PSII-42 Kinetics of methanogenesis and acetogenesis under normal rumen conditions

Abstract

Abstract Researchers are investigating numerous methods to reduce enteric methane emissions and improve the sustainability of US animal agriculture. Methane emissions from the rumen depend on principles of kinetics, where the profile of fermentation products depends on relative rates of competing reactions, substrate concentrations or enzymatic activities. We investigated the kinetics of methanogenesis and related VFA production under increasing concentrations of infused hydrogen. The H2 (0, 0.3, 0.5, 1, 2, 4, 6, 8, 10 mL) was added to Hungate tubes containing 10 mL of rumen fluid with 10 mL of CO2 headspace gas. Then H2, CH4 and CO2, and volatile fatty acid (VFA) concentrations were measured after 0 and 1 h incubation at 39 °C. Data were fit to a non-linear Michaelis-Menten (M-M) model to estimate Vmaxand Km. For initial rumen fluid without added H2gas, there was an increase in CH4, slight decrease in H2, and disappearance of acetate and propionate. When H2was added, H2disappearance increased linearly, and methane, acetate and propionate increased non-linearly. For example, methanogenesis Vmax was 0.209 mM 10mL-1minute-1 with Km of 67.2 mM (RMSPE = 0.09, n = 9). The ratio of appearance of CH4 to disappearance of H2 was 0.15, showing 60% of H2 disappearance was accounted for by methanogenesis. The H2 addition shifted fermentation from VFA degradation to synthesis via saturable pathways (e.g. at Vmax). Results show that rumen fluid has enzyme activity such that as H2 gas is made available to rumen fluid, most H2is used for methane and VFA synthesis, and rates and proportions of H2 utilization for different pathways can be measured in vitro.