Global warming increasingly threatens organisms in equatorial regions, where temperatures often exceed physiological limits. Rumen methanogens are a major biological source of anthropogenic methane, a potent greenhouse gas. Therefore, ruminal methane mitigation strategies that preserve animal productivity are urgently needed. Our In vitro analysis of Holstein steer rumen fluid—integrating gas production, volatile fatty acid (VFA) profiles, and metagenomic data—demonstrated that kombucha, a fermented beverage, effectively reduces methane emissions by modulating ruminal fermentation. Rumen fluid was incubated for 60 h under three treatments (control, 3-NOP, and kombucha). During the initial 30 h, kombucha reduced methane by 15.07 % compared to the control but was 17.54 % higher than 3-NOP. In the subsequent 30 h, kombucha achieved sustained reductions of 34.72 % versus the control and 26.28 % versus 3-NOP, highlighting its uniquely sustained methane-reducing effect. A metagenomics-guided screening and in vitro validation identified Komagataeibacter intermedius SLAM-NK6B as a key strain underlying the methane-reducing effect of kombucha. The genome of SLAM-NK6B encodes biosynthetic gene clusters for cellulose, malate, citrate, and methanobactin—metabolites that can modulate the rumen microbiota. SLAM-NK6B supplementation reduced methanogen abundance by 53.32 % and increased hydrogen pressure, shifting microbial metabolism. Excluding acetate, VFA production increased significantly, with propionate levels elevated by 15.39–43.81 %. Metagenomic data further indicated activation of alternative hydrogen sink pathways, including citrate-to-propionate and malate-to-propionate conversions. This study proposes a novel microbial metabolic strategy for methane mitigation, enabling both methane reduction and enhanced fermentation efficiency. Such metabolic guidance of the rumen microbiome offers a sustainable approach to low-emission ruminant production.

more >> https://doi.org/10.1016/j.scitotenv.2025.179793