Modeling of Cd uptake and efflux kinetics in metal-resistant bacterium Cupriavidus metallidurans.

The Model of Uptake with Instantaneous Adsorption and Efflux, MUIAE, describing and predicting the overall Cd uptake by the metal-resistant bacterium Cupriavidus metallidurans CH34, is presented. MUIAE takes into account different processes at the bacteria-medium interface with specific emphasis on the uptake and efflux kinetics and the decrease in bulk metal concentration. A single set of eight parameters provides a reasonable description of experimentally determined adsorbed and internalized Cd, as well as the evolution of dissolved Cd concentrations with time, for an initial Cd concentration between 10(-8) and 10(-4) M, covering the situation of contaminated environments and heavily polluted effluents. The same set of parameters allowed successful prediction of the internalized and adsorbed Cd as a function of the measured free Cd ion concentration in the presence of natural and anthropogenic ligands. The findings of the present study reveal the key role of Cd efflux and bulk depletion on the overall Cd uptake by C. metallidurans, and the need to account for these processes to understand and improve the efficiency of the metal removal from the contaminated environment.

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 x 10(-5) M Cd. In parallel, the free Cd ion concentrations ([Cd(2+)]) were measured by a hollow fiber permeation liquid membrane. Obtained results demonstrated that Cd uptake by bacterium Sinorhizobium meliloti was related to [Cd(2+)] in the solution. Addition of different complexing organic substances reduced [Cd(2+)] 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 x 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 twofold decrease of the intracellular Cd, but no clear trend was observed in the presence of 10(-7) to 10(-6) M 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.