Growth substrate limitation enhances anaerobic arsenic methylation byParaclostridium bifermentansstrain EML

Abstract

Microbial arsenic (As) methylation is established as a detoxification process under aerobic conditions (converting arsenite) to monomethylarsonic acid (MMAs(V))) but proposed to be a microbial warfare strategy under anoxic conditions due to the toxicity of its main product monomethylarsonous acid (MMAs(III)). Here, we leveraged a recently isolated anaerobic As methylator, Paraclostridium bifermentans strain EML to gain insights into this process. Strain EML was inoculated into a series of media that consisted of systematic dilutions of Reinforced Clostridial Broth (RCB) supplied with 25 μM arsenite to assess the impact of growth substrate availability on As methylation. Growth curves evidenced the sensitivity of strain EML to arsenite, and As speciation analysis revealed the production of MMAs(III). Concentrations of MMAs(III) were found to be positively corelated with the RCB medium dilution, suggesting that growth substrate availability plays an important role in the regulation of MMAs(III) production. Further, reverse-transcription quantitative PCR evidenced arsM gene transcription increased with RCB dilution. Thus, growth substrate limitation is interpreted to enhance arsM gene expression and the associated anaerobic As methylation. We propose that growth substrate competition between microorganisms may also lead to an increase in As methylation, providing support for a microbial warfare function for this process.

Repression of Microbial Arsenite Uptake and Methylation by Dissolved Organic Carbon

“…While production of DMA and TMAO was roughly two and 4-fold greater in the 100% strength R2A compared to the 10% strength R2A, respectively, the more than 6-fold increase in OD 600 from 10% to 100% strength R2A led to a sharp decrease in biomass-normalized methylation efficiencies (Figure S11). This result is similar to recent findings in Qiao et al, where biomass-normalized arsenic methylation by Paraclostridium bifermentans strain EML was inhibited at higher media concentrations.…”

“…To our knowledge this is the first report to link carbon source quantity and quality to the control of As methylation through a CCR-like mechanism, but a close examination of recent studies reveals findings that are consistent with this theory. For example, Qiao et al showed that biomass-normalized As methylation by P. bifermentans decreased with higher media carbon concentrations, while Leon Ninin et al reported a plateau followed by a decrease in methylated As in paddy soil incubations as DOC concentrations increased. We acknowledge that the literature generally suggests that organic matter stimulates As methylation, ,, but these prior observations may be caused by enhanced biomass growth in carbon-rich systems that obscures the repression of methylation on a per-cell basis.…”