Consequences of inhibiting methanogenesis on the biohydrogenation of fatty acids in bovine ruminal digesta

Abstract

Methanogenesis and the biohydrogenation of unsaturated fatty acids by ruminal microorganisms both utilize molecular (H2) or metabolic (2 H) hydrogen. The purpose of this study was to investigate the interdependence of the two metabolic activities, the first of which results in the emission of a potent greenhouse gas, the second causing ruminant products to have a low content of health-promoting unsaturated fatty acids. in vitro incubations were conducted using ruminal digesta taken post mortem from three beef cattle receiving a barley-based concentrate diet (C) and three grazing fresh grass (G). Inhibitors of methanogenesis, bromoethanesulfonate (BES), sodium nitrate and sodium nitrite, with or without added linoleic acid (LA; cis-9, cis-12-18:2), were added to determine the effects of inhibiting methanogenesis on biohydrogenation. These inhibitors have different modes of action and affect H2 metabolism differently. BS inhibits methanogenesis directly, and leads to the accumulation of H2, while nitrate and nitrite are alternative hydrogen sinks. BES inhibited 24-h CH4 formation by >96% and increased H2 accumulation with both diets; biohydrogenation and VFA production were unaffected, except for a small accumulation of trans-9-18:2 in G. Sodium nitrate inhibited methanogenesis by 82 and 58% with C and G, respectively, with no H2 accumulation; disappearance of LA was unaffected, though conjugated LA (CLA) isomers tended to accumulate in C but not G. Butyrate production was strongly inhibited by nitrate. Sodium nitrite inhibited accumulation of both H2 and CH4, but also volatile fatty acids, indicating that the overall fermentation was severely inhibited. qPCR of 16S rRNA genes indicated that members of the Butyrivibrio/Pseudobutyrivibrio clade increased in abundance during the incubation, while methanogens declined in C. It was concluded that inhibiting methanogenesis does not affect overall biohydrogenation, though nitrate may lead to lower butyrate production and increased CLA passage from the rumen.