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

One strategy for CO mitigation is using photosynthetic microorganisms to sequester CO under high concentrations, such as in flue gases. While elevated CO 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 CO concentrations and yet stable pH around 7.5. The growth rate of the wild type (WT) at 200 µmol photons m s and a gas phase containing 30% CO was 2.7-fold lower compared to 4% CO. Using a CRISPR interference mutant library, we identified genes that, when repressed, either enhanced or impaired growth under 30% or 4% CO. 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% CO, while repression of key regulators of photosynthesis (pmgA) and CO capture and fixation (ccmR, cp12, and yfr1) increased growth inhibition under 30% CO. Experiments confirmed that WT cells were more susceptible to light inhibition under 30% than under 4% CO and that a light-harvesting-impaired ΔcpcG mutant showed improved growth under 30% CO compared to the WT. These findings suggest that enhanced fitness under very high CO 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% CO. 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 CO. • Inhibition of growth under very high CO was light dependent. • Repression of photosynthesis genes improved growth under very high CO.