Cyanobacteria form a procarboxysome-like structure in response to high CO ₂

Fixing 25% of CO ₂ globally, cyanobacteria are integral to climate change efforts. The cyanobacterial CO ₂ concentrating mechanism (CCM) features the carboxysome, a bacterial microcompartment which houses their CO ₂ fixing machinery. The proteinaceous shell of the carboxysome restricts diffusion of CO ₂ , both inward and outward. While necessary for CCM function in air (0.04% CO ₂ ), when grown in high CO ₂ levels (3% CO ₂ ) representative of early earth, the shell would harmfully limit CO ₂ fixation. To understand how carboxysomes change form and function in response to increased CO ₂ conditions, we used a Grx1-roGFP2 redox sensor and single cell timelapse fluorescence microscopy to track subcellular redox states of Synechococcus sp. PCC 7002 grown in air or 3% CO ₂ . 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% CO ₂ , procarboxysome-like structures formed and mirrored cytosolic redox states, indicating that a more permeable shell architecture may be favorable when [CO ₂ ] is high. This work represents a step in understanding how cyanobacteria respond to changing CO ₂ concentrations and the selective forces driving carboxysome evolution.