Structural insights into the divergent evolution of a photosystem I supercomplex in Euglena gracilis

Photosystem I (PSI) forms supercomplexes with light-harvesting complexes (LHCs) to perform oxygenic photosynthesis. Here, we report a 2.82-angstrom cryo–electron microscopy structure of the PSI-LHCI supercomplex from Euglena gracilis, a eukaryotic alga with secondary green alga-derived plastids. The structure reveals a PSI monomer core with eight subunits and 13 asymmetrically arranged LHCI proteins. Euglena LHCIs bind diadinoxanthin, which is one of the carotenoids typically associated with red-lineage LHCs and is not present in the canonical LHCI belt found in green-lineage PSI-LHCI structures. Phylogenetic analysis shows that the Euglena LHCIs originated from LHCII-related clades rather than from the green-lineage LHCI group and that the nuclear-encoded PSI subunit PsaD likely originated from cyanobacteria via horizontal gene transfer. These observations indicate a mosaic origin of the Euglena PSI-LHCI. Our findings uncover a noncanonical light-harvesting architecture and highlight the structural and evolutionary plasticity of photosynthetic systems, illustrating how endosymbiotic acquisition and lineage-specific adaptation shape divergent light-harvesting strategies.