Do Exudates Mitigate Cd Biouptake by Rhizobacterium Sinorhizobium melilotii?

To better understand the role of exudates in Cd speciation and biouptake by bacterium Sinorhizobium meliloti, the content of different exudate components including siderophores, proteins and polysaccharides were quantified in absence and presence of 10M Cd at pH of 5.0 and 7.0. Obtained results demonstrated that the release of exudates by S. meliloti is constitutive process rather than induced by the presence of cadmium. Nonetheless, exudates complex cadmium and reduced significantly cadmium free ion concentration. Cd bioavailability to S. meliloti was characterized by the amount of the adsorbed and intracellular Cd. Adsorbed Cd at pH 5.0 was higher then that at pH of 7.0, in qualitative agreement to the higher free cadmium concentration and lower amounts of exudates released at that pH. The observed reduction of intracellular cadmium at pH of 5.0 with respect to pH of 7.0 was attributed to the prevailing competition between proton for cadmium for transport sites.