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Carbonic anhydrases (CAs) are ubiquitous enzymes that accelerate the reversible conversion of CO 2 to HCO 3 À . The Arabidopsis genome encodes members of the a-, band c-CA families, and it has been hypothesized that bCA activity has a role in photosynthesis. In this work, we tested this hypothesis by characterizing the two plastidial bCAs, bCA1 and bCA5, in physiological conditions of growth. We conclusively established that both proteins are localized in the chloroplast stroma and that the loss of bCA5 induced the expression of bCA1, supporting the existence of regulatory mechanisms to control the expression of stromal bCAs. We also established that bCA1 and bCA5 have markedly different enzymatic kinetics and physiological relevance. Specifically, we found that bCA5 had a first-order rate constant ~10-fold lower than bCA1, and that the loss of bCA5 is detrimental to growth and could be rescued by high CO 2 . Furthermore, we established that, while a bCA1 mutation showed near wild-type growth and no significant impact on photosynthetic efficiency, the loss of bCA5 markedly disrupted photosynthetic efficiency and light-harvesting capacity at ambient CO 2 . Therefore, we conclude that in physiological autotrophic growth, the loss of the more highly expressed bCA1 does not compensate for the loss of a less active bCA5, which in turn is involved in growth and photosynthesis at ambient CO 2 levels. These results lend support to the hypothesis that, in Arabidopsis,bCAs have non-overlapping roles in photosynthesis and identify a critical activity of stromal bCA5 and a dispensable role for bCA1.

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