Conductive material engineered direct interspecies electron transfer (DIET) in anaerobic digestion: Mechanism and application

Abstract Biological syntrophy can be defined as a nutritional condition where various syntrophic microorganisms mutually synergize their metabolic activities to break down complex organic substrates that cannot be otherwise catabolized by either individually. Direct interspecies electrons transfer (DIET) is a syntrophic metabolism wherein free electrons flow from one cell to another through shared physical, electrical connections without the requirement of reduced electron carriers (redox mediators) like molecular hydrogen or formate or protein released after cell death. In anaerobic digestion (AD), the transfer of electrons between two different syntrophic microbial communities such as bacteria and archaea is a vital process for methanogens to get control of energy barriers and catabolize complex organics which could not be digested by them alone. Studies conducted so far show that DIET can be accelerated by conductive materials like carbon nanotubes, biochar, carbon cloth, granular activated carbon (GAC), and magnetite. The conductive materials mediated DIET has showed to be highly efficient in enhancement of methane yield than indirect interspecies electron transfer (IIET) in case of conventional anaerobic digestion (AD) process. This review summarizes the studies conducted to date on the effect of conductive materials mediated DIET on methanogenesis in the AD. The different types and concentrations of conducting materials affecting (a) DIET, (b) major microbial communities required for carrying out DIET, and (c) the rate of methanation, are critically discussed in the review.