Effect of competing ions and complexing organic substances on the cadmium uptake by the soil bacterium Sinorhizobium meliloti

In an effort to improve the understanding and prediction of Cd uptake by soil bacteria, adsorbed and intracellular Cd were determined in unpolluted and highly polluted model soil solutions within a concentration range spanning from 10-9 to 5  10-5 M of cadmium. In parallel, the free cadmium ion concentrations ([Cd2+]) were measured by a hollow fiber permeation liquid membrane. Obtained results demonstrated that Cd uptake by bacterium Sinorhizobium meliloti was related to [Cd2+] in the solution. Addition of different complexing organic substance reduced [Cd2+] in the bacterial medium and decreased both adsorbed and intracellular Cd. The adsorbed Cd was considerably reduced in the presence of 10-4 to 5  10-2 M [Ca] or [Mg]. No effect on Cd adsorption was observed in the presence of Zn or Mn, even at 100-fold excess. Intracellular Cd decreased in the presence of a high excess of Ca and Zn, while no significant effect was observed in the presence of Mg. An increase of dissolved Mn from 10-9 to 10-8 M resulted in a two-fold decrease of the intracellular Cd, but no clear trend was observed in the presence of 10-7 to 10-6 M of Mn. Based on complexation and competition studies, the stability constants, necessary for quantitative description of Cd uptake by S. meliloti and the development of the bacterial biotic ligand model for Cd, were derived and validated in the solutions containing mixtures of Cd , Zn, Ca and Mg. Further numerical simulations of Cd uptake by S. meliloti exposed to soil pore waters, demonstrated the importance of Zn competition and the insignificant influence of Ca and H on Cd uptake.