Dynamic LHCI antenna and inter‐photosystem spillover enable flexible light harvesting in Euglena gracilis

SUMMARY Photosynthetic light‐harvesting antenna systems of aquatic algae show remarkable evolutionary diversity. Euglena gracilis , a unicellular flagellate that acquired its chloroplast through secondary endosymbiosis with a green alga, exemplifies evolutionary innovation in light harvesting. E. gracilis possesses a distinctive photosynthetic apparatus with loosely stacked thylakoid membranes and atypical light‐harvesting complexes (LHCs). Previous studies proposed shared antenna systems serving both photosystems I and II (PSI and PSII) and inter‐photosystem excitation energy transfer (spillover) as a regulatory mechanism. Here, we investigate the mechanisms and dynamics of light harvesting in isolated photosynthetic protein complexes and whole cells of E. gracilis by absorption, circular dichroism, fluorescence, and time‐resolved fluorescence spectroscopy. Isolated PSII and PSI–LHCI were found to have spectroscopic characteristics distinct from their green‐lineage counterparts. PSI–LHCI showed exceptionally long excitation lifetimes of 100–200 ps, resulting from energy transfer through a large mosaic LHCI antenna. Time‐resolved fluorescence of intact cells and thylakoid membranes revealed a characteristic 50 ps energy transfer component at room temperature from PSII‐associated antenna to PSI‐associated red chlorophyll forms, providing direct evidence for prevalent spillover in vivo . The mobile LHC complex, previously thought to contain both LHCI and LHCII, comprises exclusively multiple LHCI types and likely regulates excitation energy redistribution via spillover.