Toxic metal interactions with soil bacterium "Cupriavidus metallidurans"

Abstract

Living bacteria offer an ecological and cost-effective alternative to the classical physical and chemical techniques for the treatment of environmental contaminations. Although their increasing use in toxic metal removal, systematic studies on the influence of physicochemistry of the medium on metal uptake and thus on their metal removal efficiency are very limited, even though it would deserve greater environmental interest. The overall goal of the present work is to improve the mechanistic understanding of the interactions between toxic metals and microorganisms. The emphasis is on the effect of different site-specific characteristics e.g. the presence and concentrations of different complexing agents, metal mixtures and pH- on the Cd and Pb uptake by a metal-resistant bacterium (Cupriavidus metallidurans CH34). Conversely, the influence of bacteria on metal speciation and mobility in the surrounding environment is also addressed. Different processes involved in the overall metal uptake, such as the diffusion of metal species, adsorption, internalization and efflux, were quantified and described with a consistent dynamic model for a range of metal concentrations representing slightly to heavily polluted environments. Obtained results demonstrated that at 10-8 and 10-6 M of initial Cd concentrations, the bulk Cd depletion determined both adsorbed and intracellular Cd amount. Cd efflux affected Cd internalization over the whole concentration range. The findings of the present study demonstrated the key role of metal depletion in the bulk medium and the efflux on the overall metal uptake by C. metallidurans. Therefore, also the need to account for these processes in order to improve the efficiency of metal removal from contaminated environments. The relationship between Cd and Pb speciation and the metal uptake was also explored. The presence of citric, nitrilotriacetic and humic acids decreased both intracellular and total cellular metal contents. Intracellular and total cellular metal contents were directly related to the experimentally determined free cadmium and lead ion concentrations rather than the total metal concentrations, emphasizing the important role of metal speciation in uptake. Furthermore, the influence of the major cations, such as Ca and Mg, trace metals, such as Zn, Cu, Co, Mn and pH on Cd and Pb uptake were studied. Increasing concentrations of Ca, Mg, Zn or protons decreased both intracellular and total cellular Cd contents. The intracellular and total cellular Pb contents decreased significantly in the presence of Ca, while Mg decreased only total cellular metal content. In order to enhance even more the environmental relevance of the studied conditions, both metal speciation and Cd and Pb uptake by bacteria were determined in six metal contaminated soil solutions. A Michaelis-Menten based equation taking into account both speciation and competition effects was used to predict metal uptake in the soil solutions. Predictions were usually within the same order of magnitude as the experimentally measured values in the soil solutions. Cd uptake was stronger affected by the competition of other metals than Pb. In contrast Pb uptake was stronger influenced by the changes in chemical speciation than Cd, as Pb usually forms stronger complexes with dissolved organic matter. These results suggest that neglecting of metal speciation and competition effects will result in overestimation of the metal removal capacity of bacteria. Metal uptake and removal will depend on the nature and concentration of the metal of interest and ligands but also the presence and concentration of other metals. Bacteria can influence the speciation of the metals by producing exopolymeric substances (EPSs). The EPSs excreted by C. metallidurans contained components of proteinaceous and polysaccharide nature with a wide molar mass distribution. The excreted EPS formed stronger complexes with Pb than with Cd, though these complexes were up to two orders of magnitude weaker than those found with humic acids. Coupling of asymmetrical flow-field flow fractionation system to inductively coupled plasma mass spectrometer revealed that Cd bind preferentially to lower molar mass fractions, while Pb associates to both lower and higher molar mass fractions. The preference of metals to bind to different molar masse/size biopolymers is of environmental importance as it determines the mobility and fate of the metals. The results obtained in the present work emphasize the importance of taking into account chemical site-specificity of soil solutions and water, including dissolved organic ligands, pH, the presence of other metals, as well the ability of bacteria to influence metal speciation, when developing metal removal technologies by live bacteria.