Oxygenic photosynthesis invariably utilizes rubisco as the core enzyme involved in the incorporation of inorganic carbon into the skeletons of organic compounds. However,

1. the present concentration of atmospheric CO₂ does not saturate the carboxylase activity, and
2. an oxygenase activity competitive with the carboxylase activity is an intrinsic function of the enzyme.

These features can help us to visualize why many O₂-producing organisms contain concentrating mechanisms for the fixation of inorganic carbon. These mechanisms actively transport HCO₃^–, CO₂, and/or H⁺ to increase the concentration of CO₂, and the CO₂/O₂ ratio, at the active site of rubisco, thereby favoring carboxylase activity. In most land plants, the CO₂-concentrating mechanisms are C₄ photosynthesis and crassulacean acid metabolism (CAM). While present in aquatic plants, CO₂ and HCO₃^– pumps at the plasma membrane have been studied most extensively in prokaryotic cyanobacteria and eukaryotic algae.

In response to changing concentrations of inorganic carbon in aqueous environments, many photosynthetic organisms induce mechanisms for concentrating CO₂ and thus favor the carboxylation reaction of rubisco (Ogawa and Kaplan 1987; Giordano et al. 2005). Although cyanobacteria and algae exhibit symptoms of photorespiration (retarded growth, precocious senescence) when the air used to aerate their medium is switched from 5% CO₂ to 0.03% CO₂, these organisms rapidly develop the ability to concentrate inorganic carbon so that they no longer photorespire. In cyanobacteria, two cellular assemblies are essential for the concentration of inorganic carbon:

1. transporters located at both the plasma and the thylakoid membranes, and
2. carboxysomes, where most of the rubisco and the carbonic anhydrase reside.

Web Figure 8.1.A   The cyanobacterial CO₂ concentrating mechanism. Translocators located in the plasma membrane (purple box) and the thylakoid membrane (blue box) pump HCO₃^– and CO₂ into the cytosol and thylakoid of a cyanobacterium. The diffusional resistance to efflux and the internal gradient of HCO₃^– drive the inorganic carbon to the carboxysome. The carboxysomal carbonic anhydrase catalyzes the interconversion of HCO₃^– and CO₂ and, in so doing, increases the concentration of CO₂ around rubisco, facilitating the carboxylation of ribulose 1,5-bisphosphate.

The carboxysome is a unique microbody surrounded by a protein coat that contains most of the cellular rubisco and is distinct from the thylakoid membranes. At low levels of atmospheric CO₂, cyanobacteria accumulate inorganic carbon in the cytosol using HCO₃^– and CO₂ pumps associated with plasma and thylakoid membranes, respectively. The HCO₃^– diffuses from the cytosol into the carboxysome and, once inside the compartment, a carbonic anhydrase converts HCO₃^– to CO₂ for the rubisco reaction.

In short, the flow of inorganic carbon from the environment to the close proximity of rubisco requires: (1) the accumulation of HCO₃^– in the cytosol; (2) its transport to the rubisco-containing carboxysomes; and (3) the conversion HCO₃^– to CO₂ by carboxysomal carbonic anhydrase (Badger and Price 2003). The concerted enrichment of CO₂ around rubisco ultimately suppresses the oxygenation of ribulose 1,5-bisphosphate and, in so doing, eliminates the need for photorespiration.