The Roles of Carbonic Anhydrases in Carbon Concentrating Mechanisms of Aquatic Photoautotrophs

Abstract

The article surveys the role of carbonic anhydrases (CAs) in inorganic carbon (Ci) acquisition by cyanobacteria, microalgae, and macrophytes under Ci limiting conditions. Slow Ci diffusion in aquatic environments imposes the need for carbon concentrating mechanisms (also named CO2 concentrating mechanisms, CCMs) in aquatic photoautotrophs, to transport Ci against the gradient and ensure a CO2 supply for photosynthesis. There are common requirements for efficient CCM functioning in cyanobacteria, algae, and aquatic angiosperms, including active transport of HCO3- to the Ci-concentrating compartment and CO2 generation from the HCO3- pool in the Rubisco-enriched subcompartment. Facilitating Ci diffusion in aqueous solutions and across lipid bilayers, CAs play essential roles in CCMs that are best studied in cyanobacteria, green algae, and diatoms. Roles of CAs in CCMs depend on their localization and include facilitation of active transmembrane Ci uptake by supplying C1 at the outer surface (Role 1) whilst removing it at the inner surface (Role 2), as well as accelerating CO2 production from HCO3- near Rubisco (Role 3) in a specialised CO2-tight compartment, known as the carboxysome in cyanobacteria or pyrenoid in microalgae. The compartmentalization of CAs is also critical because, if activated in the HCO3- -concentrating compartment, they can easily eliminate the Ci gradient created by the CCMs.