Uncover co-evolution of arsenic transformation and methanogens in landfills

Landfills are a significant source of arsenic (As) pollution, and the toxicity of As is heavily influenced by its chemical speciation. While As methylation serves as a crucial detoxification mechanism, demethylation can remobilize As. Methanogens play a central role in the degradation of organic matter in landfills; however, research on their influence on As transformation is limited, and little is known about this interaction. This study simulated a landfill saturation zone (LSZ) scenario to explore methanogen-mediated As transformation and elucidate the coupling mechanisms between carbon metabolism and As cycling. Results revealed that methanogens were the driving force behind the conversion of methylated As (MeAs) species to inorganic As (iAs). Suppressing methanogen activity with the specific inhibitor 2-bromoethanesulfonate (BES, 15 mmol kg-1) resulted in a 1.19-fold increase in MeAs levels compared to the control (uninhibited methanogens), indicating impaired demethylation of dimethylarsenic (DMAs) to monomethylarsenic (MMAs) and iAs. Concurrently, the absolute abundances of mcrA and arsI genes were also significantly lower at this BES concentration than in the control. Analysis based on KEGG pathway annotation identified the methylotrophic methanogenesis module (M00356) as a key pathway for methanogen-driven As demethylation, facilitating the conversion of MeAs to iAs and counteracting As methylation processes.