Presentation Effect of biotic interaction to the behavior of technetium-99 in the aquatic ecosystem

石井, 伸昌  ,  田上, 恵子  ,  内田, 滋夫

Technetium-99m (99mTc) is the most commonly used medical radioisotope and decays by isomeric transition to technetium-99 (99Tc). 99Tc is also produced by thermal neutron induced fission of U-235 with a relatively high yield of about 6% and has a half-life of 2.1 × 10^5 years. These radionuclides have been released to the environment through uses of unclear energy and an unanticipated accident. In fact, Tc-99m, which was released by the Fukushima Daiichi Nuclear Power Plant accident, was detected in Tsukuba, Japan.Tc is normally present as the pertechnetate anion (TcO4-) under aerobic conditions, and this chemical species is soluble and mobile. Some of environmental bacteria, however, affect the behavior of Tc. For example, Escherichia coli accumulate Tc(VII) and reduced it to a black insoluble precipitate. In aquatic ecosystem, bacteria are generally ingested by protozoa and released as excrement. However it is not known the effect of prey-predator interactions on the behavior of Tc. In this study, 99Tc-tagged E. coli was cultured with and without a ciliate, Tetrahymena thermophila for 7 days. To collect Tc in the dissolved fraction, the culture was passed through a 0.2-µm-pores filter. The abundance of T. thermophila increased 3.7 times of initial abundance for the first 2 days of incubation. Because T. thermophila cells cannot survive without their foods, the increase in the abundance demonstrated that T. thermophila ingested E. coli as food. Concentrations of Tc in the dissolved fraction were gradually increased with time for both cultures. The maximum concentrations were 168 Bq mL-1 for the E.coli alone culture and 352 Bq mL-1 for the mixed culture with T. thermophila, respectively. It should be noted that the significant increase in the dissolved Tc in the mixed culture was found during the growth period of T. thermophila. These results suggest that ingestion behavior by T. thermophila promoted the production of the dissolved Tc.This work has been partially supported by the Agency for Natural Resources and Energy, the Ministry of Economy, Trade and Industry (METI), Japan.

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