Subterranean microbial oxidation of atmospheric methane in cavernous tropical karst

- Subterranean tropical karst sequesters atmospheric CH4 via methanotrophy.
- Steeply exposed karst in Vietnam may oxidize ca. 15 × 107 kg CH4 annually.
- Atmospheric CH4 subsidizes metabolism of subterranean methanotrophs.
- CH4 sink in tropical karst should be included in global greenhouse gas balance.

Subterranean methanotrophy is a potentially important but overlooked sink for the atmospheric greenhouse gas methane (CH4). This study documents a microbial CH4 sink in tropical subterranean karst cavities in Vietnam's northern karst province where porosity, steep topography, and scarce soil and vegetation cover foster the exchange of subterranean air with the atmosphere. Our data are based on (i) surveys of CH4, carbon dioxide, and radon concentrations in the air of 11 caves, (ii) in situ mesocosm experiments in caves, as well as (iii) laboratory mesocosm measurements using sediment and rock from caves. The extent of CH4 depletion in cave air depends on the ventilation rate and the availability of moisture to provide a habitat for CH4-oxidizing bacteria, both of which are seasonally variable in northern Vietnam and in part depend on monsoonal activity. Mesocosm experiments using fresh versus sterilized rock and sediment confirmed the role of microbial methanotrophy towards uptake of CH4 from cave air. Our results also suggest that within-cave heterogeneity of environmental variables like salinity may affect rates of CH4 oxidation. We conservatively estimate that 150,000 metric tons of atmospheric CH4 are microbially oxidized annually in the ~ 29,000 km2 of Vietnamese tropical karst, which would compensate for ~ 7% of Vietnam's agricultural CH4 emissions from rice farming and livestock. Future studies estimating the global fluxes of the atmospheric greenhouse gas CH4 should consider subterranean karst as a potentially important CH4 sink.

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