Advances in microbial community, mechanisms and stimulation effects of direct interspecies electron transfer in anaerobic digestion

Anaerobic digestion (AD) has been proven to be an effective green technology for producing biomethane while reducing environmental pollution. The interspecies electron transfer (IET) processes in AD are critical for acetogenesis and methanogenesis, and these IET processes are carried out via mediated interspecies electron transfer (MIET) and direct interspecies electron transfer (DIET). The latter has recently become a topic of significant interest, considering its potential to allow diffusion-free electron transfer during the AD process steps. To date, different multi-heme c-type cytochromes, electrically conductive pili (e-pili), and other relevant accessories during DIET between microorganisms of different natures have been reported. Additionally, several studies have been carried out on metagenomics and metatranscriptomics for better detection of DIET, the role of DIET's stimulation in alleviating stressed conditions, such as high organic loading rates (OLR) and low pH, and the stimulation mechanisms of DIET in mixed cultures and co-cultures by various conductive materials. Keeping in view this significant research progress, this study provides in-depth insights into the DIET-active microbial community, DIET mechanisms of different species, utilization of various approaches for stimulating DIET, characterization approaches for effectively detecting DIET, and potential future research directions. This study can help accelerate the field's research progress, enable a better understanding of DIET in complex microbial communities, and allow its utilization to alleviate various inhibitions in complex AD processes.