Health impacts of large releases of radionuclides. Roles of micro-organisms in the environmental fate of radionuclides.

Micro-organisms play important roles in the environmental fate of radionuclides in both aquatic and terrestrial ecosystems, with a multiplicity of physico-chemical and biological mechanisms effecting changes in mobility and speciation. Physico-chemical mechanisms of removal, which may be encompassed by the general term 'biosorption', include adsorption, ion exchange and entrapment. These are features of living and dead organisms as well as their derived products. In living cells biosorptive processes can be directly and indirectly influenced by metabolism, and may be reversible and affected by changing environmental conditions. Metabolism-dependent mechanisms of radionuclide immobilization include metal precipitation as sulfides, sequestration by metal-binding proteins and peptides, and transport and intracellular compartmentation. Chemical transformations of radionuclide species, particularly by reduction, can result in immobilization. Microbial processes involved in solubilization include autotrophic and heterotrophic leaching, complexation by siderophores and other metabolites, and chemical transformations. Such mechanisms are important components of natural biogeochemical cycles for radionuclides and should be considered in any analyses of environmental radionuclide contamination. Several micro-organism-based biotechnologies, e.g. those based on biosorption or precipitation, are of potential use for the treatment of radionuclide contamination.