Effect of microbial interaction on the fate of technetium in aquatic environments

Technetium-99 (99Tc) is a long-lived fission product with a half-life of 2.1x105 years. In the environment, it is normally present in two oxidation states, Tc(IV) and Tc(VII). The latter as the pertechnetate ion (TcO4-) interacts with various bacterial species. For instance, Escherichia coli is able to reduce Tc(VII) as an electron acceptor and to deposit it in the cell.In aquatic environments, bacteria are ingested by protozoa such as heterotrophic nanoflagellates and ciliates, suggesting that prey-predator interactions have potential effect on the fate of Tc. However, there is little information on the fate of the Tc that is ingested by protozoan cells together with bacterial cells. In the present study, release of Tc through the interaction between Tc-tagged bacteria and ciliate was investigated. The soil sorption properties of the released Tc were also determined.Escherichia coli and Tetrahymena thermophila were used as a prey-predator model. E. coli cells were tagged with 99TcO4-, and the Tc-tagged cells were cultured with T. thermophila cells for 15 days at 25C under oxic conditions. To determine the concentration of dissolved 99Tc, these cultures were filtered with 0.2-um pores.A rapid increase in the concentration of dissolved 99Tc was observed from day 2 to day 4 of incubation. This period was corresponded to the active predation period. To investigate the soil sorption properties, the dissolved 99Tc was mixed with paddy soil and shake-incubated for 3 days. 70% of the dissolved 99Tc sorbed on the soil particles, while a slight sorption was observed when 99TcO4- was used instead of the dissolved 99Tc. These results suggested that prey-predator interactions facilitate sorption of Tc on soil particles.This work has been partially supported by the Agency for Natural Resources and Energy, the Ministry of Economy, Trade and Industry (METI), Japan.International Union of Microbiological Societies 2011 Congres