The micro-niche of exoelectrogens influences bioelectricity generation in bioelectrochemical systems

Bioelectrochemical systems (BESs) employ exoelectrogens to degrade organic matter, and produce value-added products, such as bioelectricity, methane, acetate and hydrogen. Based on the advantages of energy saving and resource recovery, BES is expected to play an important role in sustainable wastewater treatment in the future. Although considerable progress has been made in the past two decades, the commercialization of BES still needs to overcome many technical challenges to reduce costs and increase electricity output. A comprehensive understanding of the working mechanism of exoelectrogens rooted in the defined micro-niche that covers interactions with electrolytes, electrodes and other microorganisms, is the premise of solving the problem. To provide theoretical guidance for BES performance optimization, this review summarized the defined micro-niche of exoelectrogens in BES, including the overall set environmental conditions of BES and the local microenvironment of electroactive biofilms (EABs). With the expansion of EAB, the microenvironment of EAB slightly shifts in pH gradient, metabolic activity, electron transfer pathways and synergetic growth with other microorganisms. Exoelectrogens in different niche have different contributions to current, and this can be adjusted by optimization of substrate, electrode material, electrode potential, etc. In summary, an efficient and stable micro-niche is the key to ensure that BES continues to produce bioelectricity and remove pollutants, providing guidance for BES design and operation in the future.