Role and potential of the semi-classical/-quantum mechanism of the extracellular environment and cell envelope in Direct Interspecies Electron Transfer (DIET)-driven biomethanation

The extracellular electron transfer (EET) capability of Methanosarcina spp. in direct interspecies electron transfer (DIET) has profoundly increased our understanding of microbial kinetics and energetics in biomethanation systems. In Methanosarcina spp., such EET mechanisms occur in the cell envelope and biofilm matrix. These substances are composed of protein-like, polysaccharide-rich biomolecular structures that were previously thought to contribute only to cell support and shape; while their participation in dynamic processes remains unclear and has gathered widespread interest. This review first addresses the molecular structure and chemical characteristics of the extracellular matrix and cell wall polymers in Methanosarcina spp. Next, we focus on recent theoretical studies on the conduction and EET mechanisms of the extracellular matrix and cell wall polymers: tunnelling, hopping, proton-activated electron transfer and voltage-dependent electron transport. We conclude this review by discussing the state-of-the-art electrochemical techniques and experiments and the associated challenges, i.e., the kinetic isotope effect and on–off resonance switching. The border impacts of such conductive pathways may offer a semi-classical/quantum perspective on microbiology and mark the renaissance of anaerobic biotechnology.