ASSESSMENT OF REDUCTIVE ACETOGENESIS WITH INDIGENOUS RUMINAL BACTERIUM POPULATIONS AND ACETITOMACULUM-RUMINIS

The objective of this study was to evaluate the role of reductive acet ogenesis as an alternative H-2, disposal mechanism in the rumen, H-2/C O2-supported acetogenic ruminal bacteria were enumerated by using a se lective inhibitor of methanogenesis, 2-bromoethanesulfonic acid (BES). Acetogenic bacteria ranged in density from 2.5 x 10(5) cells/ml in be ef cows fed a high-forage diet to 75 cells/ml in finishing steers fed a high-grain diet. Negligible endogenous acetogenic activity was demon strated in incubations containing ruminal contents, (NaHCO3)-C-13, and 100% H-2, gas phase since [U-C-13]acetate, as measured by mass spectr oscopy, did not accumulate. Enhancement of acetogenesis was observed i n these incubations when methanogenesis was inhibited by BES and/or by the addition of an axenic culture of the rumen acetogen Acetitomaculu m ruminis 190A4 (10(7) CFU/ml), To assess the relative importance of p opulation density and/or H-2, concentration for reductive acetogenesis in ruminal contents, incubations as described above were performed un der 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 demo nstrate that ruminal methanogenic bacteria limited reductive acetogene sis by lowering the H-2, partial pressure below the level necessary fo r H-2, utilization by A. ruminis 190A4,