Influence of Soil Rhizobacteria on Heavy Metal Bioavailability

Soil rhizobacteria can directly influence metal solubility by changing the heavy metal speciation in the rhizosphere. The toxic effects of heavy metals on soil microorganisms depend on their bioavailability. Although heavy metal bioavailability is mainly dependent on soil properties (pH and organic matter), bacteria can also directly influence the solubilities of heavy metals by altering their chemical properties.

Microorganisms have developed several mechanisms that can immobilize, mobilize or transform heavy metals. These processes include: (1) extracellular precipitation; (2) intracellular accumulation; (3) oxidation and reduction reactions; (4) methylation and demethylation, and; (5) extracellular binding and complexation (Brierley 1990). The exploitation of these bacterial properties for the remediation of sites contaminated with heavy metals has been shown to be a promising bioremediation alternative (Lovely and Coates 1997; Lloyd and Lovley 2001). However, at high concentrations, bioavailable heavy metals are toxic to a great number of soil microorganisms and soil microbial processes, which in turn will result in severe ecosystem disturbance.

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