Gene

Electron transport chain

Chemical physics

Biology

Photochemistry

Photosystem I

Chlamydomonas

Biochemistry

Botany

Electron transfer

Chlamydomonas reinhardtii

Energy transfer

Photosystem

Mutant

Photosystem II

Photosynthesis

Chemistry

Light-harvesting complex

Biophysics

Nature Plants

During oxygenic photosynthesis, photosystems I and II (PSI and PSII) are essential for light-driven electron transport. Excitation energy transfer in PSI occurs extremely quickly, making it an efficient energy converter. In the alga Chlamydomonas reinhardtii (Cr), multiple units of light-harvesting complex I (LHCI) bind to the PSI core and function as peripheral antennae, forming a PSI–LHCI supercomplex. CrPSI–LHCI shows significantly larger antennae compared with plant PSI–LHCI while maintaining highly efficient energy transfer from LHCI to PSI. Here, we report structures of CrPSI–LHCI, solved by cryo-electron microscopy, revealing that up to ten LHCIs are associated with the PSI core. The structures provide detailed information about antenna organization and pigment arrangement within the supercomplexes. Highly populated and closely associated chlorophylls in the antennae explain the high efficiency of light harvesting and excitation energy transfer in CrPSI–LHCI. In Chlamydomonas reinhardtii, multiple units of light-harvesting complex I bind to the photosystem-I core, forming larger antennae than in plants. The structure, solved by cryo-electron microscopy, reveals details of antenna organization and functioning.

Antenna arrangement and energy transfer pathways of a green algal photosystem-I–LHCI supercomplex