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Figure 5 Generalized pathway of carbohydrate fermentations in the rumen. Fermentation products in dark bordered boxes are maintained in substantial concentrations in the normal rumen. Fermentation products in light bordered boxes are produced and excreted by some organisms but do not accumulate under normal conditions. Abbreviations: [2H], pairs of reducing equivalents; ADP and ATP, adenosine di- or triphosphate; GDP and GTP. guanosine di- and triphosphate; PEP, phosphoenolpyruvate; AcCoA, acetyl coenzyme A. Reactions coded by a circled letter are restricted to a few species, as follows: A, fibrolytic or amylolytic microbes; B, lactate utilizers, ^

particularly Selenomonas ruminantium and Megasphaera elsdenii; C, Butyrivibio fibrisolvens; D, Ruminococcus r albus, S. ruminantium, Streptococcus bovis; E, homoacetogenic bacteria (e.g., Acetitomaculum ruminis); F, sulfate-reducing bacteria; G, methanogenic archaea; H, S. ruminantium and Succiniclasticum ruminis.

Figure 5 Generalized pathway of carbohydrate fermentations in the rumen. Fermentation products in dark bordered boxes are maintained in substantial concentrations in the normal rumen. Fermentation products in light bordered boxes are produced and excreted by some organisms but do not accumulate under normal conditions. Abbreviations: [2H], pairs of reducing equivalents; ADP and ATP, adenosine di- or triphosphate; GDP and GTP. guanosine di- and triphosphate; PEP, phosphoenolpyruvate; AcCoA, acetyl coenzyme A. Reactions coded by a circled letter are restricted to a few species, as follows: A, fibrolytic or amylolytic microbes; B, lactate utilizers, ^

particularly Selenomonas ruminantium and Megasphaera elsdenii; C, Butyrivibio fibrisolvens; D, Ruminococcus r albus, S. ruminantium, Streptococcus bovis; E, homoacetogenic bacteria (e.g., Acetitomaculum ruminis); F, sulfate-reducing bacteria; G, methanogenic archaea; H, S. ruminantium and Succiniclasticum ruminis.

2. Protozoa

Because of their large size (100 |m or more in length), protozoa are readily observed microscopically and thus were first described in 1843. Many species of ruminal protozoa have been identified, primarily based on morphological criteria (Hungate, 1966). These can be classified into flagellates and ciliates. Flagellates dominate the ruminal protozoan population of young animals, but they are gradually displaced by the ciliates with aging. The ciliates contain two main groups: the relatively simple holotrichs (e.g., Isotricha or Dasytricha) or the structurally more complex oligotrichs (e.g., Entodinium and Diplodinium). The populations of protozoa in the rumen vary widely, but they are usually in the range of 102-106/mL. These densities are much lower than those of the bacteria; however, because of their large size, the protozoa may in fact represent up to half of the microbial biomass in the rumen (Van Soest, 1994; Jouany and Ushida, 1999).

All of the ruminal protozoa appear to have a strictly fermentative metabolism. Relative to the bacteria, much less is known regarding the physiology and biochemistry of the protozoa for two reasons. First, the protozoa are rather difficult to cultivate in the laboratory (Coleman et al. 1963); ruminal protozoa generally die within hours of transferring mixed rumen microflora into most laboratory culture environments. Second, many protozoa in a variety of habitats contain intracellular or surface-attached bacterial symbionts that engage in syntrophic interactions with their hosts (Fenchel et al., 1977; Vogels et al., 1980). Thus, even when ''pure'' cultures of protozoa (i.e., single protozoal species in the absence of free-living bacteria) are established and maintained, it is difficult to evaluate the potential contribution of the associated bacteria to the metabolic activities of the protozoa. Some continuous culture systems have successfully maintained protozoa by including a floating-mat matrix that allows the protozoa to resist washout from the vessel at fluid dilution rates similar to those operating in the rumen (Abe and Kurihara, 1984), and it is likely that ruminal protozoa associate in vivo with the ruminal mat or the ruminal wall in a similar manner. Populations of different protozoal species vary among individual animals and within the same animal fed different diets (Faichney et al., 1997).

