Microbial Influences on the Mobility and Transformation of Radioactive Iodine in the Environment

Abstract

Long-lived radioactive iodine (129I, half-life: 1.57 × 107 y) has been released into the environment from nuclear fuel reprocessing plants. 129I may also be released from ground storage of nuclear waste. Given its long half-life, a better understanding of the behavior of iodine in the environment is necessary to ensure the safety of humans and the health of the environment. In this report, we summarize our recent results and new experimental data about microbial influences on the mobility and transformation of iodine. Microbial volatilization of organic iodine was observed in soil slurries and seawater samples, and various species of aerobic bacteria were considered to play a significant role through methylation of iodide (I-) to form methyl iodide (CH3I). The volatilization of iodine was also found in iodide-rich natural gas brine water, where iodide concentration is approximately 2,000 times higher than that in seawater. In this case, however, a significant amount of molecular iodine (I2) was produced together with organic iodine compounds. Iodide-oxidizing bacteria, which oxidize iodide to I2, were isolated from seawater and natural gas brine water. Phylogenetically, they were divided into two groups within the a-subclass of the Proteobacteria (Roseovarius sp. and unidentified bacteria), and they produced not only I2 but also diiodomethane (CH2I2) and chloroiodomethane (CH2ClI). Iodide-accumulating bacteria, which accumulate iodide to concentrations 5,500-fold over that of the medium, were also isolated from marine sediment. They were closely related to Arenibacter troitsensis, and iodide uptake was mediated by an active transport system. Our results suggest that the fate of iodine can be affected by microorganisms, particularly by bacteria, through processes such as volatilization, oxidation, and accumulation.