pH dependence of quinone-mediated extracellular electron transfer in a bioelectrochemical system

Quinone-mediated extracellular electron transfer (EET) is a well-known and important microbial respiration process in many natural and engineering systems. While it has been recognized that both the speciation of quinones and cell metabolism are pH-sensitive, the pH dependence of quinone-mediated EET is still unclear. In this study, pH effects in the range of 6.2 to 7.8 were investigated in a bioelectrochemical system using 9,10-anthraquinone-2-sulfonic acid (AQS) as a model quinone. The results showed that the current generation increased at pH 6.2–6.8 and then tended to stabilize, with a slight decrease at pH 7.0–7.8. The open circuit voltage (OCV) changed in a similar manner as a function of pH. The cell growth after current generation at different pH values was also indicated by the total DNA, which increased at pH 6.2–6.8 and then decreased at pH 7.0–7.8. Thermodynamic calculations and cyclic voltammetry measurements indicated that the redox potentials of AQS were negatively correlated with pH. At pH 6.2–7.0, both cell growth and the AQS redox properties had positive effects; at pH 7.0–7.8, while the AQS redox properties still had a positive impact on the EET capacity, the decline in the cell density slowed the increase of the EET capacity. These results provide a fundamental understanding of quinone-mediated EET processes and emphasize the importance of pH.