Isolated chloroplasts (from plants and algae) and photosynthetic microorganisms, including algae and cyanobacteria, can export electrons/reducing equivalents upon illumination in a phenomenon called ‘exoelectrogenesis’. Biophotovoltaic systems (BPVs) are devices in which exoelectrogenesis of photosynthetic microorganisms can be harnessed to convert solar energy and water into electricity. It has been calculated that power outputs may be achievable from BPVs that are comparable to outputs from abiotic photovoltaics and biofuel crops. In addition, using living organisms as biocatalysts in BPVs lends advantages as the photosynthetic microorganisms can be sourced from the local environment, can reproduce and self-repair. However, at present, the highest reported outputs from BPVs are significantly lower than both their calculated maximum and the outputs from abiotic photovoltaics. Substantial efforts are underway to increase BPV outputs by, for example, optimising the electrode design and manufacture. However, further rational optimisation of the biological materials used in BPVs is being held back by limited understanding of the bio-electrochemical mechanism of exoelectrogenesis of photosynthetic microorganisms. This research aimed to elucidate the bio-electrochemical mechanism of exoelectrogenesis for future applications of renewable electricity generation in BPVs.

First, a robust microbiological and photoelectrochemical protocol was developed to measure exoelectrogenesis of photosynthetic microorganisms analytically. A number of biotic and abiotic parameters were tested, including the electrode configuration, electrode architecture, direction of illumination, light intensity, temperature, and species and growth phase of the cyanobacteria used. Under standardised conditions, a reproducible ‘photocurrent profile’ (current output over time under light/dark cycles) with characteristic complexity (i.e. a pattern of peaks and troughs before reaching a steady state) was measured from Synechocystis sp. PCC6803 loaded onto inverse-opal indium tin oxide electrodes. It was hypothesised that features of the complex photocurrent profile could be assigned to different biotic or abiotic charge transfer processes and pathways that make up photosynthesis, respiration and exoelectrogenesis. The protocol was used to probe exoelectrogenesis systematically under different conditions, to aid in elucidating the bio-electrochemical mechanism.

The protocol was applied to identify the source and exit from photosynthetic and respiratory electron transport chains of the electrons eventually exported out of the cell during exoelectrogenesis. The majority of electrons eventually exported out of the cell during exoelectrogenesis originated from water oxidation by PSII, consistent with earlier proposals, and another yet unknown source, probably respiration. The electrons exited the intermingled PETC and RETC at the shared PQ pool and downstream of PSI. These electrons contributed to the net photocurrent output and features in the complex photocurrent profile.

Next, the route of the electrons eventually exported across the cell boundary to the cell exterior was probed by systematically comparing sub-cellular fractions lacking different extracellular appendages or layers of the cell boundary. The periplasmic space or outer membrane was identified as significantly contributing to the complex photocurrent profile of cells, ‘gating’ exoelectrogenesis.

The complexity was also unravelled with the identification of a putative competing charge transfer process accompanying exoelectrogenesis using a combined experimental photoelectrochemistry and modelling approach.

Exoelectrogenesis was also explored as a protective behaviour against ‘excess’ electrons generated during stress conditions. High light or rapidly changing light, iron limitation, carbon limitation and high salt stresses were tested. Evidence was found of exoelectrogenesis being in competition with CO2 fixation in response to immediate levels of inorganic carbon available, and of the putative competing charge transfer process being linked to the availability of the cell to import inorganic carbon.

Finally, the mechanism of mediation by exogenous redox mediators was studied, with the aim of understanding how to ‘wire’ (i.e. electrochemically connect) photosynthetic microorganisms efficiently to electrodes in a semi-artificial photosynthesis system. Using ultra-fast spectroscopic measurements on live cyanobacteria cells it was discovered that DCBQ can be reduced by the P680 and P700 primary donors on the picosecond timescale.