How Do Algae Concentrate CO2 to Increase the Efficiency of Photosynthetic Carbon Fixation?1

The ability of photosynthetic organisms to use CO 2 for photosynthesis depends in part on the properties of Rubisco. Rubisco has a surprisingly poor affinity for CO 2 , probably because it evolved in an atmosphere that had very high CO 2 levels compared with the present atmosphere. In C 3 plants the K m (CO 2 ) of Rubisco ranges between 15 and 25 m. In cyanobacteria Rubisco has an even lower affinity for CO 2 , and the K m (CO 2 ) can be greater than 200 m. In comparison, the concentration of CO 2 in water in equilibrium with air is approximately 10 m. From these numbers it becomes apparent that Rubisco is operating at no more than 30% of its capacity under standard atmospheric conditions. This is one of the reasons that C 3 plants contain such large amounts of Rubisco. Exacerbating this situation is the fact that O 2 is a competitive substrate with respect to CO 2 .