Ability of anaerobic microorganisms to associate with iodine: 125I tracer experiments using laboratory strains and enriched microbial communities from subsurface formation water

Iodine-129 (129I) is a fission product with a half-life of 15.7 million years. Because of its long half-life, high mobility, and high affinity to the human body, 129I is considered as one of the most problematic radionuclides in nuclear waste disposals in deep geological formation. In this study, 16 strains of anaerobic microorganisms, including nitrate-reducing bacteria, iron-reducing bacteria, sulfate-reducing bacteria, and methanogens, were cultured anaerobically with a radioiodine tracer (125I) to determine whether they possess the ability to associate with iodine. We evaluated it by association efficiency, that was determined by measuring the decrease in 125I activity in the culture supernatant. It was found that the efficiency of about half of the strains was below detection limit (1.0%), and that of the remaining strains varied from 1.0% to 6.3%, although it was not statistically significant. Similar experiments were conducted by using anaerobic microbial communities inhabiting the iodine-rich subsurface formation water collected from the Minami-kanto gas field in Japan. The specific uptake of iodine by the microbial communities was estimated to be 0.71–2.0 μg g−1 dry weight of biomass, indicating that the association ability was, if present, very limited. These results suggest that anaerobic microorganisms, in contrast with aerobic microorganisms, neither enhance nor repress the mobility of 129I, in the case of discharge of this radionuclide from disposal facilities into the surrounding environment.