Effects of Organic Carbon Source and Light Exposure on Glycogen Mitigation in Synechocystis sp. PCC 6803 Mutant Lacking PSII for Microbial Electro-Photosynthesis

Biofuels are a carbon-neutral energy source proving to be a sustainable alternative to greenhouse gas-emitting fossil fuels that are accelerating the detrimental effects of anthropogenic climate change. A developing system aimed at more efficiently producing biofuels is called Microbial Electro-Photosynthesis (MEPS). In MEPS, a Synechocystis sp. PCC 6803 mutant lacking photosystem II (PSII) receives electrons by a hydroduroquinone (DQH2) mediator from a more efficient water-splitting electrochemical cell, rather than splitting water itself using PSII. However, growth of the Synechocystis cells prior to use in MEPS requires an organic carbon source, leading to internally-stored electron sources, namely glycogen, that compete with preferred DQH2 mediator-delivered electrons. In this study, the effects of organic carbon source (pyruvate, acetate, glucose, and no carbon source) and light condition (light or dark) on the physiology and P700+ reduction kinetics of photoheterotrophically grown Synechocystis mutants were studied with the hope of identifying a maintenance culturing method that allowed for both cell viability and mitigated glycogen storage. While no significant decreases in internal electron-sources were found with these methods, it was observed that Synechocystis cells fed pyruvate in the light had most successfully reduced competition between internal electron sources and preferred DQH2-delivered electrons. This study suggests that these experiments be re-run after removing exogenous carbon sources and that the nutrients available to the cells and their effects on pyruvate and acetate uptake be further investigated.