Advances in direct interspecies electron transfer and conductive materials: Electron flux, organic degradation and microbial interaction

Direct interspecies electron transfer (DIET) via electrically conductive pili (e-pili) and c-type cytochrome between acetogens and methanogens has been proposed as an essential pathway for methane production. Supplements of conductive materials have been extensively found to promote methane production in microbial anaerobic treatment systems. This review comprehensively presents recent findings of DIET and the addition of conductive materials for methanogenesis and summarizes important results through aspects of electron flux, organic degradation, and microbial interaction. Conductive materials improve DIET and methanogenesis by acting as either substitute of e-pili or electron conduit between e-pili and electron acceptors. Other effects of conductive materials such as the change of redox potential may also be important factors for the stimulation. The type and organic loading rate of substrates affect the occurrence of DIET and stimulating effects of conductive materials. Geobacter, which can participate in DIET, were less enriched in anaerobic systems cultivated with non-ethanol substrates, suggesting the existence of other syntrophs with the capability of DIET. The coupling of communication systems such as quorum sensing may be a good strategy to achieve the formation of biofilm or granule enriched with syntrophic partners capable of DIET.