Assessment of Reductive Acetogenesis with Indigenous Ruminal Bacterium Populations and Acetitomaculum ruminis

The objective of this study was to evaluate the role of reductive acetogenesis as an alternative H 2 disposal mechanism in the rumen. H 2 /CO 2 -supported acetogenic ruminal bacteria were enumerated by using a selective inhibitor of methanogenesis, 2-bromoethanesulfonic acid (BES). Acetogenic bacteria ranged in density from 2.5 × 10 5 cells/ml in beef cows fed a high-forage diet to 75 cells/ml in finishing steers fed a high-grain diet. Negligible endogenous acetogenic activity was demonstrated in incubations containing ruminal contents, NaH 13 CO 3 , and 100% H 2 gas phase since [U- 13 C]acetate, as measured by mass spectroscopy, did not accumulate. Enhancement of acetogenesis was observed in these incubations when methanogenesis was inhibited by BES and/or by the addition of an axenic culture of the rumen acetogen Acetitomaculum ruminis 190A4 (10 7 CFU/ml). To assess the relative importance of population density and/or H 2 concentration for reductive acetogenesis in ruminal contents, incubations as described above were performed under a 100% N 2 gas phase. Both selective inhibition of methanogenesis and A. ruminis 190A4 fortification (>10 5 CFU/ml) were necessary for the detection of reductive acetogenesis under H 2 -limiting conditions. Under these conditions, H 2 accumulated to 4,800 ppm. In contrast, H 2 accumulated to 400 ppm in incubations with active methanogenesis (without BES). These H 2 concentrations correlated well with the pure culture H 2 threshold concentrations determined for A. ruminis 190A4 (3,830 ppm) and the ruminal methanogen 10-16B (126 ppm). The data demonstrate that ruminal methanogenic bacteria limited reductive acetogenesis by lowering the H 2 partial pressure below the level necessary for H 2 utilization by A. ruminis 190A4.