James Moroney, Louisiana State University, Carbonic anhydrases in photosynthetic organisms: locations and physiological roles in CO2 acquisition

The relatively low CO2 concentration in the earth’s atmosphere has led to adaptations by photosynthetic organisms that increase the CO2 concentration for Rubisco. These adaptations include C4 photosynthesis in terrestrial plants and CO2 concentrating mechanisms (CCMs) in aquatic photosynthetic organisms. Carbonic anhydrases, zinc-containing metalloenzymes that catalyze the interconversion of CO2 and HCO3-, are important components of these CCMs. The location of these carbonic anhydrases, as well as the other components of the CCMs, is essential to the proper functioning of the CCM. In cyanobacteria, a carbonic anhydrase is needed in the carboxysome to convert accumulated HCO3- to CO2 for fixation. The unicellular green alga Chlamydomonas reinhardtii has nine genes encoding carbonic anhydrases. These carbonic anhydrases are localized in many different organelles within cell, including the cell wall, chloroplast and mitochondria. Two of these CAs, CAH3 and CAH6, are localized to the chloroplast, and their locations within the chloroplast are necessary for the proper functioning of the CCM. Higher plants, even those that have C3 photosynthesis, also encode a large number of CAs. Arabidopsis has 14 genes that could encode CA, and at least eight of these genes are expressed in leaves. In this talk, the locations and physiological roles of the carbonic anhydrase isoforms of C. reinhardtii and Arabidopsis will be discussed.