Comparative effects of monochromatic red and broad-spectrum white light on biophotovoltaics: Stability, efficiency, and application potential

Biophotovoltaic (BPV) systems offer a sustainable strategy for solar energy conversion by harvesting the oxygenic photoautotrophic electrons via the extracellular electron transfer (EET) pathway. The photosynthetic pigments absorb light to drive water splitting and generate reducing equivalents for both growth and EET, which thus defines BPV performance is closely tied to light quality. This study evaluated a ferricyanide-mediated BPV system under two light conditions: monochromatic 620-nm red light and broad-spectrum white light, each applied at low (50 μmol photons m^-2 s^-1) and high (300 μmol photons m^-2 s^-1) intensities. Ferricyanide was chosen as the most stable and practical mediator for long-term, outdoor applications. At low intensity, both light types yielded similar photocurrents. However, under high-intensity white light, ferricyanide degraded into toxic cyanide, disrupting cell viability and system function. In contrast, red light preserved mediator stability and significantly enhanced EET, even under very high intensity (up to 1200 μmol photons m^-2 s^-1). While stronger light did not improve cell growth, it boosted EET, emphasizing its role as a potential energy dissipation pathway. These findings highlight the importance of matching light quality to both microbial pigment absorption and mediator stability, and moreover, provide a viable strategy for outdoor BPV deployment.