DEGRADATION OF METHYL-BROMIDE IN ANAEROBIC SEDIMENTS

Methyl bromide (MeBr) was anaerobically degraded in saltmarsh sediment s after reaction with sulfide. The product of this nucleophilic substi tution reaction was methanethiol, which underwent further chemical and bacterial reactions to form dimethyl sulfide. These two gases appeare d transiently during sediment incubations because they were metabolize d by methanogenic and sulfate-reducing bacteria. A second, less signif icant reaction of MeBr was the exchange with chloride, forming methyl chloride, which was also susceptible to attack by sulfide. Incubation of C-14-labeled methyl iodide as an analogue of MeBr resulted in the f ormation of (CH4)-C-14 and (CO2)-C-14 and also indicated that sulfate- reducing bacteria as well as methanogens metabolized the methylated su lfur intermediates. These results suggest that exposed sediments with abundant free sulfide, such as coastal salt-marshes, may constitute a sink for atmospheric MeBr.