Latin American Consensus: Children Born Small for Gestational Age

72-87Cuatrimestra

Methane Emission from Enteric Fermentation: Methanogenesis and Fermentation

Rumen fermentation of carbohydrates plays a fundamental role in ruminant metabolism as the main source of energy. Acetic, propionic and butyric acids (namely, volatile fatty acids, VFA) are the main products of the rumen fermentation of structural and nonstructural carbohydrates con- tained in the ruminant’s diet. The metabolic pathways involved in VFA production are strictly linked to methane emission, because hydrogen is actively produced during the fermentation of structural carbohydrates, and it is rapidly metabolised by methanogens, in order to maintain the optimal thermodynamic condition for the metabolism of the microbe consortium in the rumen. Hydrogen plays also a fundamental role in the maintenance of the equilibrium among VFA pathways and in their interconversion. In this chapter, after a brief chemical description of dietary carbohydrates, individual VFA pathways are described in order to put in evidence the thermodynamic control points of each pathway and the production of energy and reductive equivalent. Then, the relationship between hydrogen, VFA and methane production has been reviewed, considering the role of some dietary factors, the substrate competition between different meta- bolic pathways and the models of VFA estimation

Thermodynamic Driving Force of Hydrogen on Rumen Microbial Metabolism: A Theoretical Investigation

Hydrogen is a key product of rumen fermentation and has been suggested to thermodynamically control the production of the various volatile fatty acids (VFA). Previous studies, however, have not accounted for the fact that only thermodynamic near-equilibrium conditions control the magnitude of reaction rate. Furthermore, the role of NAD, which is affected by hydrogen partial pressure (PH 2), has often not been considered. The aim of this study was to quantify the control of PH 2 on reaction rates of specific fermentation pathways, methanogenesis and NADH oxidation in rumen microbes. The control of PH 2 was quantified using the thermodynamic potential factor (FT), which is a dimensionless factor that corrects a predicted kinetic reaction rate for the thermodynamic control exerted. Unity FT was calculated for all glucose fermentation pathways considered, indicating no inhibition of PH 2 on the production of a specific type of VFA (e.g., acetate, propionate and butyrate) in the rumen. For NADH oxidation without ferredoxin oxidation, increasing PH 2 within the rumen physiological range decreased FT from unity to zero for different NAD+ to NADH ratios and pH of 6.2 and 7.0, which indicates thermodynamic control of PH 2. For NADH oxidation with ferredoxin oxidation, increasing PH 2 within the rumen physiological range decreased FT from unity at pH of 7.0 only. For the acetate to propionate conversion, FT increased from 0.65 to unity with increasing PH 2, which indicates thermodynamic control. For propionate to acetate and butyrate to acetate conversions, FT decreased to zero below the rumen range of PH 2, indicating full thermodynamic suppression. For methanogenesis by archaea without cytochromes, FT differed from unity only below the rumen range of PH 2, indicating no thermodynamic control. This theoretical investigation shows that thermodynamic control of PH 2 on individual VFA produced and associated yield of hydrogen and methane cannot be explained without considering NADH oxidation.</p