Acclimation of photosynthetic microorganisms to changing ambient CO2 concentration.

P microorganisms can acclimate to a wide range of CO2 concentration, from as low as 0.001% to '10% CO2 (volyvol 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 carboxylaseyoxygenase), 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 energydependent inorganic carbon uptake and accumulation of HCO3 2 within the cell. In cyanobacteria, the accumulated HCO3 2