Interspecies electron transfer in syntrophic methanogenic consortia: From cultures to bioreactors

Interspecies electron transfer is common in methanogenic consortia. Syntrophic partners cooperate by transferring electrons from one species to the other using H2, formate or other pathways, and maintaining H2 at low levels, so that the overall reactions are thermodynamically more favorable. Here we reviewed the mechanisms of interspecies electron transfer involved in methanogenic consortia, i.e., interspecies H2 transfer, interspecies formate transfer, and direct interspecies electron transfer (DIET) via pili, and/or outer membrane cytochromes. Although the relative importance of different mechanisms is uncertain, one mechanism may predominate under specific circumstances. Interspecies electron transfer is a probable mechanism in anaerobic granules formed in upflow anaerobic sludge bed (UASB) reactors, though direct evidence is limited. Studies of interspecies electron transfer without extracellular shuttles are mainly confined to Shewanella and Geobacter species. These mechanisms could be more energy-conserving than interspecies H2 or formate transfer, but the relationship between them is yet unelucidated. These studies are meaningful in that they could enable us a deeper understanding of how microorganisms degrade waste and produce biogas, and how current waste treatment and methane production strategies could be improved.