Thermodynamic and kinetic control of methane emissions from ruminants.

Abstract

Abstract CH4 emissions occur directly from animal digestion (enteric) and from animal waste that is stored under anaerobic conditions. In both regards, CH4 emissions depends on kinetic and thermodynamic factors. With kinetic control, the profile of products formed depends on the relative rates of the different competing reactions. In turn, the rates of reactions depend on substrate concentrations and enzyme activities, and these enzyme activities depend on microbial growth or enzyme synthesis. With thermodynamic control, which pathway branches are available and the direction of metabolite flow depends on the concentrations of reactants and products. Biologists have focused on controlling the kinetic elements of fermentation such as enzyme function, microbial activity, gene expression or provision of substrates. However, fermentation is often controlled by thermodynamics. In chemistry, thermodynamics is quantified using Gibbs energy calculations. Whether or not a reaction can proceed spontaneously in the forward direction is represented by the change in Gibbs energy (ΔG), which can be calculated based on the ratio of products and reactants in the system. Using this calculation, a strongly negative ΔG indicates that a reaction could proceed strongly in the forward direction without the addition of energy to the system. A strongly positive value of ΔG indicates the reaction cannot proceed in the forward direction without the addition of energy to the system, and it may even run in the reverse direction. The analysis of thermodynamics in different situations in the rumen or manure storage facility can identify when CH4 may be controlled kinetically by affecting the rates of reactions, or thermodynamically by affecting substrate or product concentrations.