How do algae concentrate CO_2 to increase the efficiency of photosynthetic carbon fixation?

摘要

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 #mu#M. In cyanobacteria Rubisco has an even lower affinity for CO_2, and the K_m(CO_2) can be greater than 200 #mu#M. In comparison, the concentration of CO_2 in water in equilibrium with air is approximately 10 #mu#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.