The degradation of complex organic matter to methane and CO₂ is a common process in anoxic environments with limited oxygen, nitrate, sulfate, or oxidized iron/manganese. It is a terminal electron-accepting process in freshwater sediments, swamps, rice paddies, and sewage treatment plants. Methanogenesis is the least exergonic process compared to aerobic degradation or alternative anaerobic respiration. The conversion of hexose to methane and CO₂ releases only 15% of the energy available in aerobic degradation. Methanogenesis is the last step in degradation after other electron acceptors are exhausted. Methane stores most of the energy from aerobic biomass conversion, which can be used by other organisms. The low energy yield of methanogenesis forces microorganisms into efficient cooperation. Syntrophic relationships are mutual dependencies where neither partner can function without the other. Syntrophism is a symbiotic cooperation between bacteria that depend on each other for substrate degradation, often for energy reasons. The "Methanobacillus omelianskii" culture is a classic example of interspecies hydrogen transfer. Strain S converts ethanol to acetate and hydrogen, while strain M.o.H. converts hydrogen to methane. The combined reaction is more exergonic than either reaction alone. The fermenting bacterium cannot grow with ethanol alone because the reaction is endergonic under standard conditions. The methanogen keeps hydrogen partial pressure low to provide energy for the fermenting bacterium. Syntrophic bacteria are difficult to cultivate, but recent studies have isolated pure cultures. The term "syntrophy" refers to mutual dependence in metabolic activity, not just adding a cosubstrate. The term "consortium" is avoided as it is often used for any cooperating enrichment culture. The Mitchell hypothesis states that ATP synthesis is coupled to proton transport across a membrane. Three protons cross the membrane per ATP hydrolyzed. The smallest metabolically convertible energy is that of an ion transported across the membrane, equivalent to one-third of an ATP unit. Bacteria in syntrophic fermentations must use about -20 kJ per mol to exploit free energy changes. In methanogenic communities, four bacterial groups are involved: primary fermenters, secondary fermenters, and two types of methanogens. Primary fermenters convert complex polymers to monomers, which are further fermented to fatty acids, succinate, lactate, and alcohols. Methanogens convert some fermentation products to methane and CO₂. Secondary fermenters convert other products to acetate, CO₂, and hydrogen, which are used by methanogens. In sulfate-rich environments, sulfate-reducing bacteria oxidize fermentation products to CO₂ and sulfide. Methanogens and sulfate-reducing bacteria can coexist, but complete oxidation to CO₂ requires two steps and does not depend on syntrophic fermentations. Primary fermenters benefit from hydrogen-oxidizing partners byThe degradation of complex organic matter to methane and CO₂ is a common process in anoxic environments with limited oxygen, nitrate, sulfate, or oxidized iron/manganese. It is a terminal electron-accepting process in freshwater sediments, swamps, rice paddies, and sewage treatment plants. Methanogenesis is the least exergonic process compared to aerobic degradation or alternative anaerobic respiration. The conversion of hexose to methane and CO₂ releases only 15% of the energy available in aerobic degradation. Methanogenesis is the last step in degradation after other electron acceptors are exhausted. Methane stores most of the energy from aerobic biomass conversion, which can be used by other organisms. The low energy yield of methanogenesis forces microorganisms into efficient cooperation. Syntrophic relationships are mutual dependencies where neither partner can function without the other. Syntrophism is a symbiotic cooperation between bacteria that depend on each other for substrate degradation, often for energy reasons. The "Methanobacillus omelianskii" culture is a classic example of interspecies hydrogen transfer. Strain S converts ethanol to acetate and hydrogen, while strain M.o.H. converts hydrogen to methane. The combined reaction is more exergonic than either reaction alone. The fermenting bacterium cannot grow with ethanol alone because the reaction is endergonic under standard conditions. The methanogen keeps hydrogen partial pressure low to provide energy for the fermenting bacterium. Syntrophic bacteria are difficult to cultivate, but recent studies have isolated pure cultures. The term "syntrophy" refers to mutual dependence in metabolic activity, not just adding a cosubstrate. The term "consortium" is avoided as it is often used for any cooperating enrichment culture. The Mitchell hypothesis states that ATP synthesis is coupled to proton transport across a membrane. Three protons cross the membrane per ATP hydrolyzed. The smallest metabolically convertible energy is that of an ion transported across the membrane, equivalent to one-third of an ATP unit. Bacteria in syntrophic fermentations must use about -20 kJ per mol to exploit free energy changes. In methanogenic communities, four bacterial groups are involved: primary fermenters, secondary fermenters, and two types of methanogens. Primary fermenters convert complex polymers to monomers, which are further fermented to fatty acids, succinate, lactate, and alcohols. Methanogens convert some fermentation products to methane and CO₂. Secondary fermenters convert other products to acetate, CO₂, and hydrogen, which are used by methanogens. In sulfate-rich environments, sulfate-reducing bacteria oxidize fermentation products to CO₂ and sulfide. Methanogens and sulfate-reducing bacteria can coexist, but complete oxidation to CO₂ requires two steps and does not depend on syntrophic fermentations. Primary fermenters benefit from hydrogen-oxidizing partners by