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 .In the atmosphere, where the O 2 level is 21% and the CO 2 level is 0.035%, the competition by O 2 accounts for as much as 30% of the reactions catalyzed by Rubisco.A number of photosynthetic organisms have developed ways to increase the level of CO 2 at the location of Rubisco in the plant.This results in an increase in CO 2 fixation and a decrease in the deleterious oxygenation reaction.An excellent example of a CO 2 -concentrating mechanism in higher plants is C 4 photosynthesis, which has arisen independently in a number of plant families.Aquatic photosynthetic organisms such as the microalgae have also adapted to low CO 2 levels by concentrating CO 2 internally.This Update will focus on CO 2 -concentrating mechanisms in the microalgae.For more detailed reviews of the CO 2 concentration by algae, the reader is referred to the special issue of the Canadian Journal of Botany (1998, Vol.76) and the article by Raven (1997).