Cyanobacterial carboxysomes: microcompartments that facilitate CO2 fixation

Area of Science:

• Microbiology
• Biochemistry
• Evolutionary Biology

Background:

• Carboxysomes are protein microcompartments central to the carbon-concentrating mechanism (CCM) in cyanobacteria and proteobacteria.
• They encapsulate ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), the primary CO2-fixing enzyme, crucial for cellular productivity.

Purpose of the Study:

• To elucidate the evolutionary strategies and functional significance of distinct carboxysome types (alpha and beta) in cyanobacteria.
• To understand how these microcompartments enhance CO2 fixation despite RuBisCO's kinetic limitations.

Main Methods:

• Comparative analysis of carboxysome structures and protein components.
• Phylogenetic studies of RuBisCO and carboxysome types.
• Investigation of carboxysome roles in CO2 concentrating mechanisms.

Main Results:

• Two distinct carboxysome types, alpha and beta, evolved in parallel, encapsulating different RuBisCO forms (1A and 1B, respectively).
• Alpha-carboxysomes are found in oceanic cyanobacteria, while beta-carboxysomes are prevalent in freshwater/estuarine species.
• Both types evolved to address fluctuating atmospheric CO2 and RuBisCO's poor kinetics through compartmentalization, CO2 concentration, and oxygen exclusion.

Conclusions:

• Carboxysomes represent a key adaptation for efficient photosynthesis in diverse aquatic environments.
• Distinct evolutionary paths for alpha and beta carboxysomes highlight convergent evolution driven by environmental CO2 availability.
• Understanding carboxysome function is critical for appreciating global primary productivity in cyanobacteria.