Bias-dependent charge transport in Limnospira indica-based Biophotoelectrochemical cells

Understanding how photosynthetic microorganisms exchange electrons with electrodes is central to advancing biophotoelectrochemical systems. Here, we investigate bias-dependent photocurrent generation in biohybrid electrodes containing Limnospira indica immobilized in a PEDOT:PSS composite on fluorine-doped tin oxide (FTO). By systematically varying irradiance, applied potential, oxygen availability, and mediator composition, we show that photocurrent arises from multiple electron-transfer pathways whose engagement depends sensitively on electrical polarization. Under anodic polarization (+0.5 to +0.9 V), photocurrent shows a trend of increase with light intensity while photon-to-current efficiency declines systematically, consistent with regulatory constraints within the photosynthetic chain. In contrast, cathodic polarization (-0.5 to -0.9 V) induces distinct multiphasic transient behavior: at -0.5 V, reproducible spike-peak-trough sequences and delayed post-illumination currents reflect competition between photosynthetic electron-transfer pathways and oxygen-dependent metabolic sinks, and oxygen removal simplifies but reduces the cathodic response. At -0.9 V, cathodic currents become largely oxygen-independent yet remain strictly PSII-dependent, as DCMU abolishes both photocurrent development and dark recovery. These results demonstrate that L. indica engages multiple potential-selective electron-transfer regimes and that transient photocurrent features carry mechanistic information inaccessible from steady-state measurements, informing the rational design of improved biohybrid photoelectrodes.