Because their relatively large size permits microscopic identification of species and behavioral examination, much of our knowledge of these organisms has come from study of samples withdrawn directly from the rumen itself, particularly for comparisons of faunated animals (i.e., those having a natural protozoal population) and defaunated animals (i.e., those whose protozoal populations have been nearly or completely removed, usually by treatment with chemical agents such as 1,2-dimethyl-5-nitroimidazole or dioctyl sodium sulfosuccinate).

The holotrichs appear to be adapted to growth purely on soluble carbohydrates. On the other hand, microscopic observations have revealed that the entodi-niomorphs can engulf plant particles or can attach to the cut ends of plant fiber and can obtain their nutrition from engulfed starches and apparently some structural polysaccharides as well. Despite the observed associations of protozoa and particulate feeds, it is widely held that the primary ecological role of the entodinio-morph protozoa is the grazing of bacteria (Clarke, 1977; Hobson and Wallace, 1982). Using phase-contrast microscopy, these protozoa can be observed rapidly to ingest free bacteria (i.e., those not attached to plant fiber), and bacterial cell concentrations are approximately 10-fold higher in rumen samples from defau-nated than faunated animals. Numerous studies (reviewed by Hobson and Wallace, 1982) have thus far not identified any specific predatory relationships between particular species of protozoa and bacteria. Protozoal grazing of bacteria can reduce the availability of microbial protein to ruminants, which is a notion reflected by lower weight gain in faunated than in defaunated cattle and lambs when tests were conducted with protein-deficient diets—an effect that disappears at higher levels of feed protein. On the other hand, protozoa do appear to provide some benefits to the ruminal microflora (Jouany and Ushida, 1999). By engulfing starch granules and fermenting them more slowly than do bacteria, and by converting lactic acid to the weaker propionic acid, protozoa can help attenuate acidosis and thereby maintain fibrolytic activity of pH-sensitive cellulolytic bacteria.

Protozoa are not the only agents that control bacterial numbers; the rumen maintains substantial populations of bacteriophages (viruses that infect bacteria). Characterization of phage DNAs from rumen contents by pulsed-field electrophoresis (Swain et al., 1996) has revealed that individual animals harbor their own unique populations of phages. Regardless of these differences among host animals, phage populations (as measured by total phage DNA) follow diurnal population cycles related to the populations of the bacterial hosts, with minima and maxima at approximately 2 h and 10-12 h postfeeding, respectively.

3. Fungi

Orpin (1975) demonstrated that several microorganisms originally thought to be flagellated protozoa were actually the zoospore stage of anaerobic fungi. These fungi alternate between a freely motile zoospore stage and a particle-associated thallus. Fungal populations in rumen contents range from 104 to 105 thallus-form-ing units per gram of ruminal fluid (Theodorou et al., 1990). Approximately 24 species of these fungi have now been identified on the basis of morphology and 16S rRNA sequences (Trinci et al., 1994). Much of our understanding of the metabolic capabilities of the ruminal fungi has been derived from a single species, Neocallimastix frontalis.

