International Symposium on Ruminant Physiology: Rumen protozoa and viruses-New insights into their diversity and potential roles through omics lenses; A review.

The rumen microbiome is essential for breaking down indigestible plant material, supplying ruminants with most of their ME and MP. Whereas research has primarily focused on bacteria and archaea, protozoa and viruses (phages) have only gained attention in recent years. Protozoa contribute to feed digestion and fermentation, but as predators, they regulate microbial populations by lysing large quantities of microbial cells (the primary protein source for ruminants) and influence the amount of microbial protein reaching the small intestines, along with other mechanisms of interactions. Although rumen viruses (or phages) are abundant and diverse, they remain the least understood component of the rumen ecosystem. They can profoundly affect the rumen microbiome by directly lysing their hosts and reprogramming host metabolism through multiple mechanisms, including gene transfer and alteration of central carbon metabolism. Recent advances in omics technologies have deepened our understanding of these viruses, revealing their complex roles in rumen function. This review integrates current knowledge and recent discoveries from omics studies, highlighting the transformative effect of omics-based approaches. It also identifies critical knowledge gaps and outlines future research directions, including selective inhibition of rumen protozoa, development of phages as potential intervention tools to manage specific undesirable rumen microbes, and the causal effects of rumen viruses on microbial dynamics and animal productivity.