The Posidonia oceanica Large PSI-LHCII supercomplex reveals the molecular basis of PSI spectral diversification in higher plants

Plant Photosystem I (PSI) can capture long-wavelength photons and convert them into chemical energy through low-energy chlorophylls known as the red forms. Although higher-plant PSI retains a largely conserved architecture, the spectral properties of the red forms diversified during evolution, extending or restricting far-red absorption across species. However, the molecular basis of this diversification remains unknown. The Mediterranean seagrass Posidonia oceanica evolved a large PSI-light-harvesting complex I and II (L-PSI-LHCI-LHCII) with blue-shifted absorption and strongly attenuated red-forms. Here, we report the 1.9 A cryo-EM structure of this unique blue-adapted photosystem. It comprises a PSI core with four canonical Lhca proteins (Lhca1-4), a canonically bound LHCII trimer, and an additional Lhca1-Lhca4 dimer anchored through amino-acid residues largely conserved in both marine and land angiosperms. The sub-2 A map further revealed blue-light-associated pigment substitutions in the LHCs, together with amino-acid sites shaping local red-form environments. Comparison with land plants revealed substitution patterns at those sites consistent with the spectral diversification of PSI. These findings reveal a shared structural basis for red-form tuning, accounting for both seagrass adaptation to seawater light and the broad range of far-red absorption in higher plants.