Abstract

Photosynthetic organisms cope with changes in light quality by balancing the excitation energy flow between photosystems I (PSI) and II (PSII) through a process called state transitions. Energy redistribution has been suggested to be achieved by movement of the light-harvesting phycobilisome between PSI and PSII, or by nanometre scale rearrangements of the recently discovered PBS-PSII-PSI megacomplexes. The alternative ‘spillover’ model, on the other hand, states that energy redistribution is achieved by mutual association/dissociation of PSI and PSII. State transitions have always been studied by changing the redox state of the electron carriers using electron transfer inhibitors, or by applying illumination conditions with different colours. However, the molecular events during natural dark-to-light transitions in cyanobacteria have largely been overlooked and still remain elusive. Here we investigated changes in excitation energy transfer from phycobilisomes to the photosystems upon dark-light transitions, using picosecond fluorescence spectroscopy. It appears that megacomplexes are not involved in these changes, and neither does spillover play a role. Instead, the phycobilisomes partly energetically uncouple from PSI in the light but hardly couple to PSII.

Highlights

  • To optimize their light-harvesting capacity, photosynthetic organisms adapt rapidly to changing light conditions and/or metabolic demands by regulating the distribution of absorbed light energy between photosystems I and II (PSI, PSII)

  • It was shown that the exposure of Synechocystis PCC 6803 cells to strong light leads to energetic uncoupling and detachment of PBSs from the reaction centres, at least from those of PSII13–15

  • The spectra of the dark and light state were in both cases normalized to the fluorescence signal of Rhodamine B that was added to the sample for calibration purposes

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Summary

Introduction

To optimize their light-harvesting capacity, photosynthetic organisms adapt rapidly to changing light conditions and/or metabolic demands by regulating the distribution of absorbed light energy between photosystems I and II (PSI, PSII). An increased fluorescence in the 630–670 nm region was observed, which must be entirely attributed to PC and APC660 in PBSs. the spectral differences cannot only be explained by PBS migration from PSI to PSII, if migration lifetimes from PBS to PSs stay the same.

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