Fluxomic Topology of Cyanobacteria: Rigidity and Plasticity

Synechocystis sp. strain PCC 6803 is a model cyanobacterium that has been widely used as photo-biorefinery chassis. Like fast-growing E.coli, this species contains a complete TCA cycle that is predicted to be highly active for glucose fermentation in absence of photosynthesis. Via ¹³C-metabolic flux analysis and isotope dilution mass spectrometry for metabolite pool measurements, we observe that Synechocystis 6803 has a minimal flux through the TCA cycle (relative flux < 5% of glucose uptake), while its pentose phosphate pathway operates in a cyclic mode for complete glucose oxidation under dark condition. We further demonstrate that cyanobacteria can adapt its metabolism towards different light and carbon sources via flexible fluxes through sugar phosphate pathways and ATP charges, while its fluxes and metabolite pool sizes in the TCA cycle are maintained at very low levels. For example, its acetyl-CoA pools remain 5~10 folds lower than E.coliunder phototrophic or heterotrophic conditions, minimizing cell growth and acetate overflow capability. Flux analysis also reveals a 6.9 mmol/g/h of non-growth-associated ATP maintenance requirements for Synechocystis 6803 and the essentiality of oxidative phosphorylation for ATP generations under light insufficient conditions. The results explain why most cyanobacteria are unable to grow anaerobically with glucose under dark or low light conditions. In summary, enzyme machineries and thermodynamic favorability constrain cyanobacterial fluxome for optimal biosynthesis with organic carbon sources. Lessons from this study also include Synechocystis 6803 as a good chassis for the production of chemicals derived from its sugar phosphate pathways rather than its TCA cycle.