Directed Reaction Engineering Boosts Succinate Formation of Synechocystis sp. PCC 6803_Δsll1625

It is known that Synechocystis sp. PCC 6803 carrying a partial deletion of the succinate dehydrogenase (Synechocystis_∆sll1625) secretes succinate during aerobic cultivation with continuous illumination and in the presence of CO₂ . Maximal succinate titers of 2 mM (236 mg L^-1 ) are reported. CO₂ is identified as a crucial parameter for product formation, however, a detailed characterization of different cultivation conditions is still missing. Here the focus is on further reaction engineering to improve the photoautotrophic production of succinate using Synechocystis_∆sll1625. Therefore the impact of light availability, illumination regimes, nutrient availability, and external pH on product formation are investigated. Results obtained in this study reveal the importance of these parameters on the formation of succinate and cultivation with light/dark cycles increases the succinate concentration to 3 mM (354 mg L^-1 ) after 28 days of cultivation. Furthermore, cultivation in unbuffered medium under ambient CO₂ conditions even doubled the final succinate titer to 4 mM (472 mg L^-1 ) after 28 days. Taking biomass concentrations into account, a maximal yield of succinate on biomass of 215 mg_Succ  g_CDW ^-1 is achieved, which is the highest so far reported for the production of succinate utilizing Synechocystis as host organism.