Structure of a photosystem I supercomplex from <i>Galdieria sulphuraria</i> close to an ancestral red alga

Red algae exhibit unique photosynthetic adaptations, characterized by photosystem I (PSI) supercomplexes containing light-harvesting complexes (LHCs), forming PSI-LHCI supercomplexes. In this study, we solved the PSI-LHCI structure of <i>Galdieria sulphuraria</i> NIES-3638 at 2.19-angstrom resolution using cryo-electron microscopy, revealing a PSI monomer core associated with seven LHCI subunits. Structural analysis uncovered the absence of phylloquinones, the common secondary electron acceptor in PSI of photosynthetic organisms, suggesting adaptation to a benzoquinone-like molecule. Phylogenetic analysis suggests that <i>G. sulphuraria</i> retains traits characteristic of an ancestral red alga, including distinctive LHCI binding and interaction patterns. Variations in LHCI composition and interactions across red algae, particularly in red-lineage chlorophyll <i>a</i>/<i>b</i>-binding-like protein and red algal LHCs, highlight evolutionary divergence and specialization. These findings not only deepen our understanding of red algal PSI-LHCI diversification but also enable us to predict features of an ancestral red algal PSI-LHCI supercomplex, providing a framework to explore evolutionary adaptations from an ancestral red alga.