Methane biogeochemical turnover constrains arsenic transformation in groundwater systems: Organic molecular signatures and microbial functional networks.

Arsenic (As) contamination of groundwater is primarily driven by microbially mediated redox processes and the dynamic evolution of dissolved organic matter (DOM). The influence of cycled methanogenesis and methane oxidation processes on As species transformation in geogenic As-contaminated groundwater, however, remain mechanistically elusive. In this study, quantitative relationships among DOM molecular characteristics, microbial functional networks, and As speciation were established using sediment microcosm experiments, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and metagenomic sequencing. The results indicate that rates of methanogenesis and methane oxidation are regulated by thermodynamic properties of DOM. Labile DOM promoted As(III) mobilization at a rate of 1.04 μg kg⁻¹ d⁻¹ through methyl-related metabolism. Remarkably, enhanced methane oxidation further elevated the As(III) generation rate to 3.30 μg kg⁻¹ d⁻¹, underscoring the accelerating effect of methane cycling on As release. In contrast, humified DOM decoupled the geochemical linkage between iron and As. Microbial succession governed the redox transitions, as the proliferation of methanogens substantially increased methane production (up to 7.23 mg kg⁻¹ d⁻¹), while methanotrophs enhanced oxidation rates from 94.99 to 190.76 mg kg⁻¹ d⁻¹. This microbial progression coupled sulfate and As(V) reduction through the up-regulation of key functional genes (dsrAB, arsC). Energy conversion during DOM biodegradation governs As migration stages. These findings highlight the interactive constraints on As speciation dynamics by molecular characteristics of DOM and microbial functional networks during methane biotransformation processes in groundwater systems. This research provides new mechanistic insights into As biogeochemical cycling in geogenic contaminated groundwater.