Inhibition of photosynthesis by intracellular carbonic anhydrase in microalgae under excess concentrations of CO2

When cells of Chlorococcum littorale that had been grown in air (air-grown cells) were transferred to extremely high CO2 concentrations (>20%), active photosynthesis resumed after a lag period which lasted for 1–4 days. In contrast, C. littorale cells which had been grown in 5% CO2 (5% CO2-grown cells) could grow in 40% CO2 without any lag period. When air-grown cells were transferred to 40% CO2, the quantum efficiency of PS II (ΦII) decreased greatly, while no decrease in ΦII was apparent when the 5% CO2-grown cells were transferred to 40% CO2. In contrast to air-grown cells, 5% CO2-grown cells showed neither extracellular nor intracellular carbonic anhydrase (CA) activity. Upon the acclimation of 5% CO2-grown cells to air, photosynthetic susceptibility to 40% CO2 was induced. This change was associated with the induction of CA. In addition, neither suppression of photosynthesis nor arrest of growth was apparent when ethoxyzolamide (EZA), a membrane-permeable inhibitor of CA, had been added before transferring air-grown cells of C. littorale to 40% CO2. The intracellular pH value (pHi) decreased from 7.0 to 6.4 when air-grown C. littorale cells were exposed to 40% CO2 for 1–2 h, but no such decrease in pHi was apparent in the presence of EZA. Both air- and 5% CO2-grown cells of Chlorella sp. UK001, which was also resistant to extremely high CO2 concentrations, grew in 40% CO2 without any lag period. The activity of CA was much lower in air-grown cells of this alga than those in air-grown C. littorale cells. These results prompt us to conclude that intracellular CA caused intracellular acidification and hence inhibition of photosynthetic carbon fixation when air-grown C. littorale cells were exposed to excess concentrations of CO2. No such harmful effect of intracellular CA was observed in Chlorella sp. UK001 cells.