Photosynthesis in Synechocystis sp. PCC 6803 is not optimally regulated under very high CO₂

One strategy for CO₂ mitigation is using photosynthetic microorganisms to sequester CO₂ under high concentrations, such as in flue gases. While elevated CO₂ levels generally promote growth, excessively high levels inhibit growth through uncertain mechanisms. This study investigated the physiology of the cyanobacterium Synechocystis sp. PCC 6803 under very high CO₂ concentrations and yet stable pH around 7.5. The growth rate of the wild type (WT) at 200 µmol photons m^-2 s^-1 and a gas phase containing 30% CO₂ was 2.7-fold lower compared to 4% CO₂. Using a CRISPR interference mutant library, we identified genes that, when repressed, either enhanced or impaired growth under 30% or 4% CO₂. Repression of genes involved in light harvesting (cpc and apc), photochemical electron transfer (cytM, psbJ, and petE), and several genes with little or unknown functions promoted growth under 30% CO₂, while repression of key regulators of photosynthesis (pmgA) and CO₂ capture and fixation (ccmR, cp12, and yfr1) increased growth inhibition under 30% CO₂. Experiments confirmed that WT cells were more susceptible to light inhibition under 30% than under 4% CO₂ and that a light-harvesting-impaired ΔcpcG mutant showed improved growth under 30% CO₂ compared to the WT. These findings suggest that enhanced fitness under very high CO₂ involves modifications in light harvesting, electron transfer, and carbon metabolism, and that the native regulatory machinery is insufficient, and in some cases obstructive, for optimal growth under 30% CO₂. This genetic profiling provides potential targets for engineering cyanobacteria with improved photosynthetic efficiency and stress resilience for biotechnological applications. KEY POINTS: • Synechocystis growth was inhibited under very high CO₂. • Inhibition of growth under very high CO₂ was light dependent. • Repression of photosynthesis genes improved growth under very high CO₂.