IODINE AND MICROBIAL INTERACTIONS IN AN ORGANIC SOIL

Iodine-129 in groundwater discharging from a geologic disposal vault c ould accumulate in wetlands by chemical sorption onto low pH, highly o rganic solid surfaces or by direct or indirect microbial processes. Pr evious work indicated that saturation of anion sorption sites, microbi al toxicity, or swamping of the I reduction/oxidation reaction decreas ed the retention of a wetland sphagnum for iodine with increased iodin e porewater concentrations. Bog water and peat of an iodine-rich bog w ere studied to elucidate the role of micro-organisms in the retention and accumulation of iodine in a temperate wetland. Agar plate culture of a wide spectrum of microbes, including yeasts and moulds, with bog groundwater showed anaerobes to be more sensitive to high iodine conce ntrations than aerobes. Toxicity to anaerobes may occur at 110 mg Ilit re(-1) groundwater. Although iron-related and slime-forming bacteria w ere not affected at 2000 mg Ilitre(-1), iodine was toxic to sulphate-r elated bacteria (SRB), as indicated by Biological Activity Reaction Te sts. A 50% reduction in the SRB population occurred at groundwater con centrations of 75 mg Ilitre(-1). Microbe enumeration in iodine-rich bo g groundwater, using acridine orange staining, showed that the native wetland microbe population was sensitive to concentrations in excess o f 200 mg Ilitre(-1) groundwater. Three independent experiments confirm the toxicity threshold concentration for anaerobic and native microbe s is similar to 100 mg Ilitre(-1) groundwater. Iodine sorption to fres h peat was slightly slower and more limited under anoxic conditions. A utoclaving the peat, reinoculation following autoclaving and a sucrose addition all inhibited iodine sorption. These results suggest that mi crobes may only play a minor and indirect role in iodine sorption thro ugh the decomposition of organic matter.