Microbial contributions to subterranean methane sinks

Abstract

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

Isotopic evidence for the migration of thermogenic methane into a sulfidic cave, Cueva de Villa Luz, Tabasco, Mexico

“…This difference may be due to the lack of atmospheric connectivity at the site in Movile Cave, which is mostly waterfilled. We did not observe CH 4 concentrations below 1.5 ppmv, as have been observed in many epigenic, non-sulfidic caves in Gibraltar, Australia, the United States, and Spain (Mattey et al, 2013;Fernandez-Cortes et al, 2015;McDonough et al, 2016;Webster et al, 2016;Lennon et al, 2016). Our results demonstrate that if in-situ CH 4 oxidation processes were operating in CVL, they were not strong enough to react all of the CH 4 in the collected samples.…”

Geochemical Fingerprinting of Rising Deep Endogenous Gases in an Active Hypogenic Karst System

“…A key reference point in the data interpretation is that the background atmosphere usually has around 1.8 ppm of CH 4 and its carbon and hydrogen isotopic composition (δ 13 C CH4 ≈ −47‰ VPDB, δ 2 H CH4 ≈ −100‰ VSMOW) is a product of inputs from an isotopically wide range of sources. The CH 4 concentration of cave air in epigenetic caves and, in general, in well-ventilated caves independently of their speleogenesis mechanisms are often depleted, confirming that subterranean environments may represent an overlooked sink for atmospheric CH 4 [23,[38][39][40][41][42][43][44] and, further, it is rapidly consumed in caves on time scales ranging from hours to days [23,39]. On the opposite case, underground air of some hypogene caves may contain unusually high levels of methane (up to 3%, e.g., Movile Cave) related to the action of chemoautotrophic bacteria [45], and others have moderate CH 4 concentrations, just above the atmospheric background, related to CH 4 outgassing from spring water in sulphuric acid hypogenic caves (e.g., <4 ppm CH 4 at Cueva Villa Luz [7]).…”

Radiolysis via radioactivity is not responsible for rapid methane oxidation in subterranean air

“…At the same time, two fragments of pitchblende (containing uraninite as a radiation source) were placed into the terrarium blank experiments with no artificially enhanced radiation to demonstrate the sensitivity of our setup to detect CH 4 -losses. In addition, we conducted (iii) two experiments with moist soils in the absence of added radon isotopes to assess the potential for environmental microorganisms (i.e., MOB) to remove CH 4 as has been demonstrated elsewhere by members of our research team [14,18]. The comparisons among experiments covered a common range of CH 4 concentration and thus only differed in the lengths of their time windows needed to lower the CH 4 concentration from the upper to the lower threshold (i.e.…”

“…The study by Haynes and Kebarle [16] determined that α-radiation has a slow effect on pure CH 4 and mixed hydrocarbon gas in the absence of air, making it difficult to extrapolate results to CH 4 in air in the presence of ions and radicals from heteromolecules. Some studies, however, have raised questions about the relative importance of abiotic CH 4 oxidation based on theoretical considerations of kinetics, the inability of α-radiation from metallic uranium and radon to trigger fast oxidation of CH 4 [15,18]. Laboratory and field experiments implicated MOB with the rapid decline in cave CH 4 concentrations [18], while isotopically uncharacterized radon was unable to remove CH 4 from air in an Australian cave [15].…”