Ruminal fungi are strictly anaerobic and have a catabolism based on fermentation of carbohydrate. All described species can digest cellulose and/or hemicelluloses via extracellular enzymes that are produced in low titer but have very high specific activities (Wood et al., 1986). The major products of carbohy drate fermentation are acetate, formate, and H2 with lesser amounts of lactate (primarily the D isomer), CO2, and traces of succinate (Borneman et al., 1989). H2 production occurs via hydrogenosomes, which are intracellular organelles containing high levels of the enzyme hydrogenase. In pure culture, the amounts of soluble and gaseous fermentation products essentially equal the amount of carbohydrate consumed (Borneman et al., 1989); suggesting that the yield of fungal mycelia is very small. This notion is in accord with direct measurements that indicate the ruminal fungi contribute little to the total microbial biomass in the rumen (Faichney et al., 1997). However, the ruminal fungi appear to have specific roles not readily duplicated by bacteria. For example, there is considerable evidence that fungi can attach to and physically disrupt plant tissue (particularly the more recalcitrant tissues such as sclerenchyma and vascular bundles) during growth by penetration through cell walls and expansion into the pit fields between cells (Akin et al., 1989). This physical disruption is thought to make the plant material more easily broken apart during rumination and thus more available to bacteria, which are more efficient at digesting the individual plant cell components such as cellulose. Fungal populations are highest in animals fed diets high in fibrous stem materials; perhaps because of the latter's long ruminal retention time that coincides with the slow growth rate of the fungi.

  1. Microbial Fermentations in the Rumen
  2. Structural Carbohydrates

Plant cell walls (the fibrous component of most forages) are composed primarily of cellulose, hemicellulose, pectin, and lignin. These polymers are differentially localized into the different layers of the cell wall (Fig. 6). The architecture of the plant cell wall varies greatly with cell type (Harris, 1990). Some cell types such as mesophyll and collenchyma are thin walled and essentially unlignified, and thus are easily digested. Other cell types such as sclerenchyma and xylem tracheary elements display more complex architectures with clearly distinct structures. Groups of these cell types are separated from one another by a middle lamella, which is a highly lignified region that is also rich in pectin. Interior to the middle lamella is the primary wall, the region where wall growth initiates; it is composed primarily of xyloglucans and other hemicelluloses as well as various wall-associated proteins. The secondary wall is laid down later in development and is very thick in mature plants. This region, which contains mostly cellulose with smaller amounts of hemicelluloses and lignin, can be further differentiated into layers (S1, S2, S3) based on the orientation of the cellulose microfibrils.

a. Cellulose Cellulose is the major component of forage fiber, comprising 35-50% of dry weight. Individual cellulose molecules are linear polymers of P-1,4-linked D-glucose molecules. These chainlike molecules are assembled via

Figure 6 Schematic cross-sectional view of the cell wall of two plant cell types. Abbreviations: ML, middle lamella; PW, primary wall; SW, secondary wall; L, lumen, which in the living cell contains the cytoplasm but is replaced with ruminal fluid during ruminal digestion. (Left panel) Mesophyll cell, characterized by a thin, essentially unlignified primary cell wall that is digested rapidly from both the outer and inner (luminal) surface. The middle lamella is thin and unlignified, and is usually separated from the middle lamellae of adjacent cells by air spaces. (Right panel) Sclerenchyma cell, characterized by a thin primary wall and thick, secondary walls consisting primarily of cellulose but also containing moderate amounts of hemicelluloses and lignin. Adjacent cells are separated by middle lamellae having a high lignin content. As a result, sclerenchyma cell walls are digested only from the luminal surface outward, and at a relatively slow rate and incomplete extent.

Figure 6 Schematic cross-sectional view of the cell wall of two plant cell types. Abbreviations: ML, middle lamella; PW, primary wall; SW, secondary wall; L, lumen, which in the living cell contains the cytoplasm but is replaced with ruminal fluid during ruminal digestion. (Left panel) Mesophyll cell, characterized by a thin, essentially unlignified primary cell wall that is digested rapidly from both the outer and inner (luminal) surface. The middle lamella is thin and unlignified, and is usually separated from the middle lamellae of adjacent cells by air spaces. (Right panel) Sclerenchyma cell, characterized by a thin primary wall and thick, secondary walls consisting primarily of cellulose but also containing moderate amounts of hemicelluloses and lignin. Adjacent cells are separated by middle lamellae having a high lignin content. As a result, sclerenchyma cell walls are digested only from the luminal surface outward, and at a relatively slow rate and incomplete extent.

extensive intrachain and interchain hydrogen bonds to form crystalline microfibrils that in turn are bundled into larger cellulose fibers. The packing of cellulose chains within the microfibrils is so tight that even water cannot penetrate. Cellulose fibers thus have a fairly low ratio of exposed surface to volume. Ruminal cellulose digestion appears to follow first-order kinetics with respect to cellulose concentration (i.e., the rate of cellulose digestion is limited by the availability of cellulose rather than by any inherent property of the cellulolytic microbes themselves [Waldo et al., 1972; Van Soest, 1973]).

