The many types of carbonic anhydrases in photosynthetic organisms
Carbonic anhydrases (CAs) are enzymes that catalyze the interconversion of CO and HCO. 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 concentrating mechanisms (CCMs) which enhance the delivery of CO 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 to CO 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 for photosynthesis, other metabolic roles of CA have likely changed as the Earth's atmospheric CO level decreased. Since the algal and plant lineages diverged well before the decrease in atmospheric CO, it is likely that plant, algae and photosynthetic bacteria all adapted independently to the drop in atmospheric CO. 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 supply for photosynthesis and how CAs may help in the delivery of HCO for other metabolic reactions.