Flux balance analysis for overproduction of organic acids by Synechocystis sp. PCC 6803 under dark anoxic condition

Cyanobacteria have recently been considered as potential organisms for metabolic engineering with an objective to improve its ability to synthesize biofuel precursors and other value-added products. A modified version of the genome-scale metabolic model of Synechocystis sp. PCC 6803 model organism was used to theoretically demonstrate the flux distribution in the central carbon metabolism towards optimal accumulation of organic acids, namely succinate, acetate, lactate and malate. We further validate the model using experimental data reported by Hasunuma et al. Met. Engg. Comm., 2016, 3, 130-141 for organic acid synthesis under dark anoxic conditions from glycogen accumulated during photoautotrophic growth conditions by sequestering atmospheric CO2. We show that key limiting factors are redox balance causing gluconeogenesis, availability of CO2 and channeling of carbon flux to TCA cycle, in addition to the rate of breakdown of glycogen. Sodium bicarbonate supplementation enhances succinate production flux by eliminating the CO2 limitations for the phosphoenolpyruvate (PEP) to oxaloacetate biochemical reaction catalyzed by PEP carboxylase. A combined strategy of supplementation with NaHCO3 and increased temperature can only offer similar to 25% of the theoretical maximum succinate production. We suggest possible metabolic pathway interventional approaches to improve the succinate productivity.