Redirecting rumen fermentation to reduce methanogenesis

Methane production in ruminants has received global attention in relation to its contribution to the greenhouse gas effect and global warming. In the last two decades, research programs in Europe, Oceania and North America have explored a variety of approaches to redirecting reducing equivalents towards other reductive substrates as a means of decreasing methane production in ruminants. Some approaches such as vaccination, biocontrols (bacteriophage, bacteriocins) and chemical inhibitors directly target methanogens. Other approaches, such as defaunation, diet manipulations including various plant extracts or organic acids, and promotion of acetogenic populations, seek to lower the supply of metabolic hydrogen to methanogens. The microbial ecology of the rumen ecosystem is exceedingly complex and the ability of this system to efficiently convert complex carbohydrates to fermentable sugars is in part due to the effective disposal of H2 through reduction of CO2 to methane by methanogens. Although methane production can be inhibited for short periods, the ecology of the system is such that it frequently reverts back to initial levels of methane production though a variety of adaptive mechanisms. Hydrogen flow in the rumen can be modelled stoichiometrically, but accounting for H2 by direct measurement of reduced substrates often does not concur with the predictions of stoichiometric models. Clearly, substantial gaps remain in our knowledge of the intricacies of hydrogen flow within the ruminal ecosystem. Further characterisation of the fundamental microbial biochemistry of hydrogen generation and methane production in the rumen may provide insight for development of effective strategies for reducing methane emissions from ruminants.