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

Abstract

Abstract One strategy for CO 2 mitigation is using photosynthetic microorganisms to sequester CO 2 under high concentrations, such as in flue gases. While elevated CO 2 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 2 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% CO 2 was 2.7-fold lower compared to 4% CO 2 . Using a CRISPR interference mutant library, we identified genes that, when repressed, either enhanced or impaired growth under 30% or 4% CO 2 . 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 2 , while repression of key regulators of photosynthesis ( pmgA ) and CO 2 capture and fixation ( ccmR , cp12 , and yfr1 ) increased growth inhibition under 30% CO 2 . Experiments confirmed that WT cells were more susceptible to light inhibition under 30% than under 4% CO 2 and that a light-harvesting-impaired Δ cpcG mutant showed improved growth under 30% CO 2 compared to the WT. These findings suggest that enhanced fitness under very high CO 2 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 2 . 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 2 . • Inhibition of growth under very high CO 2 was light dependent. • Repression of photosynthesis genes improved growth under very high CO 2 . Graphical Abstract

Topics & Concepts