Microbial contributions to subterranean methane sinks

Area of Science:

• Biogeochemistry
• Microbial Ecology
• Environmental Science

Background:

• Methane (CH₄) sources and sinks are crucial for global biogeochemical cycles and climate change.
• Cave ecosystems often show depleted CH₄ concentrations, suggesting they may be CH₄ sinks.
• Two hypotheses explain this depletion: abiotic radiolysis and biotic oxidation by methanotrophic bacteria.

Purpose of the Study:

• To investigate the cause of methane depletion in cave ecosystems.
• To evaluate the radiolysis hypothesis for methane loss.
• To assess the role of microbial methanotrophy in subterranean methane cycling.

Main Methods:

• Theoretical kinetic analysis of the radiolysis hypothesis.
• Laboratory experiments to validate radiolysis constraints.
• Mesocosm experiments in Vietnamese caves to quantify microbial methane oxidation.

Main Results:

• Radiolysis is a kinetically constrained process unlikely to cause rapid methane loss in caves.
• Methanotrophic bacteria in cave rocks consume methane at rates of 1.3-2.7 mg CH₄ · m^-2 · d^-1.
• Observed methane oxidation rates in caves are comparable to or exceed those in other terrestrial ecosystems.

Conclusions:

• Microbial oxidation by methanotrophic bacteria is the primary driver of methane depletion in caves.
• Subterranean microbial methanotrophy can significantly impact atmospheric methane concentrations.
• Further research is needed to understand the broader implications of subterranean methane oxidation on global methane budgets.