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Coccolithophores are a type of single-celled microalgae that fix CO₂ into organic matter and precipitate calcium carbonate, profoundly shaping ocean optics, carbon export, and long-term carbon storage. They are major contributors to ocean photosynthesis, especially in temperate and open-ocean waters.

Understanding how their photosynthetic systems work has implications for artificial photosynthesis, bioinspired solar materials, energy networks, spectral engineering, and other applications.

In a new featured on the cover of Science on September 11, Chinese researchers have revealed a breakthrough in understanding the light-harvesting efficiency of coccolithophores.

The research, led by Prof. Wang Wenda and Prof. Tian Lijin from the Institute of Botany of the Chinese Academy of Sciences, together with collaborators, revealed the first three-dimensional structure of the photosystem I–fucoxanthin–chlorophyll a/c-binding protein supercomplex (PSI-FCPI) in the coccolithophore Emiliania huxleyi.

The study shows how this giant photosynthetic machine can expand its light-harvesting cross-section by three to four times while maintaining over 95% energy conversion efficiency.

The researchers reported the structural and pigment features of PSI-FCPI in E. huxleyi and showed that it is the largest eukaryotic photosystem I complex discovered to date. PSI-FCPI comprises 51 protein subunits and 819 pigments, with a molecular mass of 1.66 MDa. Its light-harvesting capacity is four to five times greater than that of the pea PSI-LHC supercomplex.

Despite its size, the system achieves ultrafast energy transfer within 96–120 picoseconds, enabling quantum efficiency of over 95% in converting light into electrons.

The remarkable performance of PSI-FCPI relies on its unique antenna protein and pigment arrangement. Thirty-eight peripheral Eh-FCPI antennae encircle the PSI core in eight radial belts, pieced together from canonical Lhcq subunits and newly identified Lhcq-like subunits. The resulting "giant light-harvesting antenna system" greatly enlarges its absorption ability.

The FCPI antennas are enriched with chlorophyll c and fucoxanthin-type carotenoids, enabling effective absorption of blue–green (460–490 nm) and green (490–540 nm) light in ocean water. Moreover, strong excitonic coupling between chlorophyll c and chlorophyll a eliminates energy traps and builds a smooth and efficient energy transfer network, ensuring the rapid and stable delivery of excitation energy to the PSI reaction center.

Coccolithophores play an important role in the global carbon cycle. They flourished during the Cretaceous period, leaving massive "chalk" deposits in sediments due to their calcium carbonate plates. The huge PSI-FCPI represents the ultimate expansion of light-harvesting structures in the evolutionary context of red-lineage secondary endosymbiosis.

This study not only provides insights into the adaptive evolution of this ancient lineage but also deepens our understanding of photosynthetic strategies in marine organisms.