Improving cyanobacteria productivity: From theory to assay

Abstract: Bio-based production of biochemicals and biofuels holds great promises for the transition towards a more sustainable society. With increasing levels of carbon dioxide (CO2) in the atmosphere, cyanobacteria stand apart as natural catalysts directly converting CO2 and light to product. However, current product productivities and titers do not meet the standard set by the petroleum-based industry. In particular, the solar-to-product efficiency needs to be drastically improved to make the process economically more interesting. As proof of concept, this thesis puts an emphasis on identifying metabolic limitations towards increased solar-to-product efficiency using model-guided formulation of strategies and genome-wide screening, followed by novel practical implementations. It follows previous works identifying the intracellular ATP/NADPH ratio as an important variable to balance photosynthesis, carbon fixation, product synthesis and biomass formation to ensure more performant metabolic engineering designs of photoautotrophs. In Paper I, we identified in silico growth-coupled metabolic designs linking product formation to growth to increase productivity and stability of the engineered strain. In Paper II, we found computationally and experimentally that carbon rerouting gave the best results to increase product formation. In Paper III, we used the CRISPRi system to further maximize carbon rerouting to product synthesis in growth-arrest strategies. Finally, in Paper IV, we conduct a genome-wide screening using a CRISPRi library and identified key targets to improve product synthesis, product tolerance and growth. We also demonstrate experimentally some of the strategies found in Paper I. This thesis suggests that growth-arrest production is a promising avenue to maximize the solar-to-product efficiency and asserts that systems biology tools will be needed to identify and tackle the remaining strain instability associated with those designs.

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