Light and excessive CO2 uptake inhibit growth of Synechococcus sp. PCC 7002 under very high CO2 levels

Cultivation of photosynthetic microbes under elevated CO₂ is widely used in biotechnology to enhance growth. However, very high CO₂ levels inhibit growth for reasons that are not always clear. We established a method that allowed cultivation under different CO₂ concentrations and controlled pH to study the cyanobacterium Synechococcus sp. PCC 7002. Growth rates, cell composition, and transcriptomic profiles were investigated under limiting (0.04%), optimal (1–4%), and inhibitory high (15–30%) gas phase CO₂ contents, while maintaining pH ∼7.5. At intermediate light (200 μmol m⁻² s⁻¹), the growth rate at 30% CO₂ was ∼45% of that under optimal conditions. Under low light (<100 μmol m⁻² s⁻¹), growth was less inhibited by high CO₂ than at higher light intensities. Pigmentation and photosynthesis likewise decreased under high CO₂, except that carotenoids indicative of high-light stress increased. Transcriptome sequencing confirmed that the photosynthesis machinery was downregulated and stress responses were upregulated under high CO₂. Genetic knockdown of CO₂ uptake (ndhF3 and ndhF4) or overexpression of a heterologous Na⁺/H⁺ antiporter (nhaS3) improved tolerance to high CO₂, while knockdown of the CcmR transcriptional regulator reduced it. We conclude that the inhibiting species under very high inorganic carbon conditions was CO₂, and not HCO₃⁻, and that reduced light intensity or genetically suppressing active CO₂ uptake improved the tolerance of Synechococcus to very high CO₂ conditions. This work provides a basis for cultivation under a very wide range of CO₂ concentrations, thereby broadening the scope for metabolic manipulation and biotechnological applications of cyanobacteria.