Methanogen genomics to discover targets for methane mitigation technologies and options for alternative H2 utilisation in the rumen

Reducing ruminant methane emissions is an important objective for ensuring the sustainability of ruminant-based agriculture. Methane is formed in the rumen by methanogens (part of the domain Archaea), mainly from H2 and CO2. Methanogens from a wide range of habitats are being genome-sequenced to gain a better understanding of their biology and, in particular, to identify targets for inhibition technologies for gut-associated methanogens. Genome comparisons are identifying common genes that define a methanogen, while gene differences are providing an insight into adaptations that allow methanogen survival and persistence under different environmental conditions. Within the rumen microbial food web, methanogens perform the beneficial task of removing H2, which allows reduced cofactors to be reoxidised and recycled, thereby enhancing the breakdown and fermentation of plant material. Therefore, rumen methane mitigation strategies need to consider alternative routes of H2 utilisation in the absence (or decreased levels) of methanogenesis to maintain rumen function. Two main alternatives are possible: enhancing rumen microorganisms that carry out reductive acetogenesis (combining CO2 and H2 to form acetate) or promotion of organisms that consume reducing equivalents during the conversion of metabolic intermediates (malate, fumarate and crotonate) into propionate and butyrate. A better understanding of the role and scale of methane oxidation in the rumen may also lead to future options for methane mitigation.