Methanotrophic activity in a diffusive methane/oxygen counter-gradient in an unsaturated porous medium

Area of Science:

• Environmental microbiology
• Biogeochemistry
• Greenhouse gas mitigation

Background:

• Microbial methane oxidation is a key process controlling methane emissions from various ecosystems.
• Lack of suitable model systems hinders detailed study of methane oxidation under diffusional gradients.

Purpose of the Study:

• To develop and utilize a laboratory column system to investigate microbial methane oxidation in unsaturated porous media under diffusional methane/oxygen gradients.
• To quantify methane oxidation kinetics and microbial community response to changing oxygen availability.

Main Methods:

• Design and implementation of a laboratory column simulating unsaturated porous media with methane and oxygen counter-gradients.
• Analysis of steady-state gas profiles (CH4, CO2, O2) and stable carbon isotope ratios (¹³C/¹²C) of methane.
• Quantification of microbial populations using DAPI staining and kinetic modeling.

Main Results:

• Methane oxidation followed first-order kinetics with a high rate constant (approx. 30 h⁻¹) within a 15-cm active zone.
• Methanotrophic bacteria showed significant growth and a high capacity for methane oxidation, with rapid responses to oxygen fluctuations.
• Stable carbon isotope analysis confirmed microbial oxidation with a significant enrichment (approx. 7‰) and revealed large fractionation associated with the process.

Conclusions:

• The laboratory column is an effective model system for studying microbial methane oxidation under controlled diffusional gradients.
• Methanotrophic communities in unsaturated porous media exhibit high activity and adaptability to environmental changes, impacting methane emissions.
• Accurate estimation of isotopic fractionation factors is crucial for modeling methane cycling and understanding microbial contributions to greenhouse gas fluxes.