Interactions between metals and bacteria : Importance for metal speciation

Abstract: The objective of this thesis is to contribute to the understanding of the interactions between metals and microorganisms with respect to' Toxicity of metals (Cd, Cu, Hg, Zn)' Mineralization of metal-organic complexes (citric acid, histidine)' Metal adsorption (Cs, Sr, Eu, Hg, Zn, As)Most of the studies were made on the gram-negative bacterium Klebsiella oxytoca, but the work also involved a Serratia as as well as mixed cultures of microorganisms from soil systems.Most previous studies, reported in the literature, on the toxic effects of metals on microorganisms have been performed at high nutrient levels, despite the fact that microorganisms in nature normally live under energy-limited conditions. The present toxicity studies were thereforeperformed in systems with low carbon concentrations (0.01-40 mg C 1-1), more representative of natural systems. The toxicity of Cd, Cu, Hg, and Zn to K. oxytoca, as affected by glucose concentrations, was investigated, and it was shown that the nutritional state of the organism can have a profound effect on its sensitivity to metals. The toxicity of Cd, Cu, and Zn was enhanced by raising the carbon concentration from 1 mg to 10 mg C r1, while no effect could be seen on the toxicity of mercury. This illustrates the importance of considering the carbon concentrationwhen evaluating results from toxicity studies.Bacterial surfaces are generally negatively charged in the environmental pH-range and may therefore interact with metal cations, thus influencing the mobility of metals in soil and natural waters. The metal binding properties and acid capacity of starved cells of K. oxytoca were evaluated. The elements Cs(I), Sr(ll), and Eu(III) served as models for cationic elements representing different oxidation states, and the study also included Hg(Il), Zn(II), as well as the non-metal As(V). Adsorption was determined at various pH (3 to 9) at low metal concentrations (Ht8 M); it followed the order Cs<Sr,As<Zn<Hg<Eu (at pH 5 to 7). For europium adsorption drastically increased from 10% at pH 3 to almost 100% at pH 4.5. At pH above the acid equivalence point (6.9) adsorption was reduced to 10%. Thus, it is shown that bacteria may have a profound effect on metal speciation by selectively binding metals to the surfaces. This mayhave a great impact on the distribution of the metal between mobile and stationary phases and accordingly also its transport properties.In conclusion, this study stresses the importance of considering microbial processes in predictions of metal behaviour in natural systems. Evidently, there is a great need for interdisciplinary research (i.e., biology, chemistry, geology, hydrology) in order to gain a more complete understanding of the mechanisms controlling the metal cycling in nature.