Although many species of bacteria, fungi, and protozoa have been reported to digest cellulose in vitro, only three species of bacteria—Fibrobacter (formerly Bacteroides) succinogenes, Ruminococcus flavefaciens, and R. albus—are thought to be of major importance in cellulose digestion in the rumen (Dehority, 1993). In pure culture, these three species digest crystalline cellulose as a firstorder process with rate constants of 0.05-0.10 higher than those of any cellu-lolytic microbes that grow at a similar temperature in nonruminal habitats (Weimer, 1996). These relatively rapid rates of cellulose digestion derive in part from the ability of these species to attach directly to the cellulosic substrate (Fig. 7) and digest the cellulose via cell-bound enzymes; this adherence appears to be a prerequisite to rapid cellulose digestion (Latham et al., 1978; Costerton et al., 1987; Kudo et al., 1987). The cell-associated cellulolytic enzymes are apparently organized into supramolecular complexes resembling the cellulosome, an organ-

Figure 7 Stereo-optic view of the adherence of the ruminal cellulolytic bacterium Fi-brobacter succinogenes onto a particle of cellulose. Proper focusing of the eyes or use of a stereo-optic viewer permits a three-dimensional view of the subject. Bar represents 10 |lm.

elle that has been well-characterized in the nonruminal thermophilic bacterium Clostridium thermocellum (Felix and Ljungdahl, 1993). Although cellulose digestion in the rumen is more rapid than in nonruminal environments, the process is slow relative to the digestion of nonstructural carbohydrates and proteins. Because of this, forages, with their high rumen fill and slow digestion, must be supplemented with more rapidly digested cereal grains to adequately balance energy and protein requirements for high-producing dairy animals (Van Soest, 1994).

The products of cellulose hydrolysis are cellodextrins (short water-soluble P-1,4-glucosides of two to eight glucose units) that are subject to fermentation by both cellulolytic and noncelluloytic species (Russell, 1985). Although the individual cellulolytic species can compete directly for cellulose in vitro, it appears that they show differential ability to adhere to different plant cell types (Latham et al., 1978) that may indicate separate but overlapping niches in the rumen. Moreover, it appears that degradation of some plant cell types is delayed by the slow diffusion on nonmotile fibrolytic bacteria into the plant cell lumen (Wilson and Mertens, 1995). These cell types may provide a niche for motile cellulolytic species such as Butyrivibrio fibrisolvens.

The three major cellulolytic species form different fermentation endprod-ucts (Hungate, 1966). F. succinogenes produces primarily succinate (an important precursor of propionate) with lesser amounts of acetate. R. flavefaciens produces the same acids but with acetate predominating. R. albus produces primarily acetate and ethanol in pure culture, but in the rumen it produces mostly acetate and H2.

Estimation of the relative population sizes of individual cellulolytic species based on both classic determinative schemes (van Gylswyk, 1970) and probes to 16S rRNA (Weimer et al. 1999) suggest that R. albus is the most abundant of the three species, but variations in these populations appear to be more substantial among animals than within individual animals fed widely different diets (Fig. 8). Unlike other ruminal bacteria, the ratio of fermentation endproducts formed by each of the predominant cellulolytic species changes little with growth conditions (pH or growth rate). It would thus seem that the relative populations of these three species might contribute to differences in the proportions of acetate and propionate in the rumen. However, because the three species typically comprise less than 4% of the bacterial population in the rumen, their direct contribution to VFA proportions is probably modest.

