Proposed Carbon Dioxide Concentrating Mechanism in Chlamydomonas reinhardtii

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 .