Symposium review: Understanding the role of the rumen microbiome in enteric methane mitigation and productivity in dairy cows.

Ruminants are one of the largest sources of global CH emissions. This enteric CH is exclusively produced by methanogenic archaea as a natural product during microbial fermentation in the reticulorumen. As CH formation leads to a gross energy loss for the ruminant host and is also an environmental issue, several CH mitigation approaches have been investigated, but results have been inconsistent, which may be partially attributed to a lack of understanding of the mechanistic basis of methanogenesis and the effect of inhibitors on individual methanogenic lineages and other fermenting microbes in the rumen. Methanogenic archaea are obligatory anaerobes that can reduce CO, methanol, or methylamines or cleave acetate to form CH. Although methanogens work toward a common goal of generating energy through the formation of CH, individual methanogenic lineages differ in their physiological and metabolic capabilities, which can differentially affect H transactions and CH formation. Using advanced omic approaches, recent research has revealed that less abundant methanol-utilizing Methanosphaera and methylamine- and methanol-utilizing Methanomassiliicoccales lineages are positively correlated with CH emissions and may have a greater share in overall CH production compared with more abundant CO-reducing methanogens than previously thought. These data imply that the diversity as well as the abundance of methanogens is important in CH formation, and that this diversity is influenced by H availability and interactions within and between H-producing microbes in the rumen. These complex interactions between microbes and H are further influenced by variations in dietary, host, and environmental conditions. This review discusses critical knowledge gaps underlying methanogen diversity and its link to CH formation, formation of specific bacteria-archaeal cohorts, and how H production and utilization are regulated between these cohorts during normal and inhibited methanogenesis. Addressing these knowledge gaps has the potential to lead to the development of novel strategies or to complement existing strategies to effectively reduce CH formation while also improving productivity in dairy cows.