Insights into direct interspecies electron transfer mechanisms for acceleration of anaerobic digestion of wastes

Anaerobic digestion of waste organic biomass is a well-known process for conversion of biomass to bioenergy. However, global acceptability of anaerobic digestion process for renewable energy has often been undermined because of its inconsistent or lower rate of biogas production. The overall success of the anaerobic digestion process considerably depends on the interactions between the microbial communities within the digester. The electron transfer through interspecies hydrogen transfer between acetogens and methanogens is a major bottleneck for successful anaerobic digestion process. Recent studies have reported the role of biotic (pili and cytochromes) as well as abiotic (conductive materials) components to accelerate direct interspecies electron transfer between microbial communities in the digester. These transfer mechanisms via biotic components of exoelectrogenic bacteria and methanogenic archaea is thermodynamically more favorable over indirect interspecies electron transfer. However, the use of conductive materials in promoting anaerobic digestion process has been an area of research in the last few years. The process of direct interspecies electron transfer promoting anaerobic digestion process has been investigated evidently in pure microbial cultures; however, there are scanty reports on existence of direct interspecies electron transfer in mixed microbial consortia. The present review highlights the fundamentals and applications of direct interspecies electron transfer-promoted anaerobic digestion process at laboratory scale studies used to improve kinetics of methanogenesis.