Cyanobacteria form a procarboxysome-like structure in response to high CO₂

Fixing 25% of CO₂globally, cyanobacteria are integral to climate change efforts. The cyanobacterial CO₂concentrating mechanism (CCM) features the carboxysome, a bacterial microcompartment which houses their CO₂fixing machinery. The proteinaceous shell of the carboxysome restricts diffusion of CO₂, both inward and outward. While necessary for CCM function in air (0.04% CO₂), when grown in high CO₂levels (3% CO₂) representative of early earth, the shell would harmfully limit CO₂fixation. To understand how carboxysomes change form and function in response to increased CO₂conditions, we used a Grx1-roGFP2 redox sensor and single cell timelapse fluorescence microscopy to track subcellular redox states ofSynechococcussp. PCC 7002 grown in air or 3% CO₂. Comparing different levels of compartmentalization, we targeted the cytosol, a shell-less carboxysomal assembly intermediate called the procarboxysome, and the carboxysome. The carboxysome redox state was dynamic and, under 3% CO₂, procarboxysome-like structures formed and mirrored cytosolic redox states, indicating that a more permeable shell architecture may be favorable when [CO₂] is high. This work represents a step in understanding how cyanobacteria respond to changing CO₂concentrations and the selective forces driving carboxysome evolution.