81-11 Arsenic Demethylation and Detoxification by Methylotrophic Methanogens in Flooded Rice Paddies

Abstract:

Arsenic demethylation is a microbially mediated process that can decrease methylated arsenic in rice paddies, reducing the incidence of straighthead disorder in rice plants. Rice paddies are also a significant source of methane emissions globally, and recent work has established that methylotrophic methanogens contribute to arsenic demethylation. This contribution aims to expand our understanding of (i) the methylotrophic methyltransferase enzymes involved in arsenic demethylation, (ii) how methanogens detoxify toxic intermediates and products during arsenic demethylation, and (iii) whether complex organic matter can promote these important connections between methylotrophic methanogenesis and arsenic demethylation. At the microbial community level, we show that amending soil slurries with trimethylamine promoted non-specific couplings between arsenic demethylation and multiple methylotrophic methanogenesis pathways, and that several Methanosarcina metagenome assembled genomes (MAGs) were dominant in methyltransferase gene transcription (and by inference, arsenic demethylation). In response to toxic (methyl)arsenite intermediates produced during arsenic demethylation, we observed that Methanosarcina MAGs simultaneously transcribed multiple arsenic detoxification pathways in parallel with methylotrophic methanogenesis genes, with arsenite efflux and methylarsenite oxidation being the dominant detoxification pathways in terms of both community-level transcription and MAG-resolved couplings with arsenic demethylation. In rice growth experiments in the greenhouse, we demonstrate that amending natural organic matter (i.e., dried leaves) to paddy soil favored greater net arsenic demethylation in porewater and resulted in less methylated arsenic accumulation in rice grains relative to soils without organic matter amendment. Porewater correlations between dissolved methane and arsenic species were consistent with the model of methanogenesis-mediated arsenic demethylation releasing inorganic arsenic into the extracellular porewater environment. Interestingly, there was no increase in inorganic arsenic in porewater or rice grains grown in organic matter amended soils, and rice root plaque extractions are underway to determine if organic matter amendment resulted in greater inorganic arsenic adsorption to rice roots. Across multiple scales, our findings highlight new advances in the microbial connections between arsenic demethylation and methanogenesis with implications for food security and climate change.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.