Regulation of energy balance in photosystems in response to extremely-high-CO 2 (40%) and low-CO 2 (0.04%) stress was studied in extremely-high-CO 2 -tolerant green microalgae, Chlorococcum littorale and Chlorella sp. UK001. To investigate the energy input process, we assessed an F 714 /F 685 -ratio in a 77K fluorescence emission spectrum induced by 440-nm excitation in intact cells, which represents a ratio of fluorescence intensities derived from lightharvesting chlorophyll complexes in PSI

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... d PSII. The F 714 / F 685 -ratio increased in several days after transferring C. littorale cells from air to 40% CO 2 , from 3% to 40% CO 2 and from 3% to air. In all cases, the increase in the F 714 / F 685 -ratio was observed in high cell density culture, but no or a little increase was apparent in sparse cell density culture, when these cultures were illuminated at 250 m m m mmol photon m -2 s -1 . Even in the sparse culture, however, a similar increase in the F 714 /F 685 -ratio was observed when C. littorale cells were transferred from 3% to 40% CO 2 at 20 m m m mmol photon m -2 s -1 . The cell density did not affect the F 714 /F 685 -ratio when CO 2 concentration was kept at 3%. The activity of PSI electron (e -) transport was much higher in 40% CO 2 -grown cells than in 3% CO 2 -grown cells irrespective of the cell density during the culture, whereas the difference in PSII activity between them was small. The PSI activity at high cell density was higher also in air-grown cells than that in 3% CO 2 -grown cells. In both dense and sparse culture, 40% CO 2 -grown cells and air-grown cells showed higher relative quantum yield of PSI in the presence of DCMU than 3% CO 2 -grown cells, suggesting an increase in cyclic electron flow around PSI. Likewise, the increase in the F 714 /F 685 -ratio in response to the transfer to 40% CO 2 was observed also in another extremely-high-CO 2 -tolerant alga, Chlorella sp. UK001. The possible role of the increases in the F 714 /F 685 -ratio, PSI/PSII activity ratio and cyclic etransport activity in extremely-high-CO 2 acclimation is discussed in comparison with low-CO 2 acclimation. Abbreviations: air-, 3% CO 2 -and 40% CO 2 -grown cells, algal cells grown in air and in air enriched with 3% and 40% CO 2 , respectively; CA, carbonic anhydrase; . II , quantum efficiency of PSII; LHC, light-harvesting complex.