Microbial influences on radionuclide behaviour – an example of less-understood problems and how to solve them

The potential ambivalent effects of microorganisms on radionuclide transport in the near and far -field of nuclear waste repositories up to the biosphere is discussed. Issues such as microbially induced redox processes of radionuclides, the role of se-creted microbial bioligands, of biosorption and biocolloids, the in-situ kinetics of mi-crobial reactions and the quantification of their products are addressed.
Besides the prominent processes influencing the migration of actinides in the envi-ronment, e.g. sorption onto mineral surfaces, there is growing attention to the influ-ence of indigenous microorganisms on actinide speciation. The concept of geological disposal comprises a detailed knowledge concerning potential host rock formations also in terms of such microorganisms. It is well known that respective bacteria can affect the speciation and hence the mobility of actinides. Thus, dominant bacterial strains from sites destined for future nuclear waste deposition have to be investigated regarding their interaction mechanisms with soluble actinide ions. The understanding of the speciation and the structure of the radionuclide complexes formed in presence of indigenous bacteria over a range of geochemical parameters (e.g., pH, metal concentration) becomes indispensable for eventually predicting the safety of a planned nuclear waste repository. The impact of bacteria on radionuclide speciation will be discussed by taking three examples:
a) the influence of the Mont Terri Opalinus Clay isolate Sporomusa sp. on the pluto-nium speciation; b) curium(III) interaction with cells of the Äspö -groundwater bacte-rium Pseudomonas fluorescens; and c) curium(III) complexation/mobilization with pyoverdins secreted by the Äspö -strain P. fluorescens.