Methanogenesis of the Microbial Community in Ruminants and Methane Mitigation Strategies: A Review

Enteric methane (CH ₄ ) emission from ruminant livestock is a significant source of greenhouse gases and an important target in climate-change research. The biological synthesis of methane by methanogenic archaea in the rumen is accomplished through a reduction of carbon dioxide (CO ₂ ) using reducing equivalents in particular hydrogen (H ₂ ) derived from microbial fermentation to form CH ₄ . Ruminant animals, including cattle, sheep, and goats, are one of the largest contributors to global CH ₄ emissions via enteric fermentation, and therefore this process is a focal point for climate change mitigation within agriculture. This review presents the microbial ecology of rumen methanogenesis and outlines strategies for mitigation of methane production. Methanogenic archaea are the main actors by transforming H ₂ and CO ₂ into CH ₄ . An archetypal example is the endosymbiotic relationship between methanogens and ciliate protozoa, in which the latter produce H ₂ during their fermentation that can be directly exploited by methanogens. In addition to H ₂ transfer between species, protozoa also appear to have a beneficial effect on ruminal function per se by participating in carbohydrate fermentation as an absolute contribution (volatile fatty acid [VFA] production [especially acetate, propionate, butyrate], total microbial biomass), and hence supply of crude protein nutrients to the host. So, protozoa are not a necessary factor in rumen efficiency, and controlling or eradicating them has been considered as an amelioration option, although any defaunation strategy must evaluate potential CH ₄ decreases against potential changes on fiber degradation, VFA profiles and protein nutrition affecting animal performance.