b. Hemicelluloses Hemicelluloses, a diffuse class of structural carbohydrates that may contain any of a number of monomeric units, can comprise up to one-third of plant cell wall material (Stephen, 1983). Most hemicelluloses contain a main backbone, usually having P-1,4-glycosyl or P-1,3-glycosyl linkages; various types and degrees of branching from the main chain are frequently observed. Because of the multiplicity of hemicellulose structures present in each plant species, it is extremely difficult to isolate pure substrates of known structure, which is a fact that has severely limited the laboratory study of hemicellulose digestion. Among the most abundant of the hemicelluloses are the xylans (un-branched P-1,4-linked polymers of xylose) and the arabinoxylans (xylans containing pendant arabinose side chains). The latter are particularly important, because they are thought to be covalently linked to lignin via cinnamic acid derivatives such as ferulic acid and p-coumaric acid (Hatfield, 1993).

Hemicelluloses are hydrolyzed by enzymes that may be extracellular or cell-associated depending on the species (Hespell and Whitehead, 1990). The most active hemicellulose digesters among the ruminal bacterial isolates include B. fibrisolvens and the cellulolytic species R. flavefaciens, R. albus, and F. succi-nogenes; the latter can hydrolyze hemicelluloses in vitro but cannot use the hy-drolytic products for growth (Dehority, 1973).

c. Pectic Materials Pectins are polymers of galacturonic acids, some of which also contain substantial amounts of neutral sugars (e.g., arabinose, rham-nose, and galactose). Pectins are more abundant in leaf tissue than in stems, and

Figure 8 Relative populations of the cellulolytic bacteria Ruminococcus albus, Rumino-coccus flavefaciens, and Fibrobacter succinogenes and their sums in the rumens of four cows fed the same four diets. Diets were based on alfalfa silage (AS) or corn silage (CS) at two different levels of fiber (24 or 32% neutral detergent fiber, analyzed after a-amylase treatment). Results are expressed as a fraction of the total bacterial RNA, determined using oligonucleotide probes on samples collected 3 h after feeding. Note differences in the scale of the ordinates. (From Weimer et al., 1999; used by permission of the American Dairy Science Association.)

Figure 8 Relative populations of the cellulolytic bacteria Ruminococcus albus, Rumino-coccus flavefaciens, and Fibrobacter succinogenes and their sums in the rumens of four cows fed the same four diets. Diets were based on alfalfa silage (AS) or corn silage (CS) at two different levels of fiber (24 or 32% neutral detergent fiber, analyzed after a-amylase treatment). Results are expressed as a fraction of the total bacterial RNA, determined using oligonucleotide probes on samples collected 3 h after feeding. Note differences in the scale of the ordinates. (From Weimer et al., 1999; used by permission of the American Dairy Science Association.)

they are also major components of some byproduct feeds (citrus pulp and fruit processing waste). Although purified pectins from forages are fairly water soluble, they can be considered to be structural carbohydrates, because they are localized in the plant cell wall, particularly in the middle lamellae between cells.

In many respects, pectins are an ideal substrate for ruminal fermentation. They are rapidly digested out of both alfalfa leaves and stems (rate constants of ~0.3 h_1), but unlike starch, pectins do not yield lactic acid as a fermentation product (Hatfield and Weimer, 1995). The acetate/propionate ratio resulting from fermentation of pectins is in the range of 6-12, which is well above those of most substrates and useful in maintaining milkfat levels in lactating dairy cows. Production of these acids is accompanied by consumption of the galacturonic acid moeities of the pectin, thus assisting in the maintenance of ruminal pH. Several bacterial species have been shown actively to degrade pectin, including Lachnospira multipara, B. fibrisolvens, Prevotella (formerly Bacteroides) ruminicola, some strains of the genus Ruminococcus. (Gradel and Dehority, 1972), and some spirochetes (Ziolecki, 1979).

