Effect of extremely high-CO2 stress on energy distribution between photosystem I and photosystem II in a 'high-CO2' tolerant green alga, Chlorococcum littorale and the intolerant green alga Stichococcus bacillaris

Abstract A green alga, Chlorococcum littorale , has a tolerance to extremely high-CO 2 conditions (Kodama et al., J. Marine Biotech. 1 (1993) 21–25). In order to elucidate the mechanism underlying the resistance to such high CO 2 levels, we compared the changes in excitation energy ditribution between photosystem I (PS 1) and photosystem II (PS II) by 77 K fluorescence in cells of the high CO 2 -resistant C. littorale and the non-resistant Stichococcus bacillaris . Immediately after the cell are transferred from air to 40% CO 2 , the F 714 / F 687 ratio derived from 77 K fluorescence increases in C. littorale cells, suggesting an increase of transition from state 1 to state 2. During this period, more than 80% of plastoquinone A is in the reduced form and the activity of PS I increass. Eventually the F 714 / F 687 ratio, the concentration of reduced plastoquinone A and PS I activity decrease. However, no significant increase of F 714 / F 687 ratio is observed after the transfer of S. bacillaris cells from air to 40% CO 2 . The level of reduced plastoquinone A in S. bacillaris gradually increases and the activity of PS I does not show a large change. During the transient period, the level of the D1 protein is approximately constant in C. littorale cells, but is lowered in S. bacillaris . These results suggest that, under extremely high-CO 2 conditions, PS II is protected from photoinhibition by control of the state transition in C. littorale cells, whereas such a protection mechanism does not function in the alga S. bacillaris , non-resistant to CO 2 .