absorbed more Cu than sensitive strain S112, even though adsorption was similar for these two strains. Our results support the use of Medicago sativa–sinorhizobium symbiosis for the regeneration and enrichment of moderately contaminated soils.
KeywordsHeavy metals Symbiosis Sinorhizobium sp. strains Soil regeneration Alfalfa
The authors thank Dr. Issam Nouairi in the laboratory of legumes (CBBC) for fruitful discussion. Our thanks are also addressed to Hedi Hamrouni, INGREF, and Ons Talbi, Laboratory of Extremophile Plants (CBBC), for soil analysis.
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