Carbon Acquisition Mechanisms of Algae: Carbon Dioxide Diffusion and Carbon Dioxide Concentrating Mechanisms

Abstract

At least 95{\%} of the organic carbon in cyanobacteria and algae has been fixed as CO2 by Rubisco. The kinetic properties of Rubisco are such that even the genetic variants with the highest CO2:O2 selectivity would show limited CO2 fixation and significant oxygenase activity if CO2 and O2 fluxes into cells are driven solely by diffusion. This is especially the case for submerged algae with low gas diffusion coefficients relative to diffusion boundary layer thicknesses. There is evidence from gas exchange properties and intracellular inorganic concentration that inorganic carbon concentrating mechanisms (CCMs) function in all cyanobacteria and many algae. CCMs can, in theory, operate through three categories of mechanism: the first mechanism is active transport of HCO-3 and/or CO2 across membranes, the second is CO2 pumping by a biochemical mechanism analogous to C4 and CAM (Crassulacean Acid Metabolism) pathways in higher land plants, and the third mechanism involves active transport of H+ producing an acid compartment supplied with HCO-3, which generates a high (equilibrium) CO2 concentration that supplies CO2 to Rubisco in a nearby more alkaline compartment. Most evidence favors the first mechanism as being responsible for CCM activity in algae. The CO2 (and HCO-3) permeability of membranes is crucial in defining the extent of inorganic C leakage from CCMs, and the functioning of diffusive CO2 supply to Rubisco from the medium. Like carbonic anhydrases, CCMs, are probably polyphyletic.