MICROBIAL TRANSFORMATION OF INORGANIC AND ORGANIC ARSENIC

Microorganisms readily metabolize arsenic impacting its form, speciation, mobility and toxicity. The microbial processes include oxidation, reduction, methylation, and demethylation. Over the past decade, a plethora of phylogenetically diverse arsenic-metabolizing prokaryotes have been described including chemolithoautotrophic arsenate respiring and arsenite-oxidizing species. Both inorganic (e.g., arsenate, arenite) and organic arsenicals can be metabolized. The transformations involve specific microbial species, pathways, and enzymes. While methylation has been demonstrated in fungi (aerobic) and methanogens (anaerobic), the biochemical pathway for prokaryotes remains to be fully elucidated. The mechanisms of inorganic arsenic transformation are better understood. The oxidation of arsenite is used for both detoxification and energy generation and involves an arsenite oxidase. Arsenate reduction has been exploited for both arsenic resistance and energy generation and represents two distinct mechanisms involving to very different arsenate reductases. The microbial transformation of organoarsenicals can be complex as the different side groups may be attacked resulting in a variety of products. For example, the intermediates generated by the microbial degradation of 3-nitro-4-hydroxy phenylarsonic acid (roxarsone) include 3-amino-4-hydroxy phenylarsonic acid, 4-hydroxy phenylarsonic acid, and inorganic arsenic (i.e., arsenate). The generation of the different intermediates appears to involve different microbes with specific pathways. The environmental significance of all this is that diverse microbial communities can be supported by the arsenic transformations (e.g., oxidation/reduction) as has been recently demonstrated for Mono Lake and Searles Lake, CA. While arsenic metabolizing microbes have detected in arsenic-contaminated aquifers their role in the mobilization and transport of arsenic in subsurface environments remains to be fully elucidated.