Accumulation of metals by microorganisms - processes and importance for soil systems

Metal accumulation by solid substances can counteract metal mobilization in the environment if the solid substance is immobile. Microorganisms have a high surface area-to-volume ratio because of their small size and therefore provide a large contact area that can interact with metals in the surround ing environment. Microbial metal accumulation has received much attention i n the last years due to the potential use of microorganisms for cleaning me tal-polluted water. However, considerably less attention has been paid to t he role of microorganisms for metal mobility in soil even though the same p rocesses may occur there. Therefore, this paper highlights this area. The d ifferent accumulation processes that microorganisms perform are analyzed an d their potential significance in soil systems is discussed. Different kinds of mechanisms can be involved in the accumulation of metals by microorganisms, e.g. adsorption, precipitation, complexation and active transport into the cell. Physicochemical parameters like pH and ionic comp osition, as well as biological factors are of importance for the magnitude of accumulation. Often large amounts of metals can be accumulated with vary ing specificity, and microorganisms may provide nucleation sites for minera l formation. Several studies of microbial metal accumulation have been made with differe nt methods and aims. Most of these studies concern single-component systems with one organism at a time. Data from accumulation experiments with pure cultures of microorganisms have been used to model the overall metal retent ion in soil. A further development is experimental model systems using vari ous solid soil components in salt medium. Microbial metal accumulation is difficult to study in situ, but some experi mental methods have been applied as tools for studying real soil systems, e .g. litter bags buried in soil containing microorganisms, a method where di scs with microorganisms have been put onto agar plates with soil extracts, and comparison of sterilized and non-sterilized soils or soils with or with out nutrient amendment. Different aspects of microbial metal accumulation are emphasized with the d ifferent methods applied. Single-component systems have the advantage of pr oviding excellent information of the metal binding properties of microorgan isms but cannot directly be applied to metal behavior in the heterogenous s ystems that real soils constitute. Studies focused on the behavior of metal s in real soils can, in contrast, provide information on the overall metal distribution but less insight into the processes involved. Obviously, a com bination of approaches is needed to describe metal distribution and mobilit y in polluted soil such as areas around mines. Different kinds of multi-com ponent systems as well as modelling may bridge the gap between these two ty pes of studies. Several experimental methods, complementary to each other a nd designed to allow for comparison, may emphasize different aspects of met al accumulation and should therefore be considered. To summarize, there are studies that indicate that microorganisms may also accumulate metals in soil and that the amounts may be considerable. However , much work remains to be dent, with the focus of microorganisms in soil. I t is also important to put microbial metal accumulation in relation to othe r microbial processes in soil, which can influence metal mobility, to deter mine the overall influence of soil microorganisms on metal mobility, and to be able to quantify these processes. (C) 2000 Elsevier Science B.V. All ri ghts reserved.