  1. Lignin Lignin, the third major component of the forage cell wall, is a polymer of phenylpropanoid units assembled by a random free radical condensation mechanism during cell wall biosynthesis. Lignin is indigestible under anaerobic conditions and constitutes the bulk of the indigestible material leaving the digestive tract. Moreover, the covalent linkages between lignin (or phenolic acids) and hemicelluloses reduce the digestibility of these forage components (Hatfield, 1993). Electron microscopic studies clearly reveal the recalcitrance of lignified tissues to ruminal digestion (Akin, 1979).
  2. Nonstructural Carbohydrates

Nonstructural carbohydrates are those carbohydrates in plant cells that are contained in the cytoplasm or in storage vacuoles. The most abundant of these are the starches (the linear amylose and the branched amylopectin), which are major components of cereal grains (e.g., corn) that comprise much of the diet of high-producing dairy cows.

a. Starch Starches are depolymerized fairly rapidly by extracellular enzymes (amylases and pullulanases) that produce maltodextrins (a-1,4-oligomers of glucose), which are easily converted by other a-glucosidases to glucose and maltose—substrates utilizable by almost all of the carbohydrate-fermenting microbes in the rumen (Hungate, 1966). Consequently, starches have the potential to be completely digestible, although the form of the starch is an important determinant of the rate of digestion. Wheat and barley starch are digested more rapidly than is that of high-moisture corn, which in turn is digested more rapidly than are those of dried corn or dried sorghum. The more rapidly digesting starches have first-order rate constants of digestion of ~0.25 or above.

Several bacterial species are important in starch digestion, including Rumi-nobacter (formerly Bacteroides) amylophilus, B. fibrisolvens, P. ruminicola, Suc-cinomonas amylolytica, Succinivibrio dextrinosolvens, and Streptococcus bovis. The latter species can grow extremely rapidly, particularly on glucose (minimum doubling time is 13 min), and it is the causative agent of lactic acidosis (see Sec. V.D.1). As noted above, some protozoa actively engulf starch granules but do not appear to produce lactate, thus sequestering these granules from serving as substrates for bacterial lactate production.

Even though diets high in grain content are usually preferred for high-producing cows because of their greater energy density, the presence of an adequate level of fiber in the diet is important for several reasons (Van Soest, 1994). Fiber promotes the long-term health of the ruminant animal by providing a modest rate of carbohydrate digestion and by stimulating rumination and salivation, all of which aid in maintaining ruminal pH within a range desirable for balanced microbial activity. Moreover, fiber in the diet helps the animal avoid milkfat depression, a syndrome resulting primarily from a relative deficiency in acetic acid (a precursor of short chain fatty acids in milk triglycerides) and a relative excess of propionate, which inhibits mobilization of body fat (a precursor of long chain fatty acids in milk triglycerides).

b. Soluble Sugars and Oligomers Many ruminal carbohydrate-fermenting bacteria can utilize most of the different monosaccharides that comprise the various plant polysaccharides (Hungate, 1966): D-glucose, D-xylose, D-galactose, L-arabinose, and D- or L-rhamnose. Many can also use at least some oligosac-charides that are released from the plant cytoplasm by cell wall breakage or that are produced by enzymatic hydrolysis of plant polysaccharides. The latter include cellodextrins (Russell, 1985) and xylooligosaccharides (Cotta, 1993) having seven or fewer glycosyl residues. Concentrations of soluble sugars and their oligomers are maintained at very low levels in the rumen; indicating that biopolymer hydrolysis is the rate-limiting step in digestion and that competition for soluble carbohydrates is probably an important determinant of species composition in the rumen (Russell and Baldwin, 1979a).

In the few cases that have been systematically examined, sugar fermenters have shown dramatic changes in fermentation product ratios with changes in growth rate. Both S. bovis (Russell and Hino, 1985) and Selenomonas rumi-nantium (Melville et al., 1988) carry out mixed acid fermentations at low growth rates but nearly homolactic fermentations at growth rates near their maxima.

