Evidence on the occurrence of direct interspecies electron transfer (DIET) during anaerobic digestion for the production of biomethane: A review

Anaerobic digestion is one of the most efficient technologies for the conversion of organic waste into biomethane which is a form of bioenergy. To have successful operation of the anaerobic digestion process, different studies have focused on its operational parameters, however, the efficient and stable operation of AD relies on syntrophic relationships that largely depend on the transfer of reducing equivalents for interspecies electron transfer between microbes. Hence, direct interspecies electron transfer (DIET) is a significant pathway for electron transfer among syntrophic microorganisms engaged in the anaerobic breakdown of organic volatile compounds. Despite the existing literature on DIET, the underlying mechanisms and the substantial evidence on the occurrence of DIET remain unclear. Hence, this review focuses on the methods to determine DIET including different electrochemical techniques such as microbial fuel cells, electrochemical reactors, and cyclic voltammetry, and other techniques such as microbial community analysis, biochemical approaches, gene expression analysis, metabolic flux analysis, and stable isotope labeling which can collectively suggest and support the occurrence of DIET during AD. The significance of this review paper is that it critically discusses the prospects of DIET for its further development in the direction of anaerobic digestion for potential industrial applications. Moreover, the methodologies for detecting DIET proposed herein are expected to provide a theoretical basis and reference for practical application to enhance the biomethane production process.