Genome-resolved multi-omics provide new insights into microbial nitrogen utilization by the rumen microbiota

BACKGROUND: Optimizing nitrogen (N) utilization in ruminant production systems holds both economic and environmental significance. However, traditional paradigms of N metabolism, derived primarily from well-studied model rumen bacteria, do not fully reflect the diverse and complex N metabolism in the rumen ecosystem. RESULTS: To address this gap, we utilized comparative genomics and genome-resolved multi-omics analyses using a curated set of microbial genomes to investigate N assimilation and regulation in rumen microbes. We discovered that well-established mechanisms of ammonia assimilation and regulation, such as the glutamine synthetase (GS)/glutamate synthase (GOGAT) pathways and their regulatory proteins, are absent in many of the predominant rumen microbes, which likely utilize alternative pathways for ammonia assimilation. These findings challenge the applicability of E. coli-based N regulation models to rumen bacteria in response to ammonia availability. We further linked polysaccharide utilization and ammonia assimilation across hundreds of rumen microbial species. Furthermore, we identified specific microbial species involved in ureolysis and denitrification, as well as phages carrying auxiliary metabolic genes involved in N assimilation. Using an animal trial involving 11 pairs of lamb twins in a crossover design, we demonstrated that dietary crude protein (CP) at 10% and 13% had minimal impact on rumen microbiome composition and expression of N assimilation genes. Instead, changes in concentrate levels altered N assimilation, notably increasing expression of amino acid biosynthesis pathways. CONCLUSION: These findings indicate a nuanced, species-specific microbial response to dietary interventions, highlighting the limitations of traditional N metabolism models applied to rumen microbes and the need for more granular studies of rumen microbial ecosystems.