Metal accumulation by microorganisms : Characteristics and implications for soil systems

Sammanfattning: The aim of the present study was to investigate the potential role of microbial accumulation in the mobility of metals in soil systems. As a first approach, the metal-accumulating properties of three bacterial species were studied to determine to what extent and under what conditions the accumulation occurred.For one of the bacteria, Cytophaga johnsonae, accumulation of zinc and cadmium was largely passive, although some active uptake was indicated at low zinc concentrations. Passive metal adsorption to Klebsiella oxytoca, Pseudomonas putida and C. johnsonae was highly variable related to properties of the metals and the organisms, but also depending on environmental factors like pH, ionic strength and presence of dissolved organic matter (a fulvic acid). Generally, adsorption increased with increasing pH, but decreased with increasing ionic strength, although some exceptions to this behavior were noted. The fulvic acid seemed to act as a competing ligand for metals, at least in the near-neutral range,thereby decreasing metal adsorption to bacteria.Multi-compartment systems were constructed to compare the distribution of metals among bacteria, fungi and other soil components, and to point out environmental parameters of importance for processes involved in the metal distribution. Although the microorganisms constituted only a small part of the solid mass in these systems, they accumulated considerable amounts of metals under certain conditions. The results obtained with the multi-compartment systems indicate that pH is of crucial importance.The significance of microorganisms for metal immobilization was further emphasized in studies using columns. A fungus was used in these investigations, and flow conditions were simulated to resemble natural conditions.The results suggest that microorganisms can be important metal sorbents in natural systems and should therefore be taken into account in transport modelling and mobility calculations. Further development of the employed methods and a comparison with natural soil are needed to supply further knowledge regarding the complex network of metal-microorganism interactions that exists in soil.