Short-term responses of photosynthetic reactions to extremely high-CO2 stress in a "high-CO2" tolerant green alga Chlorococcum littorale and an intolerant green alga Stichococcus bacillaris

A green alga, Chlorococcum littorale, has a tolerance to extremely high CO2 concentrations such as 40%. In order to elucidate the mechanism underlying resistance to such high CO;! concentrations, short-term changes in chlorophyll fluorescence were analyzed by using the saturating pulse method after the exposure of air-adapted cells to 40% CO2. Upon transfer of the air-adapted cells to 40% CO2, the quantum yield of PSII and O-2 evolution were suppressed within several minutes in C. littorale but not in Stichococcus bacillaris, which is intolerant to such high CO2 stress. During the stress, the air-adapted C. littorale mainly showed an increase of qT-type nonphotochemical quenching (qN), indicating a pronounced reduction in the PQ pool. The qN increase was accompanied by an increase in the 77K fluorescence derived from PSI, which is characteristic of state 2 transition. The quantum yields of PSI indicated enhanced cyclic electron flow in C, littorale. However, S. bacillaris cells did not show state transition. The PSII activity was more stable in S. bacillaris cells than in C. littorale cells during short-term exposure to 40% CO2 in light. Evidence indicated that photoinhibition under 40% CO2 was protected for a while by the xanthophyll cycle in S. bacillaris cells, but no such protection was found in C. littorale cells.