Microbial ecosystem and methanogenesis in ruminants

Abstract

Ruminant production is under increased public scrutiny in terms of the importance of cattle and other ruminants as major producers of the greenhouse gas methane. Methanogenesis is performed by methanogenic archaea, a specialised group of microbes present in several anaerobic environments including the rumen. In the rumen, methanogens utilise predominantly H 2 and CO 2 as substrates to produce methane, filling an important functional niche in the ecosystem. However, in addition to methanogens, other microbes also have an influence on methane production either because they are involved in hydrogen (H 2 ) metabolism or because they affect the numbers of methanogens or other members of the microbiota. This study explores the relationship between some of these microbes and methanogenesis and highlights some functional groups that could play a role in decreasing methane emissions. Dihydrogen ('H 2 ' from this point on) is the key element that drives methane production in the rumen. Among H 2 producers, protozoa have a prominent position, which is strengthened by their close physical association with methanogens, which favours H 2 transfer from one to the other. A strong positive interaction was found between protozoal numbers and methane emissions, and because this group is possibly not essential for rumen function, protozoa might be a target for methane mitigation. An important function that is associated with production of H 2 is the degradation of fibrous plant material. However, not all members of the rumen fibrolytic community produce H 2 . Increasing the proportion of non-H 2 producing fibrolytic microorganisms might decrease methane production without affecting forage degradability. Alternative pathways that use electron acceptors other than CO 2 to oxidise H 2 also exist in the rumen. Bacteria with this type of metabolism normally occupy a distinct ecological niche and are not dominant members of the microbiota; however, their numbers can increase if the right potential electron acceptor is present in the diet. Nitrate is an alternative electron sinks that can promote the growth of particular bacteria able to compete with methanogens. Because of the toxicity of the intermediate product, nitrite, the use of nitrate has not been fully explored, but in adapted animals, nitrite does not accumulate and nitrate supplementation may be an alternative under some dietary conditions that deserves to be further studied. In conclusion, methanogens in the rumen co-exist with other microbes, which have contrasting activities. A better understanding of these populations and the pathways that compete with methanogenesis may provide novel targets for emissions abatement in ruminant production.

The effects of feed blocks containing tomato and cucumber by-products on <i>in vitro</i> ruminal fermentation, microbiota, and methane production

“…The pH could be responsible for shifts in ruminal microbial diversity, but in the present work pH values were within physiological limits for all of the studied diets. In agreement with our results, many studies have indicated that methane production in the rumen does not depend on the abundance of methanogens, but on their diversity (Morgavi et al, 2010). The mechanism of action of FB-containing diets, which reduce methane production, does not seem to involve propionate as a sink for H 2 , but a different metabolic pathway such as homoacetogenesis, as previously mentioned.…”

Methane production and diurnal variation measured in dairy cows and predicted from fermentation pattern and nutrient or carbon flow

“…Hydrogen sinks other than CH 4 exist as H 2 can be used in reductive acetogenesis, or for reduction of nitrate or sulphate. Although considered quantitatively unimportant (Morgavi et al, 2010), these alternative H 2 sinks might explain part of the residual variation. Moss et al (2000) assumed that 90% of the H 2 surplus produced during VFA production is recovered in CH 4 .…”

Effects of disodium fumarate on ruminal fermentation and microbial communities in sheep fed on high-forage diets

“…It is suggested that DF may cause a transient effect on protozoa. Protozoa serve not only as host for methanogens, but also produce hydrogen in large quantities in a specialized organelle (hydrogenosome; Morgavi et al, 2010). This hydrogen is metabolized by methanogens that are found inside (Finlay et al, 1994) or in close association with protozoal cells (Stumm et al, 1982).…”

“…There are other potential -E-mail: liujx@zju.edu.cn electron acceptors in rumen (Wolin, 1979), such as sulfate, nitrate and fumarate, etc. (Morgavi et al, 2010). Among them fumarate is non-toxic and an intermediate of the pathways of propionate formation (Russell and Wallace, 1997), and has been extensively studied as an alternative electron sink (Castillo et al, 2004).…”