Do Exudates Affect Cadmium Speciation and Bioavailability to the Rhizobacterium Sinorhizobium meliloti?

Abstract

Environmental Context. Heavy metals such as cadmium can affect soil microbes and consequently perturb important nutrient cycles resulting in deterioration of the soil fertility. Conversely, soil bacteria might influence cadmium cycling, bioavailability and ecotoxicity by producing exudates such as proteins, polysaccharides and siderophores. These substances are shown to form complexes with cadmium, decreasing the free concentration of cadmium in soil solutions. Abstract. To better understand the role of exudates in cadmium (Cd) speciation and biouptake by the bacterium Sinorhizobium meliloti, the content of various exudate components, including siderophores, proteins and polysaccharides, was quantified in the absence and presence of 10 μM Cd at pH 5.0 and pH 7.0. The obtained results demonstrate that the release of exudates by S. meliloti is a constitutive process rather than one induced by the presence of Cd. Nonetheless, exudates complex Cd and significantly reduce its free ion concentration. Cd bioavailability to S. meliloti was characterized by the amount of adsorbed and intracellular Cd. Adsorbed Cd at pH 5.0 was higher than that at pH 7.0, which is consistent with the higher free Cd concentration and lower amounts of exudates released at that pH. The observed reduction in intracellular Cd at pH 5.0 compared with pH 7.0 was attributed to the prevailing competition between protons and Cd ions for transport sites.