METHYLATION OF ARSENIC BY ANAEROBIC MICROBIAL CONSORTIA ISOLATED FROMLAKE SEDIMENT

Anaerobic enrichment cultures, isolated from arsenic-contaminated lake sediment in the Canadian sub-arctic and grown in five selective media , methylated arsenate/arsenite to produce mono-, di- and tri-methyl ar senicals. The extent of methylation and methylarsenic species produced varied with the type of enrichment. Iron-reducing, manganese-reducing , sulfate-reducing and broad-spectrum anaerobic heterotrophic mixed cu ltures all produced methylarsenicals. Sulfate-reducing cultures produc ed higher concentrations of methylarsenicals (especially trimethyl spe cies) than iron-or manganese-reducers. There is evidence that several of the methylarsenicals, which were hydride-reactive at pH 6, were met hylarsenic(III) thiols. The organoarsenicals produced by enrichment cu ltures were the same as those detected in the porewater of the lake se diments used to initiate the enrichment cultures. Overall, this study demonstrates that microbes from anaerobic lake sediments can methylate (and demethylate) arsenic, a capability shared by manganese-, iron-, and sulfate-reducing microbial consortia.