Microbial participation in iodine volatilization from soils

The roles of microorganisms in iodine volatilization from soils were studied. Soils were incubated with iodide ion (I-), and volatile organic iodine species were determined with a gas chromatograph. Iodine was emitted mainly as methyl iodide (CH3I), and CH3I emission was sometimes enhanced by the addition of glucose. Soils were then incubated with a radioactive iodinetracer (I-125), and radioiodine emitted from soils was determined. The emission of iodine was enhanced in the presence of yeast extract but was inhibited by autoclaving of soils. The addition of streptomycin and tetracycline, antibiotics that inhibit bacterial growth, strongly inhibited iodine emission, while a fungal inhibitor cycloheximide caused little effect. Forty bacterial strains were randomly isolated from soils, and their capacities for volatilizing iodine were determined. Among these, 14 strains volatilized significant amounts of iodine when they were cultivated with iodide ion. Phylogenetic analysis based on 16S ribosomal DNA sequences showed that these bacteria are widely distributed through the bacterial domain. Our results suggest that iodine in soils is methylated and volatilized as CH3I by the action of soil bacteria and that iodine-volatilizing bacteria are ubiquitous in soil environments. The pathway of iodine volatilization by soil bacteria should be important for understanding the biogeochemical cycling of iodine as well as for the assessment of long-lived radioactive iodine (I-129) in the environment.