@misc{oai:repo.qst.go.jp:00068195,
 author = {Ishii, Nobuyoshi and Uchida, Shigeo and 石井 伸昌 and 内田 滋夫},
 month = {Oct},
 note = {Technetium-99 (99Tc) is a long-lived (half-life, 2.1 x 105 y) radioactive contaminant in the environment.  Under aerobic conditions, Tc is present in the heptavalent form as pertechnetate ion (TcO4-).  This chemical species is soluble and mobile in the environment and is readily taken up by plants.  Therefore, we would like to be able to predict the behavior of 99Tc in the environment.  Recently, we found transformation of Tc from soluble to insoluble forms in ponding water of paddy fields [1] and bacterial involvement in this transformation [2].  One of the Tc insolubilizing mechanisms would be an accumulation of Tc by bacteria as demonstrated by Lloyd et al. [3].  In aquatic environments, bacteria are ingested by heterotrophic nanoflagellates and ciliates as their food, and thus the bacterial ingestion by these protozoa would govern the fate of Tc.  Our aim, therefore, was to determine the fate of the insoluble Tc accumulating by bacterial cells after the bacterial ingestion by protozoa: Whether the insoluble Tc is concentrated and kept inside of the protozoan cells or it is excreted outside of the protozoan.
\nA ciliate Tetrahymena thermophila and a bacterium Escherichia coli AB1157 were used as a prey-predator model.  E. coli was incubated with 99TcO4- for 45 days to accumulate Tc within the cell.  After the incubation, cells were washed 5 times and then cultured with and without T. thermophila.  To estimate soluble Tc excretion by T. thermophila, the cultures were filtered through a 0.2 ?m pore-size filter, and radioactivity of 99Tc in the filtrate was measured with a liquid scintillation counter. The values of measurements were converted to concentrations (Bq mL-1).
\nThe concentration of the soluble Tc in the filtrate of the E. coli-alone culture gradually increased from 28.7 Bq mL-1 at the beginning of incubation (day 0) to the maximum concentration 168.3 Bq mL-1 at the end of incubation (day 7).  An increase in the soluble Tc concentration was also observed in the filtrate of the E. coli and T. thermophila mixed culture, with the maximum concentration, 352.1 Bq mL-1 at day 3.  The maximum concentration of the soluble Tc was 2.1-times higher in the mixed culture than in the E. coli alone culture.
The abundance of T. thermophila cells in the mixed culture increased from 1.3 x 103 to 4.7 x 103 cells mL-1 for the first 3 days of incubation.  The growth period of T. thermophila cells corresponded to the period of the significant increase in the concentration of the soluble Tc in the mixed culture.  If T. thermoplila concentrated and kept Tc inside of cells, the concentration of soluble Tc would have decrease during the growth period.  Therefore, the increase in soluble Tc suggested that the insoluble Tc was transformed to soluble forms in the food vacuoles of the T. thermophila cells and then was excreted out from the cells.
\nReferences
[1] N. Ishii, K. Tagami, and S. Uchida, Chemosphere. 57. 953 (2004).
[2] N. Ishii, and K. Tagami, Radioisotopes. 52. 475 (2003).
[3] J. R. Lloyd, J. A. Cole, and L. E. Macaskie, J Bacteriol. 179, 2014 (1997)., The 2nd International Conference on Radioactivity in the Environment},
 title = {Transformation of insoluble technetium to soluble forms by bacterivorous ciliate},
 year = {2005}
}