Background

Biophotovoltaics (BPVs) are an up-and-coming technology that use sunlight and a photosynthetic organism to generate electricity. In comparison to solar cells, BPVs have an incredibly low electricity output and much needs to be done to increase the efficiency of these systems if they are to be considered as a viable method to generate electricity. This project was centered on using directed evolution to develop a mutant of Synechocystis sp. PCC 6803, with properties that would be beneficial to a BPV cell. We started with two ideas: one focusing on increasing salt tolerance and the other on increasing the biosynthesis of carbon capture polymer PHB. As research on salt tolerance was progressing better, this was where we decided to focus the final report.

Salinity Resistance

Improving Salt Tolerance At present, cyanobacteria are attracting interest in biotechnology research areas such as BPVs and biofuels. If this technology is to be successful and we are to use cyanobacteria as a chassis in industry, it would be incredibly beneficial to have salt tolerant strains that can undertake the requirements of large scale industrial processes. In BPVs, higher concentrations of salt in the medium can generate a greater power output through an increase in current (McCormick et., al. 2011). The amount of salt that can be added to a BPV cell is limited by the salt tolerance of cyanobacteria, which can shrink under osmotic pressure and die. Working with saltwater strains can be difficult due to limited literature on cyanobacteria. Therefore, increasing the salt tolerance of a model strain would help increase the efficiency of BPV cells. Synechocystis sp. PC 6803 (hereafter referred to as Synechocystis) is a moderately halotolerant strain of cyanobacteria that produces the compatible solute glucosylglycerol (GG) in response to osmotic stress (Marin et al., 1998). The aim of this study was to randomly mutagenize ggpS to improve the salt tolerance of Synechocystis sp. PC 6803.

PHB

Conventional photovoltaic panels can only capture light energy but currently have no scalable means to store this energy. In contrast, living biophotovoltaic systems which use photosynthetic cyanobacteria are capable of both capturing and storing energy in the form of carbohydrates and other polymers. This process can further sequester or cycle carbon dioxide, helping to make our solar panel carbon negative.

Consequently, we pursued increasing the biosynthesis of polyhydroxybutyrate (PHB) a thermoplastic polyester with excellent biodegradability that is capable of being degraded by various microorganisms living in soil and saltwater. PHB acts as a carbon sink, funnelling excess metabolic flux into its production whilst sequestering CO2. PHB is native to Synechocystis sp PCC. 6803 but is only expressed in small granules in small concentrations. By undergoing error prone PCR on phaB of the phaCAB operon involved in PHB biosynthesis we hoped to generate a mutant library for the rate-limiting step to increase PHB production in E.coli initially and then transfer to  Synechocystis sp PCC. 6803.

While PHB was eventually dropped due to a shift in our narrative and project outlook, more information can still be found in the PHB section.