Historical perspective on microalgal and cyanobacterial acclimation to low- and extremely high-CO2 conditions

Reports in the 1970s from several laboratories revealed that the affinity of photosynthetic machinery for dissolved inorganic carbon (DIC) was greatly increased when unicellular green microalgae were transferred from high to low-CO2 conditions. This increase was due to the induction of carbonic anhydrase (CA) and the active transport of CO2 and/or HCO3− which increased the internal DIC concentration. The feature is referred to as the ‘CO2-concentrating mechanism (CCM)’. It was revealed that CA facilitates the supply of DIC from outside to inside the algal cells. It was also found that the active species of DIC absorbed by the algal cells and chloroplasts were CO2 and/or HCO3−, depending on the species. In the 1990s, gene technology started to throw light on the molecular aspects of CCM and identified the genes involved. The identification of the active HCO3− transporter, of the molecules functioning for the energization of cyanobacteria and of CAs with different cellular localizations in eukaryotes are examples of such successes. The first X-ray structural analysis of CA in a photosynthetic organism was carried out with a red alga. The results showed that the red alga possessed a homodimeric β-type of CA composed of two internally repeating structures. An increase in the CO2 concentration to several percent results in the loss of CCM and any further increase is often disadvantageous to cellular growth. It has recently been found that some microalgae and cyanobacteria can grow rapidly even under CO2 concentrations higher than 40%. Studies on the mechanism underlying the resistance to extremely high CO2 concentrations have indicated that only algae that can adopt the state transition in favor of PS I could adapt to and survive under such conditions. It was concluded that extra ATP produced by enhanced PS I cyclic electron flow is used as an energy source of H+-transport in extremely high-CO2 conditions. This same state transition has also been observed when high-CO2 cells were transferred to low CO2 conditions, indicating that ATP produced by cyclic electron transfer was necessary to accumulate DIC in low-CO2 conditions.