3. Conversion of Fermentation Intermediate Compounds to Volatile Fatty Acids

Microbial fermentation of both structural and nonstructural polysaccharides produces a mixture of VFAs (usually acetic with some butyric) and other fermentation acids (succinic, lactic, and formic) that are further metabolized by other ruminal microbes. Most of these bacteria require additional growth factors such as amino acids, peptides, and vitamins. Succinate is decarboxylated to propionate (see Fig. 5) by several ruminal species, including the metabolically versatile Selenomonas ruminantium and the metabolically specialized Succiniclasticum rum-inis (van Gylswyk, 1995). Lactate is converted to propionate by several bacterial species, particularly S. ruminantium, Megasphaera elsdenii, Veillonella parvula, Anaerovibrio lipolytica, and some Propionibacterium spp. (Mackie and Heath, 1979). Formate is produced in abundance in the rumen both from carbohydrate fermentation and from reduction of carbon dioxide. Formate is rapidly turned over to methane and rarely accumulates (Hungate et al., 1970).

4. H2 Consumption and Interspecies Hydrogen Transfer

Anaerobic metabolism requires that electrons (reducing equivalents) generated from biological oxidations be transferred to terminal electron acceptors other than oxygen. Most anaerobes that ferment carbohydrates dispose of these electrons by transfer to one or more organic intermediate compounds in the catabolic pathway such as pyruvate (producing lactate), acetyl coenzyme A and acetaldehyde (producing ethanol), and carbon dioxide (producing formate) (see Fig. 5). An alternative electron acceptor is the protons present in all aqueous environments, resulting in production of hydrogen gas (H2). Disposal of electrons as H2 is particularly advantageous in that it does not consume carbon-containing intermediate compounds that may be used as biosynthetic precursors. However, production of H2 is thermodynamically unfavorable unless its production is coupled to its continuous removal by H2-consuming reactions. This spatial and temporal coupling of H2 production with H2 use, referred to as interspecies H2 transfer, is one of the most important processes in the ecology of anaerobic habitats (Oremland, 1988; Wolin, 1990). Interspecies H2 transfer benefits both the H2 consumer, which directly receives its energy source, and the H2 consumer, which can channel more of its substrate into the ATP-yielding production of acetate as a fermentation endproduct (Table 6).

The dominant H2-consuming reaction in the rumen is the reduction of carbon dioxide to methane gas:

This reaction is carried out by a specialized group of organisms, the methanogens. These organisms are classified with the Archaea, a phylogenetically distinct group that represents an early evolutionary lineage distinct from both eubacteria (true bacteria) and eukaryotes (Woese and Olsen 1986). Methanogens are highly specialized metabolically. Most are restricted in their catabolism to reduction of carbon dioxide to methane, using H2 as an electron donor, whereas a few have the ability to convert one or more simple organic compounds (methanol, methylamine, formate, or acetate) to methane (Oremland, 1988). Methanol may be periodically available in the rumen from deesterification of pectins. Formate, although not a major ruminal fermentation product, is probably produced by carbon dioxide reduction in amounts sufficient to contribute slightly to ruminal methano-genesis. Acetate, although abundant in the rumen, does not support growth of

Table 6 Fermentation Products from Cellulose in Ruminococcus albus Monocultures and R. albus/Methanobreveibacter smithii Cocultures Illustrating Changes Caused by Interspecies Transfer of H2 to the Methanogen mmol/100 mmol Glucose equivalents consumed3

Table 6 Fermentation Products from Cellulose in Ruminococcus albus Monocultures and R. albus/Methanobreveibacter smithii Cocultures Illustrating Changes Caused by Interspecies Transfer of H2 to the Methanogen mmol/100 mmol Glucose equivalents consumed3

Product

R. albus alone

R. albus + M. smithii

Ethanol

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