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

Abstract

ABSTRACT: Ruminants are one of the largest sources of global CH4 emissions. This enteric CH4 is exclusively produced by methanogenic archaea as a natural product during microbial fermentation in the reticulorumen. As CH4 formation leads to a gross energy loss for the ruminant host and is also an environmental issue, several CH4 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 CO2, methanol, or methylamines or cleave acetate to form CH4. Although methanogens work toward a common goal of generating energy through the formation of CH4, individual methanogenic lineages differ in their physiological and metabolic capabilities, which can differentially affect H2 transactions and CH4 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 CH4 emissions and may have a greater share in overall CH4 production compared with more abundant CO2-reducing methanogens than previously thought. These data imply that the diversity as well as the abundance of methanogens is important in CH4 formation, and that this diversity is influenced by H2 availability and interactions within and between H2-producing microbes in the rumen. These complex interactions between microbes and H2 are further influenced by variations in dietary, host, and environmental conditions. This review discusses critical knowledge gaps underlying methanogen diversity and its link to CH4 formation, formation of specific bacteria-archaeal cohorts, and how H2 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 CH4 formation while also improving productivity in dairy cows.

Keywords

• methanogenesis
• hydrogen sinks
• hydrogenases
• microbial syntrophy