The Structure and Mechanism of Energy Transfer in Phycobilisomes.

Phycobilisomes (PBSs) are the major light-harvesting antenna of photosynthesis in cyanobacteria and red algae. Different types of PBSs exhibit a basic structure: a central core that interacts with photosystem II (PSII) and peripheral rods that are attached to the core and that expand the light-absorption cross-section area. This review summarizes recent progress in the study of PBS structures, with an emphasis on protein-bilin chromophore interactions. We describe the mechanisms of excitation energy transfer (EET) in PBSs with near-unity efficiency, as recent studies using two-dimensional electron spectroscopy showed that both Förster EET and coherent EET are involved in this process. Recent studies that provided insights into the mechanism of the PBS-thylakoid membrane association, particularly of PBS-PSII interactions, are also described. In addition, we discuss progress and some unsettled issues from studies on state transitions, which regulate energy distribution between PSII and PSI, in PBS-containing organisms.