Unveiling the hidden role of extracellular polymeric substances in microalgae-bacteria systems for enhanced electron transfer

Extracellular electron transfer (EET), a common process that is widely present in ecosystems, has broad potential applications in wastewater resource recovery and pollutant degradation. As an environmentally friendly technology, microalgae-bacteria systems (MBSs) can generate considerable quantities of extracellular polymeric substances (EPS), which play a critical role in biogeochemical cycling. However, the precise role of EPS in the EET process of MBSs remains unclear. Here, we demonstrate that EPS may deeply affect the microbial EET in MBSs. Our results reveal that, within the context of MBSs as a promising technology for wastewater biotreatment, the polysaccharide content in EPS was significantly greater than that of proteins. Additionally, the results of cyclic voltammograms show two oxidation peaks located at +287 and − 338 mV (vs. Ag/AgCl), as well as two reduction peaks at +600 and + 82 mV in the loosely-bound EPS (LB-EPS). In general, the electrochemical activity of tightly-bound EPS (TB-EPS) was lower than that of LB-EPS. In terms of electroactive substances from EPS, isoprenoids, quinones and hydroquinones were the most frequently found prenol lipids. As a type of soluble redox mediator, flavonoids were also identified. Riboflavin synthase activity increased as the wastewater treatment process progressed. Glycolysis, the pyruvate metabolic pathway and the tricarboxylic acid cycle, as well as ubiquinone and other terpenoid-quinone biosynthesis pathways were improved with the production of ATP, NADH and electron shuttles. Typical EET-related genes (such as MacA, PilA, and PilE) revealed a notably positive correlation with exopolysaccharides and proteins. These findings highlight the significance of EPS in enhanced EET and offer new insights to promote the application of EPS in biological wastewater treatment.