ENERGETICS OF SYNTROPHIC COOPERATION IN METHANOGENIC DEGRADATION

Fatty acids and alcohols are key intermediates in the methanogenic deg radation of organic matter, e.g., in anaerobic sewage sludge digesters or freshwater lake sediments. They are produced by classical fermenti ng bacteria for disposal of electrons derived in simultaneous substrat e oxidations. Methanogenic bacteria can degrade primarily only one-car bon compounds. Therefore, acetate, propionate, ethanol, and their high er homologs have to be fermented further to one-carbon compounds. Thes e fermentations are called secondary or syntrophic fermentations. They are endergonic processes under standard conditions and depend on inti mate coupling with methanogenesis. The energetic situation of the prok aryotes cooperating in these processes is problematic: the free energy available in the reactions for total conversion of substrate to metha ne attributes to each partner amounts of energy in the range of the mi nimum biochemically convertible energy, i.e., 20 to 25 kJ per mol per reaction. This amount corresponds to one-third of an ATP unit and is e quivalent to the energy required for a monovalent ion to cross the cha rged cytoplasmic membrane. Recent studies have revealed that syntrophi cally fermenting bacteria synthesize ATP by substrate-level phosphoryl ation and reinvest part of the ATP-bound energy into reversed electron transport processes, to release the electrons at a redox level access ible by the partner bacteria and to balance their energy budget. These findings allow us to understand the energy economy of these bacteria on the basis of concepts derived from the bioenergetics of other micro organisms.