Acclimation of photosynthetic microorganisms to changing ambient CO ₂ concentration

Photosynthetic microorganisms can acclimate to a wide range of CO2 concentration, from as low as 0.001% to ≈10% CO2 (vol/vol in the air in equilibrium with their environment). Some can even grow in the presence of 40% CO2 (1). Acclimation to a limiting CO2 level, well below the Km(CO2) of their carboxylating enzyme, Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase), is achieved by substantial physiological and structural changes at various cell levels (2–4). The most prominent of these is the induction of a CO2 concentrating mechanism (CCM), which raises the [CO2] in close proximity to Rubisco. The latter is for the most part located in pyrenoids or carboxysomes in eukaryotes and prokaryotes photosynthetic microorganisms, respectively (5). This CCM involves light energy-dependent inorganic carbon uptake and accumulation of HCO within the cell. In cyanobacteria, the accumulated HCO penetrates the carboxysomes where carbonic anhydrase (CA) catalyzes the formation of CO2 in the close vicinity of Rubisco. The elevated CO2 concentration is thus confined to the carboxysomes (3). This model is most likely also applicable to eukaryotic algae where pyrenoids, densely packed with Rubisco and also containing CA, may have the same function as carboxysomes (2–7). The elevated [CO2] in these bodies compensates for the relatively low affinity of Rubisco for CO2 and consequently also decreases photorespiration.