Kinetics of Methanogenesis: From Laboratory Experiments to Natural Environments.

Methanogenesis is the final step of the degradation of organic matter and hence highly important in the global cycling of carbon. In studying the occurrence and significant of methane production, a key question is how to predict the rates of methanogenesis in natural environments. Current models predict the rates using the Monod equation, a rate law that emphasizes substrate concentrations. This study shows that, in addition to substrate concentrations, we also need to consider other environmental conditions and microbiological factors, including the availability of chemical energy, the heterogeneity of natural environments, and how methanogens adapt to natural environments.

According to the theory of geomicrobial kinetics, microbial rates are controlled by thermodynamic drivers, the differences between the energy available in the environment and the energy saved by microorganisms. To illustrate the thermodynamic control, we simulated numerically the laboratory experiments of methanogenesis by pure cultures and by natural communities. The simulation shows that methanogenesis often proceeds near thermodynamic equilibrium and, as a result, has small thermodynamic drives. The results also show that the thermodynamic factor places a significant limitation on the rates of methanogenesis, more significant than substrate concentrations.

Rates of methanogenesis are also influenced by the heterogeneity of natural environments. Heterogeneity describes the spatial variations in physicochemical characteristics of the environment. It impacts the rates of methanogenesis by affecting the distribution of substrates, chemical energy, and microorganisms. We simulate methanogenesis growing in porous media across a wide range of heterogeneity in pore volume. Our results demonstrate a significant impact of medium heterogeneity: increases in spatial heterogeneity tend to increase the rates of methanogenesis.

Computing the rates of methanogenesis requires a series of kinetic parameters, including the rate constant and half-saturation constant of methanogens. Taking acetoclastic methanogenesis as an example, we analyzed the kinetics of methanogen communities from lake sediments, peatlands, and marsh soils. The results show that the half-saturation constants of methanogen communities vary significantly with acetate concentrations of the environment: where acetate concentrations are large, the half-saturation constants are also large. We hypothesize that the variations of the half-saturation constant reflect how methanogens adapt to the availability of substrate in the environment. Hence in predicting the rates of methanogenesis, the half-saturation constants determined for pure cultures under controlled laboratory conditions may not be directly applicable to the environment of interest. Instead, we should determine the half-saturation constants of natural communities using samples recovered from the environment.