Solubilization and immobilization of toxic metals by bacteria

Microorganisms play important roles in the biogeochemical cycling of metals. Some microbial processes release metals thereby increasing their mobility, which may increase bioavailability and potential toxicity, whereas other processes result in immobilization and reduce bioavailability. The relative balance between mobilization and immobilization varies depending on the microorganisms, their environment and changing physico-chemical conditions. As well as being a key component of natural biogeochemical cycles for metals, these processes may be exploited for the treatment of contaminated solid and liquid wastes. This paper describes two selected aspects, which illustrate the key importance in effecting changes in metal solubility, namely bioleaching by aerobic chemoautotrophic sulphur and ferrous oxidisers (bacteria genera Thiobacillus) and metal sulphide precipitation by anaerobic dissimilatory sulphate reducers (bacteria genera Desulfovibrio and Desulfotomaculum).