Christina Lam, Steph Slowinski, Saraswati Saraswati, Christina Smeaton, Philippe Van Cappellen (2022). Identifying methanogenic pathways using isothermal microcalorimetry. Proceedings of the GWF Annual Open Science Meeting, May 16-18, 2022.

Petroleum hydrocarbons (PHCs) are major environmental contaminants which can leach from contaminated soils to surrounding surface water or groundwater sources. Methanogenesis generates methane (CH4) and represents a key process involved in PHC biodegradation at contaminated sites under anoxic and electron acceptor-limited conditions. Hydrocarbon fermentation precedes methanogenesis, producing mainly hydrogen and acetate, which then act as reactants for the two main methanogenic pathways: hydrogen-based methanogenesis (HBM) and acetate-based methanogenesis (ABM). It has been proposed that acetate buildup may inhibit PHC biodegradation at contaminated sites by causing the ABM and hydrocarbon fermentation reactions to become thermodynamically unfavorable. That is, high acetate concentrations result in Gibbs reaction energies > 0. We conducted soil batch experiments with commercial peat amended with two acetate concentrations, 0.1 mM and 1 mM. An isothermal microcalorimeter was used to measure heat fluxes released during the incubations of peat amended with two acetate concentrations. These heat fluxes are related to the reaction enthalpies with opposite signs for HBM (endergonic) and ABM (exergonic). The heat fluxes and the gas phase concentrations of CO2 and CH4 suggested that ABM switched to HBM over the course of the experiment. As expected, the relative differences in the heat fluxes in the soils amended with two different acetate concentrations imply that the higher acetate concentration (1 mM) results in lower ABM rates overall. These experimental results provide preliminary evidence of the hypothesized thermodynamic inhibition of ABM by acetate buildup. They also demonstrate the potential for microcalorimetry as a tool for identifying methanogenic reaction pathways at contaminated sites. In this presentation, we will show our experimental results and discuss the implications of our study for the natural attenuation of PHCs at contaminated soil sites.

First Author: Christina Lam, Ecohydrology Research Group, Department of Earth and Environmental Sciences, University of Waterloo, Canada
Additional Authors: Steph Slowinski (1), Saraswati Saraswati (1), Christina Smeaton (2), Philippe Van Cappellen (1); (1) Ecohydrology Research Group, Department of Earth and Environmental Sciences, University of Waterloo, Canada; (2) School of Science and the Environment, Grenfell Campus, Memorial University of Newfoundland, Canada