Microbial Contribution to Global Iodine Cycling: Volatilization, Accumulation, Reduction, Oxidation, and Sorption of Iodine

Abstract

Iodine is an essential trace element for humans and animals because of its important role as a constituent of thyroid hormones. If the anthropogenic iodine-129 ( 129 I, half-life: 1.6×10 7 years), which is released from nuclear facilities into the environment and has a long half-life, participates in the biogeochemical cycling of iodine, it potentially accumulates in the human thyroid gland and might cause thyroid cancer. Therefore, it is necessary to obtain better information on the behavior of iodine in the environment for accurate safety assessments of 129 I. Major pathways of iodine cycling are the volatilization of organic iodine compounds into the atmosphere, accumulation of iodine in living organisms, oxidation and reduction of inorganic iodine species, and sorption of iodine by soils and sediments. Considerable geochemical evidence has indicated that these processes are influenced or controlled by microbial activities, although the precise mechanisms involved are still unclear. This review summarizes current knowledge on interactions between microorganisms and iodine, with special emphasis on newly isolated bacteria possibly contributing to the cycling of iodine on a global scale.

Surficial redistribution of fallout131iodine in a small temperate catchment

“…Instead, we attribute the increase in subsurface 137 Cs inventory to its translocation from a deeper reservoir (27) during the seasonal transition from winter dormancy to spring growth. Iodine is a micronutrient in its own right, widely implicated in soil microbial metabolism (29,30). While the enrichment pattern or process may not be identical for iodine as for cesium, as the former is derived from a new, atmospheric source and the latter from an old, subsurface source, we propose that the active enrichment of iodine in surface soils or root mat is not unexpected (31).…”

Halocarbon emissions and sources in the equatorial Atlantic Cold Tongue

“…CH 3 I and CH 2 I 2 has been observed in the field and the laboratory (Manley and Dastoor, 1988;Amachi et al, 2001;Fuse et al, 2003;Amachi, 2008). Large uncertainties regarding the production of halocarbons in the ocean remain.…”

Biogeochemical Considerations Related To The Remediation Of I-129 Plumes

“…Oceans are considered the primary source of iodine; accordingly, our understanding of biological transformations of iodine is derived largely from those processes that have been studied in marine environments. Based on thermodynamic considerations iodate/iodide levels in seawater are in disequilibrium, with iodide found at levels much higher than would be expected, especially in certain surface waters, coastal and estuary regions, deep oxygenated water, and porewater of marine sediments where I -concentration up to 40 µg L -1 have been measured (Amachi, 2008;Farrenkopf et al, 1997). The conversion of IO 3 -to I -is thought to be catalyzed primarily via microbial activity, perhaps by the enzyme nitrate reductase or release of I -from C-I or N-I bonds upon decomposition of organic matter (Farrenkopf et Organic iodine species, including CH 3 I, CH 2 ClI, CH 2 I 2 and CH 3 CH 2 I, are found at relatively low concentrations (< 5% of total I) in the open ocean, but concentrations between 5-40% of total dissolved iodine are not uncommon in coastal and estuary systems (Schwehr and Santschi, 2003;Wong and Cheng, 1998;Hou et al, 2009b).…”

“…Microorganisms can catalyze the reduction, oxidation, and alkylation of iodine (Amachi, 2008;Denham et al, 2009). Within the anoxic portion of the Ringold formation or within anaerobic pockets of the heterogeneous Hanford formation, the potential for iodate reduction to iodide is high since this activity has been readily detected among microorganisms involved in denitrification and iron and sulfur reduction (Amachi et al, 2007a;Councell et al, 1997;Farrenkopf et al, 1997).…”