Yeast culture promotes butyrate produced fibrolytic bacteria as intracellular hydrogen sink in the rumen

Background Yeast culture (YC) supplementation is widely adopted to mitigate rumen pH depression and alleviate the inhibition of fiber degradation under starch-rich diets. Yet, the underlying microbial mechanisms, particularly how yeast culture orchestrates fibrolytic communities and affects metabolic hydrogen flow in the rumen, remain a critical knowledge gap. Accordingly, elucidating the microbial basis by which yeast culture modulates fiber degradation and hydrogen utilization under starch-rich diets is of both theoretical and practical importance. Methods We conducted a study with growing lambs receiving starch-rich diets that differed only in yeast culture supplementation (CON 0%, YC 1%). We evaluated their growth performance, apparent total-tract digestibilities, rumen fermentation end-products, and the rumen metagenome. Results The YC treatment increased the lambs’ final body mass ( P = 0.02), average daily gain ( P = 0.03), digestibilities of neutral detergent fiber ( P < 0.001) and acid detergent fiber ( P < 0.001), and rumen pH ( P < 0.05), and tended to increase organic matter digestibility ( P = 0.09). In addition, total VFA concentrations, particularly butyrate, were higher at 6 h post-morning feeding ( P = 0.01). Fibrolytic and hydrogenotrophic taxa (e.g., Ruminococcus _E and Quinella ) and CAZyme families, including GH43, GH31, GH9, and GH35, were enriched by the YC treatment, as were bacteria involved in fiber degradation and butyrate production. Furthermore, none of the top five YC treatment-enriched bacterial genomes contained any hydrogenase genes, which indicates that this butyrogenic fibrolytic consortium is significantly different from the hydrogen-producing fiber-degrading microorganisms we are familiar with. Conclusion Yeast culture supplementation promoted the proliferation of a distinct butyrogenic consortium that degrades fiber while apparently disposing intracellularly metabolic hydrogen generated during fermentation, rather than releasing it as H 2 . These findings provide a microbial basis for understanding how yeast culture improves fermentation efficiency under starch-rich diets and suggest that selecting yeast culture products capable of promoting butyrogenic fibrolytic bacteria may be beneficial for ruminant performance and rumen stability.