Algal Carbon Dioxide Concentrating Mechanisms

Abstract

Photosynthetic microorganisms like cyanobacteria and most eukaryotic algae acclimate to a limited availability of carbon dioxide (CO 2 ) in their environment by inducing a process called the carbon dioxide concentrating mechanism. This process uses an active inorganic carbon (C i ; CO 2 and/or HCO 3 − ) uptake system that leads to the internal accumulation of C i to levels significantly higher than extracellular levels. Carbonic anhydrase activity converts much of the accumulated hydrogen carbonate to CO 2 , concentrating this substrate around Rubisco and thereby optimising photosynthetic efficiency even under low CO 2 conditions. The efficiency of the process is further improved by the sequestration of Rubisco into specialised structures like the cyanobacterial carboxysome or the pyrenoid in eukaryotic algae. The carbon dioxide concentrating mechanism enhances carbon dioxide fixation and growth in algae. With the increasing demands for sustainable energy sources, algae with efficient carbon dioxide concentrating mechanisms are attractive models for biotechnological and transgenic applications for biofuel and biomass production. Key Concepts The carbon dioxide concentrating mechanism helps photosynthetic algae optimise photosynthesis under limiting CO 2 conditions. Rubisco uses both carbon dioxide and oxygen as substrates. Rubisco is localised in the carboxysome in cyanobacteria and the pyrenoid in the green alga, Chlamydomonas reinhardtii . The charged hydrogen carbonate needs transporters to enter the cell and cross organellar membranes. Carbonic anhydrase is an efficient enzyme that carries out the reversible interconversion of carbon dioxide and hydrogen carbonate and this reaction proceed at a much faster rate than the uncatalysed reaction. At higher pH levels, most of the inorganic carbon is in the form of hydrogen carbonate. Most algae with carbon dioxide concentrating mechanisms can take up both carbon dioxide and hydrogen carbonate. Carbon dioxide can diffuse out of the cell so algae trap carbon dioxide in the form of the charged hydrogen carbonate anion. Algae are used for large‐scale biomass production and are currently being explored as sources of biofuel.