Co

Fig. 1. Metabiotic cooperations in defined cocultures degrading glucose (A) and trimethoxybenzoate to methane and CO (B)

Acetate

Fig. 2. Carbon and electron flow in the methanogenic degradation of complex organic matter. Groups of prokaryotes involved: 1 primary fermentative bacteria; 2 hydrogen-oxidizing methanogens; 3 acetate-cleaving methanogens; 4 secondary fermenting bacteria (syntrophs); and 5 homoacetogenic bacteria

Fig. 2. Carbon and electron flow in the methanogenic degradation of complex organic matter. Groups of prokaryotes involved: 1 primary fermentative bacteria; 2 hydrogen-oxidizing methanogens; 3 acetate-cleaving methanogens; 4 secondary fermenting bacteria (syntrophs); and 5 homoacetogenic bacteria

The interdependence among these partners may vary from an "assembly line"-type of cooperation called metabiosis in which only the later partner in the line profits from the former one but the advantage to the former members in the line by the later partners is negligible. Examples of this kind are degradation of glucose via acetate to methane by cooperation of Acetobacterium woodii and Methanosarcina barkeri (Fig. 1a; Winter and Wolfe 1979), or complete oxidation of trimethoxybenzoate via gallic acid and acetate by a triculture consisting of A. woodii, Pelobacter acidigallici, and Desulfobacter postgatei (Fig. 1b; Kreikenbohm and Pfennig 1985). Degradation of sugars and polysaccharides by clostridia is influenced positively by cooperation with hydrogen-consuming methanogens that shift the fermentation pattern to more acetate formation and, with this, to higher ATP yields (Schink 1997). Degradation of such compounds in sediments or in well-balanced sludge digestors may proceed nearly exclusively through acetate plus hydrogen, i.e., through the bold arrows in Fig. 2, with very little production of reduced side products such as butyrate and other fatty acids. Excessive production of these reduced side products is typically found in pure cultures or in unbalanced reactors receiving easily fermentable substrates at high rates that cannot be counterbal anced by sufficient growth of methanogenic partners. Cooperative interactions between fermenting bacteria and methanogenic partners have been found to be involved also in the anaerobic degradation of amino acids (Wildenauer and Winter 1986; Winter et al. 1987). The extent of cooperation between the partners degrading amino acids varies dramatically depending on the degradation pathways, from total independence of each other to obligately syntrophic relationships. Quite often, different degradation pathways are used for one specific amino acid in the presence or absence of hydrogen-scavenging partner organisms (Schink and Stams 2001). Finally, there are the strictly syntrophic relationships in which both partners depend on each other for energetic reasons and together perform a fermentation process that neither could run on its own, as is typical of syntrophic associations.

Syntrophy is a special case of symbiotic cooperation between two metaboli-cally different types of bacteria that depend on each other for degradation of a certain substrate, typically through transfer of one or more metabolic intermedi-ate(s) between the partners. The pool size of the intermediate shuttled between the partners has to be kept low to allow efficient cooperation. The term "syntro-phy" should be restricted to those cooperations in which both partners depend on each other to perform the metabolic activity observed, and in which the mutual dependence cannot be overcome by simply adding a cosubstrate or any type of nutrient. A classical example is the "Methanobacillus omelianskii" culture, which was later shown to be a coculture of two partner organisms, the S strain and the strain M.o.H. (Bryant et al. 1967). Both strains cooperate in the conversion of ethanol to acetate and methane by interspecies hydrogen transfer, as follows:

S Strain:

2 CH3CH2OH + 2 H2O ^ 2 CH3COO- + 2 H + + 4 H2 AG0' = +19 kJ per 2 mol of ethanol

Coculture:

2 CH3CH2OH + CO2 ^ 2 CH3COO- - + 2 H + + CH4 DG0' =-112 kJ per mol of methane

Thus, the fermenting bacterium cannot grow with ethanol in the absence of the hydrogen-scavenging partner because it carries out a reaction that is en-dergonic under standard conditions. The first reaction can provide energy for the first strain only if the hydrogen partial pressure is kept low enough (<10-3 bar) by the methanogen. Therefore, neither partner can grow with ethanol alone, and ethanol degradation depends on the cooperating activities of both.

In this article, we avoid the term "consortium", which is often used to describe any kind of enrichment cultures cooperating in whatever way. This term was originally coined for the structured phototrophic aggregates like "Pelochromatium" and "Chlorochromatium" and should be restricted to such spatially well-organized systems (see Chap. 2).

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