Reducing Enteric Methanogenesis through Alternate Hydrogen Sinks in the Rumen

Climate change and the urgent need to reduce greenhouse gas (GHG) emission from agriculture has resulted in significant pressure on the livestock industry for advanced practices that are environmentally more sustainable. Livestock is responsible for more than 15% of anthropogenic methane (CH4) emission via enteric fermentation and improved strategies for mitigating enteric CH4 production therefore represents a promising target to reduce the overall GHG contribution from agriculture. Ruminal CH4 is produced by methanogenic archaea, combining CO2 and hydrogen (H2). Removal of H2 is essential, as its accumulation inhibits many biological functions that are essential for maintaining a healthy rumen ecosystem. Although several other pathways occur in the rumen, including reductive acetogenesis, propionogenesis, nitrate, and sulfate reduction, methanogenesis seems to be the dominant pathway for H2 removal. Global warming is not the only problem associated with the release of CH4 from ruminants, but the released GHG also represent valuable metabolic energy that is lost to the animal and that needs to be replenished via its food. Therefore, reduction of enteric CH4 emissions will benefit not only the environment but also be an important step toward the efficient production of high-quality animal-based protein. In recent decades, several approaches, relying on a diverse set of biological and chemical compounds, have been tested for their ability to inhibit rumen methanogenesis reliably and without negative effects for the ruminant animal. Although many of these strategies initially appeared to be promising, they turned out to be less sustainable on the industrial scale and when implemented over an extended period. The development of a long-term solution most likely has been hindered by our still incomplete understanding of microbial processes that are responsible for maintaining and dictating rumen function. Since manipulation of the overall structure of the rumen microbiome is still a significant challenge targeting key intermediates of rumen methanogenesis, such as H2, and population that are responsible for maintaining the H2 equilibrium in the rumen could be a more immediate approach. Addition of microorganisms capable of non-methanogenic H2 sequestration or of reducing equivalents are potential avenues to divert molecular H2 from methanogenesis and therefore for abate enteric CH4. However, in order to achieve the best outcome, a detailed understanding of rumen microbiology is needed. Here we discuss some of the problems and benefits associated with alternate pathways, such as reductive acetogenesis, propionogenesis, and sulfate and nitrate reduction, which would allow us to bypass H2 production and accumulation in the rumen.