Cyanobacteria dynamically regulate phycobilisome-to-photosystem excitation energy transfer

In cyanobacteria and red algae, the phycobilisome (PBS) absorbs light and transfers its energy to the chlorophylls in photosystems II (PSII) and I (PSI). With the help of target analysis of time-resolved emission spectra measured at room temperature (RT) and at 77 K, we establish a general kinetic scheme for excitation energy transfer (EET) and trapping based upon a PBS-PSII-PSI megacomplex. At RT it is found that in the dark-adapted cells (State II), the terminal emitter of PBS, allophycocyanin APC680, transfers energy to PSII and PSI with equal rates of ≈50 ns^-1, and that spillover from PSII to PSI is present with rate ≈6 ns^-1. At 77 K, upon transition from State I to State II the EET rate from APC680 to PSII is constant, whereas the rate to PSI increases by 67%. This indicates that a structural change in EET distance in the PBS-PSII-PSI megacomplex underlies the state transition.