Structural basis for energy transfer and electron transport in the photosystem I-ferredoxin-NADP+ reductase supercomplex from Nannochloropsis oceanica

In photosynthetic linear electron flow (LEF), photosystem I (PSI) transfers electrons to its stromal acceptor ferredoxin (Fd), which are then delivered to ferredoxin–NADP⁺ reductase (FNR) for NADPH production. The marine stramenopile alga Nannochloropsis oceanica (N. oceanica) is noted for its high lipid content, bioenergy potential, and elevated LEF activity. Its PSI associates with a light-harvesting complex I known as the violaxanthin-chlorophyll a-protein (VCP). Here, we present structural and biochemical evidence that NoPSI-VCP directly binds FNR on the stromal side, which may account for the significantly enhanced LEF activity of N. oceanica. The cryo-EM structure of the NoPSI-VCP-FNR supercomplex, resolved at 2.89 Å resolution, shows that FNR interacts with core subunits PsaB, PsaD, PsaI, and PsaL through an extended N terminal domain, representing a previously unreported association mode. The structure also delineates the precise arrangement of eleven core subunits and nine peripheral VCP antenna proteins, collectively forming an extensive pigment network and illuminating efficient energy transfer pathways from the antenna system to the reaction center. This study elucidates a novel PSI–FNR interaction mode and provides a structural foundation for understanding the unique supramolecular organization, efficient light harvesting, and electron transport mechanisms in N. oceanica PSI.