Publication
Whole-cell biophotovoltaic (BPV) devices leverage photoautotrophic microorganisms like cyanobacteria to convert sunlight and water into bioelectricity. They are capable of growing on sequestered carbon dioxide (CO2), eliminating the need to add exogenous organic carbon. These microorganisms engage in extracellular electron transfer (EET), discharging surplus electrons in their environment. By coupling these microbes with an anode, the released electrons can be harvested, and the circuit completed with a cathode to yield a full BPV device. However, BPV performance is currently limited due to slow electron transfer dynamics between the microorganisms and electrodes. This thesis proposes two innovative strategies to enhance this interaction.
The first strategy focused on developing a sustainable carbon-based electrode to optimize microbial immobilization and extracellular charge extraction. We optimized the electropolymerization of polypyrrole (PPy) and characterized the resulting PPy layer on a graphite substrate. This modified support was interfaced with two cyanobacterial strains: Synechocystis sp. PCC6803 (Synechocystis) and Synechococcus elongatus PCC7942 (Elongatus). The bioanodes were tested electrochemically without an external redox mediator to enhance charge exchange. Notably, Synechocystis exhibited a 6-fold increase in photocurrent, while Elongatus showed no improvement. This difference was attributed to the strains' distinct surface charge characteristics, affecting their adherence to PPy, which was investigated using crystal violet staining, chlorophyll extraction, confocal microscopy, and scanning electron microscopy (SEM).
The second strategy involved in situ modification of Synechocystis using dopamine (DA), a technique previously applied to non-photosynthetic organisms. The cyanobacteria self-coated with polydopamine (PDA) nanoparticles polymerized from exogenous DA in the medium, a process sensitive to pH and DA concentration. We tuned the coating conditions to minimize light shielding while ensuring cell viability. Under optimal treatment conditions (pH 7.5, 1 mM DA), the coated cells exhibited a 3-fold increase in photocurrent extraction compared to uncoated cells on graphite and in the presence of an exogenous redox mediator. SEM imaging confirmed enhanced adherence of the coated cells to the electrode, consistent with its improved electrochemical performance.
Ultimately, the engineered anodic supports and microbes were coupled based on their complementary properties. PPy-coated anodes achieved a 2-fold increase in photocurrent when interfaced with Synechocystis and in the presence of a redox mediator. Interestingly, PDA-coated and uncoated cells showed similar surface charges and adherence to the anodic support. The observed 40 % increase in photocurrent for the PDA-coated cells is attributed to the photoresponse of the cells in addition to that of the PDA. These results suggest that electrostatic interactions are critical