Methanogenic digestion of glucose plus yeast extract by a defined bacterial consortium: Carbon balances and growth yields in chemostat culture

A methanogenic consortium of bacteria, isolated from anaerobic sewage sludge by growth on glucose and yeast extract and mineral salts, consisted of two strict anaerobes, one of which (the GD strain) degraded glucose and the other was a methanogen. In addition the consortium contained a small population of facultative anaerobes (4 types) which constituted <1% of the total biomass.In glucose-limited chemostat cultures of the consortium, the maximum methane output rate occured with a dilution rate (D) of 0.1 h⁻¹. With D = 0.10 h⁻¹ the consortium fermented both the glucose and yeast extract giving the following C balance (% C of glucose and yeast extract in the products): acetate, 34.2; biomass, 25.4; CO₂, 13.8; CH₄, 6.5; ethanol, 7.9; butyrate, 7.3; propionate, 3.2 (C recovery, 98.3%; H₂ production, 0.04 mol/Cmol substrate.The GD strain in uncontaminated culture fermented glucose only and gave the following C balance in a glucose-limited steady state chemostat culture with D=0.12 h⁻¹ (% glucose C in the products): acetate, 35.1; ethanol, 23.1; CO₂, 20.6; biomass, 12.3; butyrate, 4.4; propionate, 1.7 (C recovery, 97.2%); H₂ production, 0.293 mol/C mol glucose.The maximum growth yields (Y_G) from the C sources were 0.139 and 0.292 (C-mol biomass/C-mol substrate) for the GD organism and the consortium respectively.The maintenance energies were remarkably small compared with that typical of aerobic bacteria. This prompts the suggestion that the main function of maintenance energy substrate in aerobes is not to provide ATP but rather reducing equivalents to protect cells against O₂ damage. It is concluded that, in the technology of methanogenic conversion of wastes, besides the acidogenic and methanogenic stages, a third stage, for digestion of the biomass formed is required, otherwise the biomass can account for 25% of the substrate C supplied.