Demethylation of methylarsonic acid by a microbial community

Abstract

Summary Arsenic is one of the most widespread environmental carcinogens and has created devastating human health problems worldwide, yet little is known about mechanisms of biotransformation in contaminated regions. Methylarsonic acid [MAs(V)], extensively utilized as an herbicide, is largely demethylated to more toxic inorganic arsenite, which causes environmental problems. To understand the process of demethylation of methylarsenicals, soil samples commonly used on Florida golf courses were studied. Several soil extracts were found to demethylate MAs(V) to inorganic arsenite [As(III)]. From these extracts, a bacterial isolate was capable of reducing MAs(V) to MAs(III) but not of demethylating to As(III). A second bacterial isolate was capable of demethylating MAs(III) to As(III) but not of reducing MAs(V). A mixed culture could carry out the complete process of reduction and demethylation, demonstrating that demethylation of MAs(V) to As(III) is a two-step process. Analysis of the 16S ribosomal DNA sequences of the two organisms identified the MAs(V)-reducing and the MAs(III)-demethylating isolates as belong to Burkholderia and Streptomyces species respectively. This is the first report of a novel pathway of degradation of a methylarsenical herbicide by sequential reduction and demethylation in a microbial soil community, which we propose plays a significant role in the arsenic biogeocycle.

A C⋅As lyase for degradation of environmental organoarsenical herbicides and animal husbandry growth promoters

“…MR1 and the MAs(III)-demethylating bacterium Streptomyces sp. MD1 from Florida golf course soil (14). Together, these two activities result in degradation of the herbicide MSMA.…”

“…The molecular mechanisms of MSMA reduction, the initial reaction of the two-step MAs(V) breakdown pathway, remains unknown (14). Reduction of MSMA has been also observed in plants such as rice (31).…”

“…The total DNA extracted from Bacillus sp. MD1 (14) was partially digested with HindIII (New England BioLabs) and ethanol-precipitated. The pelleted DNA was suspended in water and layered on a discontinuous sucrose gradient (10%, 20%, 30%, and 40%, wt/vol) containing 50 mM Tris, pH 8.0, 0.1 M EDTA, and 0.1 M NaCl, and centrifuged at 210,000 × g for 5 h at 17°C.…”

“…Although microbial degradation of environmental organoarsenicals has been documented (8,9,11,13), no molecular details of the reaction have been reported. We recently demonstrated that a microbial community in Florida golf course soil carries out a two-step pathway of MSMA reduction and demethylation (14). Here we report the isolation of an environmental methylarsonous acid [MAs(III)]-demethylating bacterium Bacillus sp.…”

Arsenic Methylation Dynamics in a Rice Paddy Soil Anaerobic Enrichment Culture

“…Rapid demethylation of methylated As has been proposed as an explanation for why environmental abundances of methylated arsenic remain low. 43 Arsenic demethylation pathways by aerobes have been characterized 44,45 and a C−As lyase has been identified. 43 Studies of As demethylation under anaerobic conditions have yielded conflicting results, however.…”