2.6　The Obligate H2-producing Acetogenic Bacteria

Concept was proposed by Wolin and Bryant through their metabolic coupling analysis of species between ruminant methanobacterium and “S” organism grown on medium containing acetoketones. Metabolic relationship between species of methane-producing and non-methane-producing bacteria is expressed mainly in“hydrogen transfer between species”, i.e. hydrogen liberated in the anaerobic respiration by non-methane-producing bacteria is utilized by methane-producing bacteria serving as part of substrates to form methane. This fashion of oxidation-reduction between two bacteria or more than two of different species is known as interspecies H2-Transfer. For example, as illustrated by Bryant and Wolfe (1967),M.Omelianskii was coupled by MOH strain of methanobacteria with“S”organism to form “symbiotic body” (intergrowth, Fig.2.9), within this symbiotic body ethanol was oxidized by “S” organism forming CH3-COOH and H2(this reaction can also be inhibited by self-produced H2) and H2produced was utilized by MOH strain to synthesize CH4. In this connection, H2production was increased by “S” organism. Taking this as presupposition for intersurvival, molecular hydrogen is the intermediate body for coupling of two bacteria. That the effect of hydrogen transfer occurs between species leads to degradation and utilization of substrates by microbes under anaerobic conditions.

Fig.2.9　Interspecies H2-transfer

There also exists hydrogen transfer between methanobacterium and hydrogenproducing bacteria, such as sulfate-reducing bacteria besides what mentioned above.

In addition, to some extent, combination of methanosarcina with nonmethane-producing bacterium of several oblique anaerobes does occur and this kind of non-methane producing bacteria of these oblique anaerobes is known as satellite organisms. For example, Eubacterium limosum, Bacteroides sp. and one species of Bacteroides were totally known as the 3rd isolates. All of them are likely to be hydrogen-producing bacteria and their relationship to Methanobacterium might be that of “hydrogen transfer between species”.

In biogas fermentation, owing to the presence of “hydrogen transfer between species”between Methanobacteria and non-methanobacteria, hydrogen is provided successively by hydrogen-producing bacteria while H2yielded is used continuously by Methanobacteria, which ensures hydrogen production by H2-producing bacteria, not inhibited by accumulation of it. It is the dynamic equilibrium that keeps the biogas fermentation progressing normally.

Besides what have been discussed above, i.e. cooperative actions between them, there exist mutual inhibiting and mutual restricting actions between different biogas fermenting microbes, including metabolites themselves and species. Furthermore, the optimum pH and oxidation-reduction potentials between acidforming bacteria and Methanobacteria are markedly different. In a digesting system, it is impossible to accommodate the living demands for both microbes. Eventually there exist contradictions. In the early stage of biogas fermentation, however, the conditions are favorable to acid-forming microbes that grow more prosperously, being predominant in this stage. As time goes on, ammonium yielded by the action NH3producing bacteria causes a gradual increase in pH and a fall in oxidation-reduction potentials, which favourite the activity of Methanobacteria resulting in an increase in amount. And again, the increase of pH resulted by the action of NH3-producing bacteria restricts the action of acid-forming bacteria and collaborates on Methanobacteria, thereby an equilibrium of digestion and biochemical changes in this process from acid formation to methane-production is achieved.

The syntrophic relations are summarized in Table 2.5.

Table 2.5　Syntrophic relations between hydrogen-producing acetogenic bacteria and methanogenic bacteria