Asparagopsis taxiformis dramatically reduces methane emissions in ruminants by affecting rumen microbial composition and inhibiting methyl-coenzyme M activity

Greenhouse gas (GHG) emissions from livestock production are a significant source of atmospheric GHGs. Emissions from rumen fermentation in ruminants constitute the largest contributor within the livestock sector. Asparagopsis taxiformis (A. taxiformis) has shown great potential to mitigate methane (CH4) emissions in recent years. This study aims to evaluate the impact of A. taxiformis on methane emissions and to fill the knowledge gap regarding its mechanisms of action in affecting CH4 metabolism and rumen fermentation. The experimental design consisted of a control group (CON) and test groups supplemented with 2% (Low), 5% (Mid), and 10% (High) of dried and freeze-dried treatment A. taxiformis, respectively, for 48 h of in vitro rumen fermentation. The CH4 mitigation was evaluated by analyzing gas production parameters, and the impact on rumen fermentation was evaluated by analyzing nutrient degradation, and fermentation parameters. The CH4 mitigation mechanism was explored through metagenomics and metabolomics. 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 24.08% (P<0.01), 98.20% (P<0.01), and 99.23% (P<0.01), respectively. Freeze-dried treatment at High group significantly reduced the degradation rates of dry matter by 2.12% (P <0.05), acid detergent fiber by 4.79% (P<0.05), and crude protein by 40.79% (P< 0.001), as well as acetate concentration by 32.51% (P<0.05). 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 Mid group of A. taxiformis significantly increased the relative abundance of propionate-producing bacteria such as Prevotella, Ruminobacte, and Succinivibrio and inhibited the relative abundance of acetate-producing bacteria such as Ruminococcus, altered the pattern of volatile fatty acid (VFA) synthesis in the rumen, and reduced H2 release and promoted propionate production, indirectly alleviating CH4 production. Moreover, by suppressing the relative abundance of Methanobrevibacter, CH4 production in the rumen was directly suppressed. Furthermore, The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that A. taxiformis significantly inhibited the abundance of K00399, methyl-coenzyme M reductase alpha subunit [EC:2.8.4.1], which directly inhibited CH4 synthesis. Metabolomics analysis of A. taxiformis 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 A. taxiformis at the 5% supplementation level achieved the optimal effect of balancing the mitigating CH4 emissions and the negative effect of fermentation by changing the fermentation pattern and function of the rumen.