Demethylation of the Antibiotic Methylarsenite is Coupled to Denitrification in Anoxic Paddy Soil

Abstract

Arsenic (As) biomethylation is an important component of the As biogeochemical cycle, which produces methylarsenite [MAs(III)] as an intermediate product. Its high toxicity is used by some microbes as an antibiotic to kill off other microbes and gain a competitive advantage. Some aerobic microbes have evolved a detoxification mechanism to demethylate MAs(III) via the dioxygenase C−As lyase ArsI. How MAs(III) is demethylated under anoxic conditions is unclear. We found that nitrate addition to a flooded paddy soil enhanced MAs(III) demethylation. A facultative anaerobe Bacillus sp. CZDM1 isolated from the soil was able to demethylate MAs(III) under anoxic nitratereducing conditions. A putative C−As lyase gene (BcarsI) was identified in the genome of strain CZDM1. The expression of BcarsI in the As-sensitive Escherichia coli AW3110 conferred the bacterium the ability to demethylate MAs(III) under anoxic nitrate-reducing condition and enhanced its resistance to MAs(III). Both Bacillus sp. CZDM1 and E. coli AW3110 harboring BcarsI could not demethylate MAs(III) under fermentative conditions. Five conserved amino acid resides of cysteine, histidine, and glutamic acid are essential for MAs(III) demethylation under anoxic nitrate-reducing conditions. Putative arsI genes are widely present in denitrifying bacteria, with 75% of the sequenced genomes containing arsI, also possessing dissimilatory nitrate reductase genes narG or napA. These results reveal a novel mechanism in which MAs(III) is demethylated via ArsI by coupling to denitrification, and such a mechanism is likely to be common in an anoxic environment such as paddy soils and wetlands.

Methanogens-Driven Arsenic Methylation Preceding Formation of Methylated Thioarsenates in Sulfide-Rich Hot Springs

“…in the enrichment cultures and M. thermophila were able to methylate arsenite to MMA but could not substantially further methylate MMA to DMA nor demethylate DMA or MMA, which is contrary to the observations in paddy soils, where methanogenic enrichment cultures were reported to demethylate methylated arsenic species. ,, MMA(III) was reported to be demethylated to inorganic arsenic by some microorganisms via a C–As bond lyase, ArsI, under aerobic conditions coupled to denitrification in anoxic paddy soils . To date, ArsI in methanogens has not been found yet, but Chen et al found that 13 C-DMA was partially transformed into 13 C–CH 4 in a methanogens-bearing experimental system .…”

“…14,52,73 MMA(III) was reported to be demethylated to inorganic arsenic by some microorganisms via a C−As bond lyase, ArsI, under aerobic conditions 74 coupled to denitrification in anoxic paddy soils. 75 To date, ArsI in methanogens has not been found yet, but Chen et al found that 13 C-DMA was partially transformed into 13 C−CH 4 in a methanogens-bearing experimental system. 14 Demethylation of DMA occurring under methanogenic conditions was also observed by other authors.…”

“…The sole methanogens in the enrichment cultures and the pure strain of M. thermophila apparently did not demethylate methylated arsenic by themselves. To sum up, the demethylation of DMA and MMA in methanogens enrichment cultures observed in previous studies could be catalyzed by multiple methanogens or completed with the aid of some other microorganisms, facilitating the preliminary step for demethylation, e.g., DMA or MMA reduction, 76,77 or coupled with denitrification, 75 which were not the case of the experiments in this study.…”

Methylotrophic methanogens and bacteria synergistically demethylate dimethylarsenate in paddy soil and alleviate rice straighthead disease

“…Inoculants were precultured to the mid-log phase before being introduced to the soil. Soil solution was collected at different time points during 15 days of incubation for the determination of As species [ 25 ].…”

“…Opposite to the process of As methylation, methylated As compounds can be demethylated by some microorganisms. It has been shown that MMAs(III) is demethylated by the dioxygenase C−As lyase ArsI in some bacteria [ 24 , 25 ]. DMAs can also be demethylated, although the mechanism remains unclear.…”