Syntrophy and Interspecies Electron Transfer in Methanogenic Microbial Communities

Anaerobic digestion of municipal and other organic waste is a microbial process for conversion of complex organic substances to biogas (a renewable energy source) comprising a mixture of methane and CO2, and a stabilized sludge, which may be used as an organic fertilizer. Diverse groups of the methanogenic microbial community degrade complex organic compounds into simple fermentation products such as hydrogen, formate, acetate, short-chained volatile fatty acids, ethanol, etc. These low-molecular mass products act as the substrates and carriers involved in biogas production by syntrophic bacteria and methanogenic archaea at the methanogenesis stage, the last stage of the anaerobic process. The present review discusses syntrophic interactions between the microorganisms involved in anaerobic degradation of organic substances, as well as two types of interspecies electron transfer (IET): indirect IET (IIET, Indirect Interspecies Electron Transfer) and direct IET (DIET, Direct Interspecies Electron Transfer). DIET-based syntrophic interactions between microorganisms may be stimulated by adding conductive materials into anaerobic digesters, which may have the potential for practical applications.