The rumen contains H2-producing and H2-utilizing bacteria. Cellulolytic and
fermentative bacteria are the main H2-producing bacteria and these are found in high
numbers (Wolin et al., 1997). Much of the primary attack on the feed ingested by the ruminants is mediated by bacteria. For the continued efficient fermentation process by bacteria, the H2 released as a by-product of feed fermentation should be continually
removed from the rumen. Methanogens living in the rumen perform this function by producing methane from this released H2. This phenomenon is called as “interspecies
H2 transfer”. This relationship between H2-producing bacteria and H2-utilizing
methanogens is one of the most prominent interactions in the rumen. Whilst this relationship might appear to be well-characterized, the number and variety of bacteria thought to participate in them continues to grow (Krause & Russell, 1996). Studies performed on bacterial 16S rRNA genes in the rumen of different ruminants has revealed a vast diversity of bacterial genera and species that have not been characterized, largely because there are no cultured representatives (Brulc et al., 2009; Edwards et al., 2004; Firkins & Yu, 2006; Kocherginskaya et al., 2001; Koike et al., 2003; Tajima et al., 2007).
Increasing concern about global warming has stimulated the search for non- methanogenic sinks for the H2 produced during the fermentation process. It may be
possible to use H2-utilizing bacteria as an alternative to methanogens to reduce methane
emission from the rumen. Sulfate reducing bacteria (SRB), homoacetogenic bacteria (homoacetogens) and fumarate reducers are the main H2-utilizing bacteria in the rumen.
Homoacetogens use H2 to reduce CO2 to form acetate, SRB reduce sulphate to sulphide
(Morvan et al., 1996) and fumarate reducers reduce fumarate to succinate (Asanuma & Hino, 2000). However, in the rumen, these H2-utlizing bacteria have to compete with
methanogens for H2, and methanogenesis appears to be the dominant mechanism in the
rumen microbial ecosystem (Chaban et al., 2006). One potential way to reduce methanogenesis is to promote the activity of H2-utilizing bacteria in the rumen.
Acetate formation (acetogenesis) is a competitive pathway to methane production but current data suggests that it is insignificant in the rumen (Fonty et al., 2007; Le Van
29
et al., 1998). Conversion of H2 and CO2 into methane is thermodynamically more
favourable and yields more energy than conversion of H2 and CO2 into acetate (Le Van
et al., 1998; Nollet et al., 1997a). However, the loss of feed energy and emission of methane to the atmosphere by the production of methane could be minimized, if H2/CO2
is used to form acetate (2CO2 + 4H2 → CH3COOH + 2H2O) instead of CH4. This
acetogenesis could be beneficial to ruminants because it yields acetate, a nutrient for ruminants rather than CH4 (Joblin, 1999).
Homoacetogens have been found in ruminants from undetectable to 1.2 × 109 per gram of rumen contents and are established in the rumen before the methanogens (Doré
et al., 1995; Leedle & Greening, 1988; Le Van et al., 1998). They are among the first species to colonise the rumens of lambs and are quite abundant in newborn lambs (Morvan et al., 1996). The colonization and establishment of homoacetogens has been reported to be independent of other microbes, but when methanogens appeared, the homoacetogenic bacterial population decreased rapidly to below the detection threshold (Morvan et al., 1994). The correlation between numbers of homoacetogenic bacteria and methanogens is negative (Doré et al., 1995). Homoacetogens are outcompeted by methanogens as acetate formation yield less energy from oxidation of H2 compared to
CH4 formation. In addition, they have a lower affinity and higher threshold level for H2
the substrate (Cord-Ruwisch et al., 1988; Le Van et al., 1998; Weimer, 1998).
Rumen homoacetogen populations can be affected by diet, age of the animal and time of sampling (Doré et al., 1995; Leedle & Greening, 1988; Le Van et al., 1998). In vitro studies have shown that homoacetogenesis is stimulated after selective inhibition of methanogens (Le Van et al., 1998; Nollet et al., 1998) and later it was shown that ruminal methanogens are not essential for effective fermentation in the rumen as they can be replaced by homoacetogens (Fonty et al., 2007). The use of yeasts as ruminal feed additives (live or dead) stimulated homoacetogenic species even in the presence of methanogens in vitro (Chaucheyras et al., 1995). However, many attempts to increase the reductive acetogenesis process by increasing the number of homoacetogens or addition of non-rumen isolates have not been successful yet in in vitro or in vivo trials (Demeyer et al., 1996; Le Van et al., 1998; Lopez et al., 1999; Nollet et al., 1997a;1998). They may still fulfil the role of H2 utilization if methanogens are
30 successfully inhibited, and the rumen does contain many potential homoacetogens (Henderson et al., 2010).
Another H2-utilizing bacterial group is the fumarate reducers. In vitro studies have
shown that addition of sodium fumarate can decrease methane production (Asanuma et al., 1999; Lopez et al., 1999). According to those studies, the bacteria such as
Fibrobacter succinogens, Selenomonas ruminantium ssp. ruminantium, Selenomonas ruminantium ssp. lactilytica, Veillonella parvula and Wollinella succinogenes oxidize H2 by using fumarate as a final electron acceptor. This suggests that these bacteria could
compete with methanogens for H2. However, the affinity of these bacteria to H2 was
lower that it was for methanogens (Asanuma et al., 1999). Succinate, the product of fumarate reduction, is decarboxylated to propionate, a valuable animal nutrient (Wolin
et al., 1997). Interestingly, higher than the expected quantities of propionate were observed when fumarate was added to the rumen of lambs (Fonty et al., 2007). This suggested that fumarate metabolism in the rumen does not necessarily involve consumption of hydrogen gas but that it may simply be fermented. More studies are needed to check the potential to use of this group of bacteria as H2-consumers.
Competitive and cooperative relationships between methanogens and sulfate reducing bacteria (SRB) have been found in anaerobic environments. In anaerobic environments, in which sulfate is not limiting, SRB generally compete with methanogens for common substrates (e.g H2, formate and acetate). A cooperative
relationship between methanogens and SRB can exist and is another example of an „interspecies hydrogen transfer‟, since some SRB are capable of growth in the absence of sulfate and produce H2. The rumen contains SRB and they were identified in the
rumen by the third day after birth (Morvan et al., 1994). No apparent competition was observed between SRB and methanogens. Sulfate cannot be added at a significant level in the rumen as an alternative to ruminal methanogenesis, due to as the toxicity of sulphides for other ruminal micro-organisms and for the host animal (Morvan et al., 1996).