Reduced mediators released by cyanobacteria during exoelectrogenesis detected using differential pulse voltammetry.

Cyanobacteria generate electrical current through exoelectrogenesis (extracellular electron transfer) downstream of photosynthesis, yet the identity of the endogenous redox mediator(s) responsible remains unresolved. Here, we apply differential pulse voltammetry (DPV) to detect and characterise redox-active species released by Synechocystis sp. PCC 6803 under illumination. To enable sensitive detection, we developed an electrolyte intermediate between BG11 (Blue-Green-11) growth medium and MOPS (3-(N-morpholino)propanesulfonic acid) buffer that minimises background electrochemical interference while maintaining short-term cellular functionality. DPV revealed multiple light-enhanced oxidation peaks (0.1-0.65 V vs. saturated calomel electrode (SCE)) that were not resolvable using cyclic voltammetry, providing evidence that cyanobacteria release reduced compounds during exoelectrogenesis. These signals were partially reproduced in cell exudates and absent in controls, confirming their biological origin. The lack of corresponding reduction peaks suggests irreversible redox processes. Comparison with candidate mediators showed that NADPH and 4-hydroxybenzoate, an intermediate in plastoquinone biosynthesis, exhibit only partially similar electrochemical behaviour compared to cells and do not fully account for the measured responses. Collectively, our results indicate that exoelectrogenesis in Synechocystis sp. PCC 6803 involves a mixture of redox-active species rather than a single mediator. This study establishes DPV as a powerful tool for in situ detection of cyanobacterial mediators and provides new insights into the mechanisms of photosynthetic extracellular electron transfer.