Bacteria mediate methylation of iodine in marine and terrestrial environments

Abstract

Methyl iodide (CH(3)I) plays an important role in the natural iodine cycle and participates in atmospheric ozone destruction. However, the main source of this compound in nature is still unclear. Here we report that a wide variety of bacteria including terrestrial and marine bacteria are capable of methylating the environmental level of iodide (0.1 microM). Of the strains tested, Rhizobium sp. strain MRCD 19 was chosen for further analysis, and it was found that the cell extract catalyzed the methylation of iodide with S-adenosyl-L-methionine as the methyl donor. These results strongly indicate that bacteria contribute to iodine transfer from the terrestrial and marine ecosystems into the atmosphere.

FIG. 1

CH₃I production by resting cells of Rhizobium sp. strain MRCD 19 (●) and A. macleodii IAM 12920 (○) at various concentrations of iodide. Means and standard deviations of triplicate samples are shown, but the error bars are hidden under the symbols. Of the 12 data points depicted, only 1 point showed a standard deviation of more than 10%.

FIG. 2

The iodide-methylating reaction by cell extracts of Rhizobium sp. strain MRCD 19 is dependent on the concentrations of KI (A) and SAM (B). I⁻-methylating rates (picomoles per minute per milligram of protein) and their reciprocals were plotted against KI concentrations and their reciprocals (A) and SAM concentrations and their reciprocals (B). The reactions were carried out in potassium phosphate buffer (pH 7.0) containing KI, SAM, approximately 30 kBq of K¹²⁵I, and 50 mg of cell extract. CH₃I was collected and quantified as described in Materials and Methods. The concentration of KI was varied from 0.05 to 50 mM in the presence of 0.5 mM SAM (●), and the concentration of SAM was varied from 0.005 to 0.5 mM in the presence of 10 mM KI (○). Means of triplicate samples are shown.