Microbial contributions to subterranean methane sinks

Sources and sinks of methane (CH₄) are critical for understanding global biogeochemical cycles and their role in climate change. A growing number of studies have reported that CH₄ concentrations in cave ecosystems are depleted, leading to the notion that these subterranean environments may act as sinks for atmospheric CH₄. Recently, it was hypothesized that this CH₄ depletion may be caused by radiolysis, an abiotic process whereby CH₄ is oxidized via interactions with ionizing radiation derived from radioactive decay. An alternate explanation is that the depletion of CH₄ concentrations in caves could be due to biological processes, specifically oxidation by methanotrophic bacteria. We theoretically explored the radiolysis hypothesis and conclude that it is a kinetically constrained process that is unlikely to lead to the rapid loss of CH₄ in subterranean environments. We present results from a controlled laboratory experiment to support this claim. We then tested the microbial oxidation hypothesis with a set of mesocosm experiments that were conducted in two Vietnamese caves. Our results reveal that methanotrophic bacteria associated with cave rocks consume CH₄ at a rate of 1.3–2.7 mg CH₄ · m^?2 · d^?1. These CH₄ oxidation rates equal or exceed what has been reported in other habitats, including agricultural systems, grasslands, deciduous forests, and Arctic tundra. Together, our results suggest that depleted concentrations of CH₄ in caves are most likely due to microbial activity, not radiolysis as has been recently claimed. Microbial methanotrophy has the potential to oxidize CH₄ not only in caves, but also in smaller‐size open subterranean spaces, such as cracks, fissures, and other pores that are connected to and rapidly exchange with the atmosphere. Future studies are needed to understand how subterranean CH₄ oxidation scales up to affect local, regional, and global CH₄ cycling.