Asparagopsis taxiformis mitigates ruminant methane emissions via microbial modulation and inhibition of methyl-coenzyme M reductase

The results showed that freeze-dried treatment had better potential to mitigate CH₄ emissions than dried treatment, and supplementation of freeze-dried treatments at Low, Mid, and High groups significantly reduced CH₄ production by 32.44%, 98.53%, and 99.33%, respectively. However, the High group exhibited a huge negative impact on rumen fermentation. Therefore, subsequent analyses focused on the Low and Mid groups to explore the underlying mechanisms. Metagenomics analyses showed that supplementation of freeze-dried treatment with the Mid-level supplementation significantly increased the relative abundance of propionate-producing bacteria such as <i>Prevotella</i>, <i>Ruminobacter</i>, and <i>Succinivibrio</i>, while inhibited acetate-producing bacteria such as <i>Ruminococcus</i>, altered the pattern of volatile fatty acid (VFA) synthesis in the rumen, and reduced H₂ availability for methanogenesis and promoted propionate production, indirectly alleviating CH₄ production. Moreover, by suppressing the relative abundance of <i>Methanobrevibacter</i>, CH₄ production in the rumen was directly suppressed. Furthermore, KEGG pathway analysis showed that <i>A. taxiformis</i> significantly inhibited the abundance of K00399, methyl-coenzyme M reductase alpha subunit, which directly inhibited CH₄ synthesis. Metabolomics analysis of <i>A. taxiformis</i> supplementation significantly enriched ketoglutarate, malate, isocitrate, and melatonin, which may have reduced the release of rumen fermented H₂, thereby mitigating CH₄ emissions. In summary, freeze-dried treatment <i>A. taxiformis</i> at the 5% supplementation level achieved the optimal balance between CH₄ mitigation and rumen fermentation efficiency.