Structure and function of a huge photosystem I-fucoxanthin chlorophyll supercomplex from a coccolithophore

Photosystem I (PSI) is a pigment-protein complex, which converts light energy into chemical energy in photosynthesis. Among photosynthetic organisms, PSI-LHC (light-harvesting complex) structures exhibit substantial differences in their sizes, reflecting adaptation to different light environments. Here we report the structure of a PSI-fucoxanthin chlorophyll a/c binding protein (FCPI) supercomplex from the coccolithophore Emiliania huxleyi (Eh) at 2.79-angstrom resolution by cryo-electron microscopy, which showed a huge Eh-PSI-FCPI supercomplex containing 38 peripheral Eh-FCPI antennae and a linker protein (EhLP) in addition to the PSI core. A network of 819 pigments was found in Eh-PSI-FCPI, which functions to capture and transfer light energy with 95% quantum efficiency. This elucidates how its modular Eh-FCPI arrangement contributes to the expansion of PSI cross section and efficient light harvesting.