The many types of carbonic anhydrases in photosynthetic organisms.

Carbonic anhydrases (CAs) are enzymes that catalyze the interconversion of CO 2 and HCO 3 − . In nature, there are multiple families of CA, designated with the Greek letters α through θ. CAs are ubiquitous in plants, algae and photosynthetic bacteria, often playing essential roles in the CO 2 concentrating mechanisms (CCMs) which enhance the delivery of CO 2 to Rubisco. As algal CCMs become better characterized, it is clear that different types of CAs are playing the same role in different algae. For example, an α-CA catalyzes the conversion of accumulated HCO 3 − to CO 2 in the green alga Chlamydomonas reinhardtii , while a θ-CA performs the same function in the diatom Phaeodactylum tricornutum . In this review we argue that, in addition to its role of delivering CO 2 for photosynthesis, other metabolic roles of CA have likely changed as the Earth’s atmospheric CO 2 level decreased. Since the algal and plant lineages diverged well before the decrease in atmospheric CO 2 , it is likely that plant, algae and photosynthetic bacteria all adapted independently to the drop in atmospheric CO 2 . In light of this, we will discuss how the roles of CAs may have changed over time, focusing on the role of CA in pH regulation, how CAs affect CO 2 supply for photosynthesis and how CAs may help in the delivery of HCO 3 − for other metabolic reactions. [ABSTRACT FROM AUTHOR]