The range of interactions between microorganisms, metals, and soil components can be extremely complex, and because of this, many inferences are necessarily taken from simplified laboratory experiments, which probably do not adequately reflect the range of natural situations that may be encountered. However, it is possible to make some general observations and conclusions regarding the processes involved, their relative importance, and their involvement in metal speciation in the soil. The relative role of fungi and bacteria depends on associated biota (other microorganisms, plants, and animals) as well as on physicochemical properties of the environmental matrix (e.g., pH, water, inorganic and organic ions, molecules, compounds, colloids, and particulates).
The ability of microorganisms to influence metal speciation in the soil results from indirect and direct properties which influence the balance of metal species between soluble and insoluble phases (Figures 2.1 to 2.3). Mobilization of metals can be achieved by various mechanisms: protonation, chelation, and chemical transformations. Metal immobilization can occur by precipitation or crystallization of insoluble organic or inorganic compounds (e.g., oxalates and sulfides, respectively), or by sorption, uptake, and intracellular sequestration (Glasauer et al., 2004a). In addition, redox reactions can mobilize or immobilize metals, depending on the particular metal species and microenvironment (Figure 2.2).
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