Improved prediction of methane production requires consideration of NADH oxidation

Carbohydrates in the rumen are fermented to various volatile fatty acids (VFA). This process also yields NADH (a cofactor carrying electrons). The type of VFA formed (e.g., acetate, propionate and butyrate), determines the quantity of NADH oxidized back to NAD+ via hydrogen production, which is thermodynamically inhibited at elevated hydrogen partial pressure. Many methanogenic archaea utilize hydrogen to reduce carbon dioxide to methane, and this keeps hydrogen partial pressure at a low level, enabling NADH oxidation in bacteria. We quantified the control of hydrogen partial pressure on reaction rates of specific fermentation pathways, methanogenesis and NADH oxidation in rumen microbes using the thermodynamic potential factor. This is a dimensionless factor that corrects a predicted kinetic reaction rate for the thermodynamic control exerted.

Our results indicate that fermentation of glucose to specific types of VFA, as well as rumen methanogenesis by archaea without cytochromes (which comprise most of the rumen methanogenic population), is not affected by hydrogen partial pressure within the rumen physiological range. However, oxidation of NADH does appear to be controlled by hydrogen partial pressure. Thus, thermodynamic control of hydrogen partial pressure on individual VFA produced and on associated yield of hydrogen and methane cannot be explained without considering NADH oxidation. To improve the accuracy of prediction of methane and VFA production by rumen fermentation models, representation of the key role of the NAD⁺ to NADH ratio is required.

Please find a link to the PLOS ONE article: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0161362

For more information, please contact henk.vanlingen@wur.nl or jan.dijkstra@wur.nl