Cyanobacterial carboxysomes: Microcompartments that facilitate CO ₂ fixation

Carboxysomes are extraordinarily efficient proteinaceous microcompartments that encapsulate the primary CO₂-fixing enzyme (ribulose-1,5- bisphosphate carboxylase/oxygenase, RuBisCO) in cyanobacteria and some proteobacteria. These microbodies form part of a CO₂-concentrating mechanism (CCM), operating together with active CO₂ and HCO ₃^- uptake transporters which accumulate HCO ₃^- in the cytoplasm of the cell. Cyanobacteria (also known as blue-green algae) are highly productive on a global scale, especially those species from open-ocean niches, which collectively contribute nearly 30% of global net primary fixation. This productivity would not be possible without a CCM which is dependent on carboxysomes. Two evolutionarily distinct forms of carboxysome are evident that encapsulate proteobacterial RuBisCO form-1A or higher-plant RuBisCO form-1B, respectively. Based partly on RuBisCO phylogeny, the two carboxysome types are known either as α-carboxysomes, found in predominantly oceanic cyanobacteria (α-cyanobacteria) and some proteobacteria, or as β-carboxysomes, found mainly in freshwater/estuarine cyanobacteria (β-cyanobacteria). Both carboxysome types are believed to have evolved in parallel as a consequence of fluctuating atmospheric CO ₂ levels and evolutionary pressure acting via the poor enzymatic kinetics of RuBisCO. The three-dimensional structures and protein components of each carboxysome type reflect distinct evolutionarily strategies to the same major functions: subcellular compartmentalization and RuBisCO encapsulation, oxygen exclusion, and CO₂ concentration and fixation.