Multiple structures of photosystem I-FCPI supercomplexes from a coccolithophore alga reveal a modular antenna...

Photosystem I (PSI) converts light energy into chemical energy in photosynthesis, and forms supercomplexes with light-harvesting complexes (LHCI) in eukaryotes to enhance energy capture and transfer. Various numbers and organizations of both PSI core and LHCI subunits are observed in various organisms. A subgroup of haptophytes named coccolithophores play a major role in marine carbon cycle and CaCO₃ production, and the light-harvesting antennas of them are named FCPs (fucoxanthin-chlorophyll a/c binding protein) because they bind chlorophyll c and fucoxanthin in addition to chlorophyll a. A structure of a large PSI-FCPI supercomplex containing 38 FCPI subunits has been reported from a coccolithophore Emiliania huxleyi recently (L. Shen et al., Science 389, eadv2132, 2025). Here we solved five cryo-electron microscopy (cryo-EM) structures of PSI-FCPI supercomplexes isolated from another coccolithophore Chrysotila roscoffensis with different detergents at resolutions ranging from 2.3 to 1.7 Å. These structures represent discrete PSI-FCPIs containing 1, 4, 6, 8 and 9 FCPI subunits, with FCPIs arranged in a modular fashion. Association of each FCPI module to the PSI core, as well as the arrangement of protein subunits and pigments, are revealed. Contributions of individual antenna modules to excitation energy transfer were calculated and compared with PSI-FCPI supercomplexes from other species of coccolithophores and haptophytes. These results pinpoint the assembly of stable PSI-FCPI supercomplexes in C. roscoffensis and provide insights into how antenna modules contribute to energy transfer in coccolithophores.