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

A green alga, Chlorococcum littorale, has a tolerance to extremely high-CO2 conditions (Kodama et al., J. Marine Biotech. 1 (1993) 21-25). In order to elucidate the mechanism underlying the resistance to such high CO, levels, we compared the changes in excitation energy distribution between photosystem I (PS I) and photosystem II (PS II) by 77 K fluorescence in cells of the high CO2-resistant C. littorale and the non-resistant Stichococcus bacillaris. Immediately after the cells are transferred from air to 40% CO2, 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 increases. Eventually the F-714/F-687 ratio, the concentration of reduced plastoquinone A and PS I activity decrease. However, no significant increase of the F-714/F-687 ratio is observed after the transfer of S. bacillaris cells from air to 40% CO2. 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-CO2 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,. (C) 1998 Elsevier Science S.A. All rights reserved.