Carbonic anhydrases (CAs) are zinc metalloenzymes that catalyze the interconversion of CO2 and HCO3- and are ubiquitous in nature. Higher plants contain three evolutionarily distinct CA families, αCAs, βCAs, and γCAs, where each family is represented by multiple isoforms in all species. Alternative splicing of CA transcripts appears common; consequently, the number of functional CA isoforms in a species may exceed the number of genes. CAs are expressed in numerous plant tissues and in different cellular locations. The most prevalent CAs are those in the chloroplast, cytosol, and mitochondria. This diversity in location is paralleled in the many physiological and biochemical roles that CAs play in plants. In this review, the number and types of CAs in C3, C4, and crassulacean acid metabolism (CAM) plants are considered, and the roles of the α and γCAs are briefly discussed. The remainder of the review focuses on plant βCAs and includes the identification of homologs between species using phylogenetic approaches, a consideration of the inter- and intracellular localization of the proteins, along with the evidence for alternative splice forms. Current understanding of βCA tissue-specific expression patterns and what controls them are reviewed, and the physiological roles for which βCAs have been implicated are presented.

Aquatic photosynthetic microorganisms account for almost 50% of the world's photosynthesis (19). These organisms face several challenges in acquiring CO 2 from the environment. The first challenge is presented by the properties of ribulose bisphosphate carboxylase-oxygenase (Rubisco). Rubisco is an unusually slow enzyme with a low affinity for CO 2. At atmospheric levels of CO 2 , Rubisco can function at only about 25% of its catalytic capacity because the concentration of dissolved CO 2 is less than the K m (CO 2) of Rubisco and due to the relatively high concentration of O 2 which competes with CO 2. A second challenge these organisms face is that the diffusion of CO 2 in an aqueous solution is 10,000 times slower than the diffusion of CO 2 in air. Thus, the ability to scavenge CO 2 as quickly as it becomes available is highly advantageous to aquatic photosynthetic organisms. Third, algae often experience significant fluctuations in inorganic carbon (C i ϭ CO 2 ϩ HCO 3 Ϫ) levels and pH, which change the availability of CO 2 and HCO 3 Ϫ for photosynthesis. At an acidic pH, the vast majority of C i is in the form of CO 2 , while at an alkaline pH, C i is mostly in the form of HCO 3 Ϫ , with CO 2 making up only a small fraction of the available C i (8, 25). Algae have adapted to these challenges through the development of a CO 2 concentrating mechanism (CCM). The CCM is a biological adaptation to low carbon dioxide concentrations in the environment. It is a mechanism which augments photosynthetic productivity in algal cells by increasing levels of inorganic carbon many times over the environmental concentration of carbon dioxide. In this minireview, we aim to provide an update on the CCM and present a model on how the green alga Chlamydomonas reinhardtii concentrates CO 2 .

Significance: Models of the CO ₂ concentrating mechanism (CCM) of green algae and diatoms postulate that chloroplast CO ₂ is generated from HCO ₃ ⁻ brought into the acidic thylakoid lumen and converted to CO ₂ by specific thylakoid carbonic anhydrases. However, the identity of the transporter required for thylakoid HCO ₃ ⁻ uptake has remained elusive. In this work, 3 bestrophin-like proteins, BST1–3, located on the thylakoid membrane have been found to be essential to the CCM of Chlamydomonas . Reduction in expression of BST1–3 markedly reduced the inorganic carbon affinity of the alga. These proteins are prime candidates to be thylakoid HCO ₃ ⁻ transporters, a critical currently missing step of the CCM required for future engineering efforts of the Chlamydomonas CCM into plants to improve photosynthesis.

The removal of two cytoplasmic carbonic anhydrases, βCA2 and βCA4, from Arabidopsis causes reduced growth in plants grown in a low-CO2 environment.