Extracellular electron transfer in electroactive anaerobic granular sludge mediated by the phenothiazine derivative

Exoelectrogenic anaerobic granular sludge (EGS) is an attractive biocatalyst due to its thick biomass. The extracellular electron transfer (EET) within EGS and between EGS and electrode is crucial for oxidative reaction efficiency. Herein, a phenothiazine derivative, methylene blue (MB) is supplied as a transient mediator to enhance EET between EGS and electrode. The maximum current of MB-EGS (8.88 mA) is significantly higher than that of EGS alone (1.34 mA). MB of 8 mg/L proved to be the most efficient concentration for EET enhancement. MB induces a heterogeneous distribution of the extracellular polymeric substances, particularly more proteins, which can drive multistep electron transfer from granules to electrodes or granules nearby. Furthermore, the surface charge of MB-EGS is less negative, alleviating electrostatic repulsive interactions between granules and bacteria. The microbial community analysis confirms enrichment of exoelectrogens (i.e., Geobacter) and suppressed methanogenesis in MB-EGS. This study opens the door for engineering EGS using mediators for more efficient EET and sheds light on the development of an efficient biocatalyst for various applications of microbial electrochemical technology.