Cyanobacteria have evolved a significant environmental adaptation, known as a CO(2)-concentrating-mechanism (CCM), that vastly improves photosynthetic performance and survival under limiting CO(2) concentrations. The CCM functions to transport and accumulate inorganic carbon actively (Ci; HCO(3)(-), and CO(2)) within the cell where the Ci pool is utilized to provide elevated CO(2) concentrations around the primary CO(2)-fixing enzyme, ribulose bisphosphate carboxylase-oxygenase (Rubisco). In cyanobacteria, Rubisco is encapsulated in unique micro-compartments known as carboxysomes. Cyanobacteria can possess up to five distinct transport systems for Ci uptake. Through database analysis of some 33 complete genomic DNA sequences for cyanobacteria it is evident that considerable diversity exists in the composition of transporters employed, although in many species this diversity is yet to be confirmed by comparative phenomics. In addition, two types of carboxysomes are known within the cyanobacteria that have apparently arisen by parallel evolution, and considerable progress has been made towards understanding the proteins responsible for carboxysome assembly and function. Progress has also been made towards identifying the primary signal for the induction of the subset of CCM genes known as CO(2)-responsive genes, and transcriptional regulators CcmR and CmpR have been shown to regulate these genes. Finally, some prospects for introducing cyanobacterial CCM components into higher plants are considered, with the objective of engineering plants that make more efficient use of water and nitrogen.

SUMMARY Cyanobacteria are the globally dominant photoautotrophic lineage. Their success is dependent on a set of adaptations collectively termed the CO 2 -concentrating mechanism (CCM). The purpose of the CCM is to support effective CO 2 fixation by enhancing the chemical conditions in the vicinity of the primary CO 2 -fixing enzyme, d -ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO), to promote the carboxylase reaction and suppress the oxygenase reaction. In cyanobacteria and some proteobacteria, this is achieved by encapsulation of RubisCO within carboxysomes, which are examples of a group of proteinaceous bodies called bacterial microcompartments. Carboxysomes encapsulate the CO 2 -fixing enzyme within the selectively permeable protein shell and simultaneously encapsulate a carbonic anhydrase enzyme for CO 2 supply from a cytoplasmic bicarbonate pool. These bodies appear to have arisen twice and undergone a process of convergent evolution. While the gross structures of all known carboxysomes are ostensibly very similar, with shared gross features such as a selectively permeable shell layer, each type of carboxysome encapsulates a phyletically distinct form of RubisCO enzyme. Furthermore, the specific proteins forming structures such as the protein shell or the inner RubisCO matrix are not identical between carboxysome types. Each type has evolutionarily distinct forms of the same proteins, as well as proteins that are entirely unrelated to one another. In light of recent developments in the study of carboxysome structure and function, we present this review to summarize the knowledge of the structure and function of both types of carboxysome. We also endeavor to cast light on differing evolutionary trajectories which may have led to the differences observed in extant carboxysomes.

Compartmentalization of enzymes is a cellular strategy to regulate metabolic pathways and increase their efficiency 1 . The αand β-carboxysomes of cyanobacteria contain Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), a complex of 8 large (RbcL) and 8 small (RbcS) subunits, and carbonic anhydrase (CA) 2-4 . Since the proteinaceous carboxysome shell provides a barrier to the diffusion of CO2 but not HCO3 − (ref. 5), CA generates high concentrations of CO2 for carbon fixation by Rubisco 6 . The shell also prevents access to reducing agents, generating an oxidizing environment 7-9 . Formation of β-carboxysomes involves aggregation of Rubisco by the protein CcmM 10 , which exists in two forms: Fulllength CcmM (M58 in Synechococcus elongatus PCC7942) containing a CA-like domain 8 followed by three Rubisco small subunit-like (SSUL) modules connected by flexible linkers, and M35, lacking the CA-like domain 11 . It has long been speculated that the SSUL modules interact with Rubisco by replacing RbcS 2-4 . Here we reconstituted the Rubisco:CcmM complex and solved its structure. Contrary to expectation, the SSUL modules do not replace RbcS, but bind close to the equatorial region of Rubisco between RbcL dimers, linking Rubisco molecules and inducing phase separation into a liquid-like matrix. Disulfide bond formation in SSUL increases the network flexibility and is required for carboxysome function in vivo. Importantly, the formation of the liquid-like condensate of Rubisco is mediated by dynamic interactions with the SSUL domains, rather than by low complexity sequences, which typically mediate liquid-liquid phase separation in eukaryotes 12,13 . Indeed, within the pyrenoid of eukaryotic algae, the functional homologue of carboxysomes, Rubisco has been shown to adopt a liquid-like state via interactions with the intrinsically disordered protein EPYC1 14 . Understanding carboxysome biogenesis will be important in efforts to engineer CO2 concentrating mechanisms (CCM) in plants 15-19 .

Cyanobacteria probably exhibit the widest range of diversity in growth habitats of all photosynthetic organisms. They are found in cold and hot, alkaline and acidic, marine, freshwater, saline, terrestrial, and symbiotic environments. In addition to this, they originated on earth at least 2.5 billion years ago and have evolved through periods of dramatic O2 increases, CO2 declines, and temperature changes. One of the key problems they have faced through evolution and in their current environments is the capture of CO2 and its efficient use by Rubisco in photosynthesis. A central response to this challenge has been the development of a CO2 concentrating mechanism (CCM) that can be adapted to various environmental limitations. There are two primary functional elements of this CCM. Firstly, the containment of Rubisco in carboxysome protein microbodies within the cell (the sites of CO2) elevation), and, secondly, the presence of several inorganic carbon (Ci) transporters that deliver HCO3- intracellularly. Cyanobacteria show both species adaptation and acclimation of this mechanism. Between species, there are differences in the suites of Ci transporters in each genome, the nature of the carboxysome structures and the functional roles of carbonic anhydrases. Within a species, different CCM activities can be induced depending on the Ci availability in the environment. This acclimation is largely based on the induction of multiple Ci transporters with different affinities and specificities for either CO2 or HCO3- as substrates. These features are discussed in relation to our current knowledge of the genomic sequences of a diverse array of cyanobacteria and their ecological environments.