2025-09-12 中国科学院 (CAS)
<関連情報>
- https://english.cas.cn/newsroom/research_news/life/202509/t20250911_1054383.shtml
- https://www.science.org/doi/10.1126/science.adv2132
有孔虫由来の巨大光合成系I-フコキサンチンクロロフィル複合体の構造と機能 Structure and function of a huge photosystem I–fucoxanthin chlorophyll supercomplex from a coccolithophore
Lili Shen, Fei Ren, Yin-Chu Wang, Zhenhua Li, […] , and Wenda Wang
Science Published:11 Sep 2025
DOI:https://doi.org/10.1126/science.adv2132
Editor’s summary
Photosynthetic organisms absorb light through photosystems comprising elaborate light-harvesting complexes with many proteins and pigments to direct light energy into the reaction center. Shen et al. resolved the structure of photosystem I from the coccolithophore Emiliania huxleyi, revealing a vast supercomplex of fucoxanthin and chlorophyll antennae with 95% quantum efficiency. High levels of chlorophyll c and fucoxanthin allow fast kinetics and the absorption of blue-green and green light, suitable for an ocean-dwelling coccolithophore. —Madeleine Seale
Structured Abstract
INTRODUCTION
Photosystem I (PSI) is a crucial pigment-protein complex in thylakoid membranes and drives electron transfer for the fixation of carbon dioxide, which has almost 100% quantum efficiency in converting light energy into chemical energy. To make full use of light energy under different habitats, eukaryotic photosynthetic organisms use multiple types and numbers of light-harvesting complexes (LHCs) to collect light energy and transfer it to PSI reaction centers. Among them, Lhca serves as the light-harvesting antenna of PSI in green algae and land plants, whereas a number of Lhcr, Lhcf, or Lhcq proteins are found in red-lineage algae, including red algae, diatoms, cryptophytes, haptophytes, and dinoflagellates. These varying LHCs absorb light energy and mediate energy balance under different light conditions.
RATIONALE
The PSI core and reaction center are largely conserved across different lineages of photosynthetic organisms, but some PSI complexes of the red-lineage algae bind more than 20 LHC antenna subunits to expand cross sections for harvesting more light energy. In addition, diverse pigments [chlorophylls (Chls)] c and keto-carotenoids, such as fucoxanthins and peridinins] are contained in Lhcr, Lhcf, or Lhcq antennae, which function to harvest more blue-green (460 to 490 nm) and green (490 to 540 nm) light energy where seawater attenuates most of the blue and red light. Coccolithophores were a dominant group of red-lineage algae in the Cretaceous and are abundant in modern oceans. They contribute substantially to the global primary productivity and carbon and calcium cycles. The coccolithophore PSI-LHCI has complicated pigments and LHCI protein components, which confer it with the ability to adapt to changing light environments under water.
RESULTS
We purified a PSI-fucoxanthin chlorophyll a/c-binding protein (PSI-FCPI) supercomplex from the coccolithophore Emiliania huxleyi (Eh) and solved its structure by single-particle cryo–electron microscopy at a resolution of 2.79 Å. This monomeric supercomplex contains 12 PSI core subunits, a specific lumenal linker protein (EhLP), and 38 peripheral Eh-FCPI antennae, with 411 Chls a, 152 Chls c, 256 carotenoids, and a number of other ligands, which thus constitutes the largest PSI-antenna supercomplex known so far. Phylogenetic analysis indicated that 16 Lhcq-like Eh-FCPIs were specific to E. huxleyi, whereas 10 Lhcrs, 1 Lhcf, 1 RedCAP, and 10 Lhcq Eh-FCPIs were similar to either those in red algae or diatoms. The 38 Eh-FCPIs formed a radial arrangement of eight belt-like Eh-FCPI clusters around the PSI core. In addition, four Chl c species and four fucoxanthin derivatives were found in the structure, in addition to Chl a, diadinoxanthin, diatoxanthin, α-carotene, and β-carotene, which formed a complicated pigment network for light harvesting and energy transfer. We measured femtosecond transient absorption spectrograms and found that Eh-PSI-FCPI has an overall excitation trapping time of 96 to 120 ps, representing 95% quantum efficiency in this giant Eh-PSI-FCPI.
CONCLUSION
Our results reveal the organization of 38 FCPI antennae and a huge number of pigments that contribute to light harvesting and fast energy transfer in the Eh-PSI-FCPI. The 95% quantum efficiency of coccolithophore PSI-FCPI illustrates a highly efficient system in spite of a four to five times expansion of the light-harvesting cross section compared with PSI-LHCI of land plants.
A coccolithophore PSI-FCPI supercomplex consisting of a large number of light-harvesting antennae and pigments.
The Eh-PSI-FCPI contains 38 LHC antennae surrounding the PSI core, with a total of 819 pigments. Among them, Chl a, Chl c, and Fx (including their derivatives) enable different absorption ranges of sunlight under water. The whole system showed 95% quantum efficiency (ФPSI), illustrating its high efficiency of light-energy utilization.
Abstract
Photosystem I (PSI) is a pigment-protein complex, which converts light energy into chemical energy in photosynthesis. Among photosynthetic organisms, PSI-LHC (light-harvesting complex) structures exhibit substantial differences in their sizes, reflecting adaptation to different light environments. Here we report the structure of a PSI-fucoxanthin chlorophyll a/c binding protein (FCPI) supercomplex from the coccolithophore Emiliania huxleyi (Eh) at 2.79-angstrom resolution by cryo–electron microscopy, which showed a huge Eh-PSI-FCPI supercomplex containing 38 peripheral Eh-FCPI antennae and a linker protein (EhLP) in addition to the PSI core. A network of 819 pigments was found in Eh-PSI-FCPI, which functions to capture and transfer light energy with 95% quantum efficiency. This elucidates how its modular Eh-FCPI arrangement contributes to the expansion of PSI cross section and efficient light harvesting.
