According to conventional knowledge, in this anaerobic food web ~30% of the methane is generated via hydrogen (with CO2 as the electron acceptor) by hydrogenotrophic methano — gens and ~70% is generated via acetate by acetoclastic methanogens. The latter process is referred to as acetate cleavage (Smith and Mah 1980), and occurs under close-to-optimum environmental conditions without reactor upsets (Mechanism I in Table 4.1). When the envi­ronmental conditions are not as favorable (e. g., stressed conditions), an alternative mechanism exists to convert acetate into methane, reducing the percentage of methane that is generated from acetate through acetate cleavage. This is referred to as acetate oxidation as part of a two-step process in which acetate is first oxidized to H2 and CO2 by homoacetogenic bacteria (in reverse) and H2/CO2 are then converted to CH4 by hydrogenotrophic methanogens (Mechanism II in Table 4.1; Barker 1936; Zinder and Koch 1984; Zinder 1994).

The first step ofbacterial acetate oxidation has a positive standard free energy (Mechanisms II[a] in Table 4.1), resulting in a thermodynamically unfavorable reaction at biological stan­dard conditions. At homoacetogenic nonequilibrium cases with the presence of methanogens in the anaerobic food web, the reaction sum becomes thermodynamically favorable provided

that an extremely low hydrogen partial pressure is maintained by the second step (Mechanisms II[b] in Table 4.1). Therefore, the relationship between an acetate-oxidizing bacteria and a hydrogenotrophic methanogen is a syntrophic relationship. Even though thermodynamically Mechanism II becomes as favorable as Mechanism I, the overall energy yield has to be shared between the two syntrophic members. Syntrophic acetate oxidation associations have been observed in a variety of anaerobic systems (Petersen and Ahring 1991; Schnhrer et al. 1994, 1999; Shigematsu et al. 2004; Schwarz et al. 2007). In such systems, acetate-oxidizing bacteria play an extremely important role guaranteeing the proper functioning of the anaero­bic digestion by preventing acetate accumulation. This alternative pathway is, therefore, important to maintain stability in engineered systems.