# [CO2] Alters Cyanobacterial Carboxysome Encapsulation and Redox State

## TL;DR

This study shows how cyanobacteria adjust their CO2-fixing structures in response to different CO2 levels, revealing new insights into their adaptability.

## Contribution

The study reveals that carboxysomes dynamically adjust their redox state and encapsulation under varying CO2 conditions.

## Key findings

- Carboxysome redox state is dynamic and changes with CO2 levels.
- Under 3% CO2, partially encapsulated procarboxysomes form, exposing contents to the cytosol.
- These findings suggest carboxysomes are adaptable to environmental CO2 concentrations.

## Abstract

Responsible for fixing 25% of CO2 globally, cyanobacteria use carboxysomes to house their CO2 fixing machinery. The formation and permeability of the proteinaceous shell of carboxysomes is an area of active study. While necessary in air (0.04% CO2), the shell is not required when cyanobacteria are in high CO2 levels representative of early Earth. To understand how the carboxysome shell responds to increased CO2 conditions, we used a Grx1-roGFP2 redox sensor and single cell timelapse fluorescence microscopy to track subcellular redox states of Synechococcus sp. PCC 7002. Comparing different levels of compartmentalization, we targeted the cytosol, a shell-less carboxysomal assembly intermediate called procarboxysomes, and carboxysomes. Carboxysome redox state was dynamic, and, under 3% CO2 conditions, procarboxysome-like structures formed which were only partially encapsulated and exposed the carboxysome contents to the cytosol. This work expands the adaptability of carboxysomes to environmental conditions and builds understanding of the selective forces that initially drove carboxysome evolution.