Ruminal acetogens and their potential to lower ruminant methane emissions

Ruminant methane is a major contributor to the anthropogenic greenhouse gas inventories of Australia and New Zealand. Direct intervention in the rumen offers one means for controlling ruminant methane emissions. In this respect, acetogenic bacteria (acetogens) normally present in the rumen are of interest because they have the potential to provide an alternative sink for H2, an essential intermediate in the formation of methane. Although little is known about the populations of acetogens in grazing ruminants, studies on ruminants fed diets containing concentrates or conserved forages indicate that the rumen contains a diversity of acetogens and that some of these have the potential to act as hydrogenotrophs in place of methanogens. This paper describes the current understanding of ruminal acetogens and outlines potential applications of acetogens in methane mitigation strategies. Strategies which use acetogens to outcompete and displace methanogens are considered less likely to be successful than strategies which use acetogens to maintain low H2 levels in the rumen following suppression of methanogens. However, the former cannot be completely discounted at present.