a Mean values from continuous culture trials conducted at five different dilution rates. b Equivalent to 300 mmol H2 consumed (at a stoichiome-

try of 4 mol H2 consumed per mol CH4 formed). Source: Pavlostathis et al., 1990.

a Mean values from continuous culture trials conducted at five different dilution rates. b Equivalent to 300 mmol H2 consumed (at a stoichiome-

try of 4 mol H2 consumed per mol CH4 formed). Source: Pavlostathis et al., 1990.

''aceticlastic'' methanogens, whose growth rates even under ideal conditions are well below dilution rates of both liquids and solids in the rumen. Most methanogens are also autotrophs; that is, they can obtain all of their cell carbon from carbon dioxide. Thus, they can produce microbial protein for the ruminant host without consumption of otherwise useful organic matter.

The energy associated with the reduction of the abundant ruminal carbon dioxide to methane is sufficient to permit both growth of the methanogens and thermodynamic displacement or ''pulling'' of the reduction of protons to H2. As a result, the concentration of H2 in ruminal fluid is very low—normally near 1 |M with only occasional excursions to approximately 20 |M for a few minutes postfeeding (Smolenski and Robinson, 1988). Thus, ruminal methanogenesis, which is viewed unfavorably by nutritionists as a loss of ~8% of the metaboliz-able energy of the feed, in fact has an important thermodynamic function that permits an adequate rate and extent of carbohydrate fermentation.

Representatives of another group of bacteria, the carbon dioxide-reducing homoacetogens, have been isolated from the rumen and appear to be present at low cell densities. Like the methanogens, these eubacteria can reduce carbon dioxide with H2, but according to the stoichiometry

The homoacetogens have attracted interest as potential competitors of the metha-nogens in that they could, in principle, remove fermentatively produced H2 while at the same time producing acetic acid, an energy source and biosynthetic precursor that the ruminant is well equipped to use (Mackie and Bryant, 1994). Unfortunately, numerous in vitro studies have shown that the acetogens are ineffective competitors of the methanogens because of the latter's superior affinity for low concentrations of H2. The actual role of the acetogens in the rumen is presently unclear; because this metabolically diverse group is capable of sugar fermentation and removal of methoxyl groups from some feed constituents, its members may fill several niches.

A third group of H2 utilizers, the sulfate-reducing bacteria, can couple the oxidation of H2 or certain organic compounds such as lactate to reduction of sulfate (Odom and Singleton, 1993):

Sulfate-reducing bacteria have an affinity for H2 that even surpasses that of the methanogens; indeed, sulfate reduction is the dominant means of disposal of excess electrons in a sulfate-rich environment (e.g., ocean sediments). Sulfate-reducing bacteria have the unusual capacity to act as H2 consumers when sulfate is abundant or as H2 producers (from lactate) when sulfate is absent (Bryant et al., 1977). In the latter situation, the sulfate reducers may be maintained in the rumen by a symbiotic interaction with methanogens wherein the sulfate reducers oxidize lactate to H2, whose concentration is kept low by methanogenic activity.

5. Nitrogen Metabolism in the Rumen a. Protein Degradation Availability of protein to the ruminant is determined by the amount of protein in the feed, its loss in the rumen from microbial fermentation, and the efficiency of microbial protein synthesis that occurs in the rumen. It is estimated that approximately 35-80% of the protein of most forages and grains is degraded by ruminal fermentation and is thus not directly available for intestinal absorption (National Research Council, 1985). Hydrolysis of protein depends on several factors—particularly solubility, which determines both its availability to ruminal microbes and its rate of escape from the rumen. The generalized scheme of protein degradation (see Fig. 9) suggests some similarities to polysaccharide degradations. Proteins are hydrolyzed extracellularly or at the mi-crobial cell surface to produce soluble oligomers that serve as the actual growth substrates. Major proteolytic species in the rumen are B. fibrisolvens, S. bovis, and P. ruminicola. These species also have important roles in carbohydrate fermentation.

The fermentation of amino acids and peptides released from protein hydrolysis is carried out by a number of ruminal species. The most active appear to

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