Iodide-oxidizing bacteria (IOB), which oxidize iodide (I-) to molecular iodine (I2), were isolated from iodide-rich (63 microM to 1.2 mM) natural gas brine waters collected from several locations. Agar media containing iodide and starch were prepared, and brine waters were spread directly on the media. The IOB, which appeared as purple colonies, were obtained from 28 of the 44 brine waters. The population sizes of IOB in the brines were 10(2) to 10(5) colony-forming units (CFU) mL(-1). However, IOB were not detected in natural seawaters and terrestrial soils (fewer than 10 CFU mL(-1) and 10(2) CFU g wet weight of soils(-1), respectively). Interestingly, after the enrichment with 1 mM iodide, IOB were found in 6 of the 8 seawaters with population sizes of 10(3) to 10(5) CFU mL(-1). 16S rDNA sequencing and phylogenetic analyses showed that the IOB strains are divided into two groups within the alpha-subclass of the Proteobacteria. One of the groups was phylogenetically most closely related to Roseovarius tolerans with sequence similarities between 94% and 98%. The other group was most closely related to Rhodothalassium salexigens, although the sequence similarities were relatively low (89% to 91%). The iodide-oxidizing reaction by IOB was mediated by an extracellular enzyme protein that requires oxygen. Radiotracer experiments showed that IOB produce not only I2 but also volatile organic iodine, which were identified as diiodomethane (CH2I2) and chloroiodomethane (CH2ClI). These results indicate that at least two types of IOB are distributed in the environment, and that they are preferentially isolated in environments in which iodide levels are very high. It is possible that IOB oxidize iodide in the natural environment, and they could significantly contribute to the biogeochemical cycling of iodine.

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To estimate radiocesium intake due to eating mushrooms, about 100 samples belonging to 11 species were analyzed to establish representative values for 137Cs and 40K in common edible mushrooms available in food markets. Concentration ranges were <0.047-39 Bq kg(-1) (wet wt) for 137Cs and 30-210 Bq kg(-1) (wet wt) for 40K. The median concentrations were 1.3 Bq kg(-1) (wet wt) for 137Cs and 97 Bq kg(-1) (wet wt) for 40K. The 137Cs concentrations in cultivated mushrooms were markedly lower than those in wild mushrooms. The annual intake of 137Cs per person through mushrooms was calculated (using analytical results and food consumption data in Japan) to be 6.0 Bq for 137Cs, which is about 32% of the total dietary intake of this nuclide. The effective dose equivalent of 137Cs through mushroom was estimated to be 7.7 x 10(-8) Sv (range estimated from the standard deviation: 3.0 x 10(-8) - 2.0 x 10(-7)).