The effect of fumarate used as a feed additive on the reduction of methanogenesis in the rumen was evaluated by in vitro experiments. The addition of fumarate to the culture of mixed ruminal microorganisms that were fermenting hay powder and concentrate reduced methane production. Most fumarate was metabolized to propionate, and a slight increase was noted in other volatile fatty acids. Fumarate was utilized by mixed bacteria but not by mixed protozoa. Fibrobacter succinogenes, Selenomonas ruminantium ssp. ruminantium, Selenomonas ruminantium ssp. lactilytica, Veillonella parvula, and Wollinella succinogenes oxidized H-2 by using fumarate as a final electron acceptor, suggesting that these bacteria compete with methanogens for H-2, which is the main substrate for methanogenesis in the rumen. However, the affinity of these bacteria to H-2 was lower than their affinity to methanogens. These fumarate-utilizing bacteria metabolized malate to products that were similar to those from fumarate, suggesting the possession of fumarate dehydratase. Fibrobacter succinogenes, V. parvula, and W. succinogenes utilized formate, another substrate for methanogenesis, as an electron donor for fumarate reduction. The affinity of these bacteria to formate was higher than the affinity methanogens have for formate. When methanogens were cocultured with an equal cellular amount of each of the fumarate-utilizing bacteria, methane production was markedly decreased, not only from formate, but also from H-2. These results suggest that the addition of fumarate to ruminant feed reduces methanogenesis and enhances propionate production in the rumen.
