Energy Transfer in Cyanobacteria and Red Algae: Confirmation of Spillover in Intact Megacomplexes of Phycobilisome and Both Photosystems

Cyanobacteria and red algae control the energy distributions of two photosystems (PSI and PSII) by changing the energy transfer among phycobilisome (PBS), PSI, and PSII. However, whether PSII → PSI energy transfer (spillover) occurs in the intact megacomplexes composed of PBS, PSI, and PSII (PBS–PSII–PSI megacomplexes) in vivo remains controversial. In this study, we measured the delayed fluorescence spectra of PBS-selective excitation in cyanobacterial and red algal cells. In the absence of spillover, 7% of the PBS (at most) would combine with PSII, inconsistent with the PBSs’ function as the antenna pigment–protein complexes of PSII. Therefore, we conclude that spillover occurs in vivo in PBS–PSII–PSI megacomplexes of both cyanobacteria and red algae.