Iodide Oxidizing Activity of Bacteria from Subsurface Sediments of the Savannah River Site, SC, U.S.A

129I is a major by-product of nuclear fission that is of concern because of its extremely long half-life (~16 million yrs), perceived toxicity through bioaccumulation, and the increasing inventory of this radionuclide worldwide. Relatively high concentrations of iodate (27.3%) and organoiodine (23.9%) are present in a 129I-conatminated aquifer at the Savannah River Site (SRS), SC, U.S.A. To determine if microorganisms could influence 129I speciation in this system, iodide oxidizing activity was evaluated in bacteria isolated from SRS sediments. All strains isolated directly from sediment material (n = 325) and from sediment enrichment cultures containing 1 mM iodide (n = 29) tested negative for iodide oxidation on R2A agar plates containing iodide and starch. However, tubes from 2/24 enrichment cultures exhibited a distinct yellow coloration, indicative of iodide oxidation to I2 and/ or volatile organic iodine species, after 22 weeks incubation. Analysis of iodine speciation in the enrichment culture supernatant from these two tubes revealed transformation of iodide to iodate and organo-iodide. Screening of 84 strains for iodide oxidizing activity using a combination of triiodide (I3 –) formation, radiography and a recently developed, sensitive iodine speciation assay revealed that 44 of these strains were capable of iodide oxidation. These results indicate that iodide oxidation, albeit at very slow rates, can be supported by a variety of terrestrial bacteria.