Isotopic Labeling Reveals Microbial Methane Oxidation Coupled to Fe(III) Mineral Reduction in Sediments from an As-Contaminated Aquifer

Abstract

Although arsenic (As) groundwater contamination in South and Southeast Asia is a threat to human health, mechanisms of its release from sediment to groundwater are still not fully understood. In many aquifers, Fe(III) minerals are often the main hosting phases for As and their stability is crucial for As mobility. Recently, a new mechanism for As mobilization into groundwater was proposed with methane (CH4) serving as an electron donor for microbially mediated reductive dissolution of As-bearing Fe(III) minerals. To provide unequivocal evidence for the occurrence of Fe(III)-coupled methanotrophy, we incubated sediments from an As-contaminated aquifer in Hanoi (Vietnam) anoxically with isotopically labeled 13CH4. Up to 35% of the available Fe(III) was reduced within 232 days with simultaneous production of 13CO2 demonstrating anaerobic oxidation of 13CH4 with Fe(III) as the electron acceptor. The microbial community at the end of the incubation was dominated by archaea affiliating with Candidatus Methanoperedens, implying its involvement in Fe(III)-dependent CH4 oxidation. These results suggest that methanotrophs can contribute to dissolution of As-bearing Fe(III) minerals, which eventually leads to As-release into groundwater.