Photoautotrophic overproduction of mannitol in Synechocystis sp. PCC6803 using osmotic pressure as a driving force

Mannitol is a C(6) polyol with a wide range of applications in the food and pharmaceutical industry. The sustainable production of mannitol, especially via the direct conversion of CO₂ by photosynthetic cyanobacteria, has become increasingly appealing. However, previous work aiming to achieve mannitol production in cyanobacteria proved to be genetically unstable. To stabilize mannitol production in Synechocystis sp. PCC6803, we first validate that mannitol can work as well as the native compatible solutes (e.g., sucrose and glucosyl-glycerol) in conferring salt resistance. Under this background, the mannitol production system was proven to be stable under salt stress condition in the mutants which lack sucrose and glucosyl-glycerol in their arsenal against osmotic pressure (Chapter II). However, even with this stabilized mannitol production system, its production in Synechocystis sp. PCC6803 is still very low, and therefore, it is unable to meet the requirements of industrial scale. In the following studies, we precisely control the expression level of the mannitol cassette using different promoters in chapter III and proceed to select a super mannitol producer from adaptive laboratory evolution under increasing salt pressure in Chapter IV. Both methods are used to successfully increase the mannitol production in cells. The lastly in Chapter V, we explore a new pathway of mannitol expression in Synechocystis sp. PCC6803. This thesis aims to contribute a solid theoretical basis for achieving mannitol production in Synechocystis sp. PCC6803 at a commercial scale, and possibly, inspire researchers to develop highly efficient photosynthetic cell factories for other valuable compounds with similar properties.