The phycobilisome is the primary light-harvesting antenna
in cyanobacterial
and red algal oxygenic photosynthesis. It maintains near-unity efficiency
of energy transfer to reaction centers despite relying on slow exciton
hopping along a relatively sparse network of highly fluorescent phycobilin
chromophores. How the complex maintains this high efficiency remains
unexplained. Using a two-dimensional electronic spectroscopy polarization
scheme that enhances energy transfer features, we directly watch energy
flow in the phycobilisome complex ofSynechocystissp. PCC 6803 from the outer phycocyanin rods to the allophycocyanin
core. The observed downhill flow of energy, previously hidden within
congested spectra, is faster than timescales predicted by Förster
hopping along single rod chromophores. We attribute the fast, 8 ps
energy transfer to interactions between rod-core linker proteins and
terminal rod chromophores, which facilitate unidirectionally downhill
energy flow to the core. This mechanism drives the high energy transfer
efficiency in the phycobilisome and suggests that linker protein–chromophore
interactions have likely evolved to shape its energetic landscape.