Microbial contributions to subterranean methane sinks

Abstract

Understanding the sources and sinks of methane (CH4) is critical for predicting and managing global biogeochemical cycles. Recent studies have reported that CH4 concentrations in cave ecosystems are depleted and that these subterranean environments may act as a daily sinks for atmospheric CH4. It has been hypothesized that this CH4 depletion may be caused by radiolysis, an abiotic process whereby CH4 is oxidized via interactions with ionizing radiation derived from radon decay. Alternatively, the depletion of CH4 concentrations 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 experimental results to support this claim. We tested the microbial oxidation hypothesis in a set of mesocosm experiments that were conducted in Vietnamese caves. Our results reveal that methanotrophic bacteria associated with cave rocks consume CH4 at a rate of 1.33 - 2.70 mg CH4 {middle dot} m-2 {middle dot} 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. As such, microbial methanotrophy has the potential to significantly oxidize CH4 in caves, but also 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 regional and global CH4 cycling.