The Impact of Redox Mediators on the Electrogenic and Physiological Properties of <i>Synechocystis</i> sp. PCC 6803 in a Biophotovoltaic System

Biophotovoltaics (BPV) is a novel biohybrid solution to utilize solar energy potentially at high energy efficiency, by exploiting the water splitting in oxygenic photoautotrophs and electrochemical electron harvest. Unlike model electrogens, known phototrophic microbes benefit from redox mediators for extracting the photosynthetic electrons and transferring them to the external electron sink for further utilization. In this work, three representative mediators, i.e., 1,4-benzoquinone (BQ), [Co(bpy)₃]²⁺ (CoBP), and ferricyanide, are chosen and systematically evaluated for their impacts on the microbial physiology and electrogenic activity of Synechocystis sp. PCC6803. This work aimed to generate a knowledge base to guide future mediator selection and design. The results suggest ferricyanide remains the best option, as being the only mediator that promoted long-term current output. However, both BQ and CoBP produce higher current densities than ferricyanide, albeit only for a short time. Comprehensive analysis of the photosystem using fluorometric methods suggests that BQ strongly increases the PQ/PQH₂ ratio, while CoBP inhibits the electron flow from plastoquinone to photosystem I at high concentrations. Both mediators interrupt the photosynthetic electron flow and consequently cell growth. Eliminating the contribution of storage carbon to the intracellular electron flux demonstrates that all three chemicals can extract electrons originating from water splitting.