Photocatalytic performance of an α-Fe2O3 electrode and its effects on the growth and metabolism of Citrobacter freundii.

Electronic exchanges occur between semiconductor minerals and microorganisms. However, researchers have focused on the photocatalytic degradation of pollutants by semiconductor minerals, and there is a limited amount of studies on semiconductor photogenerated electrons that influence the growth and energetic mechanisms of bacteria. Bioelectrochemical systems (BES) are important new bioengineering technologies for investigating the mechanisms by which bacteria absorb electrons. In this work, we built a BES that used α-Fe2O3 nanorods as a photoanode and Citrobacter freundii as bio-cathode bacteria to explore the effect of photoelectrons on C. freundii growth and metabolism. The photoanode was prepared by a hydrothermal synthesis method. As confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), the photoanode was made of α-Fe2O3. Corresponding scanning electron microscope (SEM) images showed that α-Fe2O3 nanorod arrays formed with a diameter of 50 nm, and the band gap was 2.03 eV, as indicated by UV-vis diffuse reflectance spectroscopy (UV-vis DRS). The C. freundii growth metabolism changed significantly because of photoelectrons; under light conditions, the growth rate of C. freundii significantly accelerated, and as inferred from the three-dimensional fluorescence spectrum, the protein, humic acid, and NADH concentrations were significantly higher at 72 h. According to the changes in the organic acid content, photoelectrons participated in the reductive tricarboxylic acid cycle (rTCA) to enhance growth and metabolism. The results of the study have broad implications for advancing fields that study the effects of semiconductor minerals on electroactive microorganisms and the semiconductor-photoelectronic transport mechanisms of electroautotrophic microorganisms. KEY POINTS: • For the first time, A BES was built that used α-Fe2O3 nanorods as a photoanode and C. freundii as a bio-cathode bacteria. • Photoelectrons produced by α-Fe2O3 photoelectrode promote the growth of C. freundii. • Effects of photoelectrons on C. freundii metabolism were conjectured by the changes of organic acids and NADH.