Journal: Geobiology

Article Title: Microbial contributions to subterranean methane sinks.

PMID/MRN: 27671735

Publication Date: 2016-Sep-27

Abstract:

Sources and sinks of methane (CH 4 ) are critical for understanding global biogeochemical cycles and their role in climate change. A growing number of studies have reported that CH 4 concentrations in cave ecosystems are depleted, leading to the notion that these subterranean environments may act as sinks for atmospheric CH 4 . Recently, it was hypothesized that this CH 4 depletion may be caused by radiolysis, an abiotic process whereby CH 4 is oxidized via interactions with ionizing radiation derived from radioactive decay. An alternate explanation is that the depletion of CH 4 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 CH 4 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 CH 4 at a rate of 1.3-2.7ÊmgÊCH 4 ÊáÊm -2 ÊáÊd -1 . These CH 4 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 CH 4 in caves are most likely due to microbial activity, not radiolysis as has been recently claimed. Microbial methanotrophy has the potential to oxidize CH 4 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 CH 4 oxidation scales up to affect local, regional, and global CH 4 cycling. © 2016 John Wiley & Sons Ltd.