Photoautotrophic production of succinate via the oxidative branch of the tricarboxylic acid cycle influences glycogen accumulation in Synechocystis sp. PCC 6803

Abstract

Cyanobacteria are interesting biocatalysts for the sustainable production of value-added compounds, but a functional link of biocatalytic efficiency and cell physiology is missing. Especially the role and structure of the tricarboxylic acid (TCA) cycle in cyanobacteria is not fully understood. Succinate dehydrogenase (SDH) is an essential enzyme linking the TCA cycle to the quinone pool and its substrate succinate is an important platform chemical. We utilized Synechocystis sp. PCC 6803 as a host organism for the photoautotrophic production of succinate via the oxidative branch of the TCA cycle. In Synechocystis sp. PCC 6803 SDH is described to be encoded by three open reading frames (ORF). Four SDH deletion mutants were created and characterized regarding the formation of succinate. Synechocystis_∆sll1625 was the best performing strain accumulating 420 mg L−1 succinate during cultivation in a 1.8 L-photobioreactor under constant illumination. Furthermore, deletion of the SDH resulted in a non-bleaching phenotype and concomitant accumulation of glycogen and succinate during growth at 5% CO2. This substantially influenced cell physiology of the mutant, reflected in decreased cell numbers and higher cell volumes. Thus, the TCA cycle seems to have a key role in controlling carbon fluxes from CO2 into biomass, storage compounds like glycogen, and other carbon sinks.