Iodide Accumulation by Aerobic Bacteria Isolated from Subsurface Sediments of a 129I-Contaminated Aquifer at the Savannah River Site, South Carolina

I-129 is of major concern because of its mobility in the environment, excessive inventory, toxicity (it accumulates in the thyroid), and long half-life (similar to 16 million years). The aim of this study was to determine if bacteria from a I-129-contaminated oxic aquifer at the F area of the U.S. Department of Energy's Savannah River Site, SC, could accumulate iodide at environmentally relevant concentrations (0.1 mu M I-). Iodide accumulation capability was found in 3 out of 136 aerobic bacterial strains isolated from the F area that were closely related to Streptomyces/Kitasatospora spp., Bacillus mycoides, and Ralstonia/Cupriavidus spp. Two previously described iodide-accumulating marine strains, a Flexibacter aggregans strain and an Arenibacter troitsensis strain, accumulated 2 to 50% total iodide (0.1 mu M), whereas the F-area strains accumulated just 0.2 to 2.0%. Iodide accumulation by FA-30 was stimulated by the addition of H2O2, was not inhibited by chloride ions (27 mM), did not exhibit substrate saturation kinetics with regard to I- concentration (up to 10 mu M I-), and increased at pH values of <6. Overall, the data indicate that I- accumulation likely results from electrophilic substitution of cellular organic molecules. This study demonstrates that readily culturable, aerobic bacteria of the F-area aquifer do not accumulate significant amounts of iodide; however, this mechanism may contribute to the long-term fate and transport of I-129 and to the biogeochemical cycling of iodine over geologic time.