Uncovering a unique light-harvesting structure in marine algae

Photosynthesis, the natural process of converting sunlight energy into chemical energy and generating molecular oxygen, is a remarkable natural phenomenon that not only forms the basis for sustaining almost all living organisms on Earth but also provides a blueprint for artificial photosynthesis. For decades, researchers have been working on technologies that could replicate this process artificially. Artificial photosynthetic systems use technology to store solar energy in chemical bonds, potentially creating sustainable fuels like hydrogen. A deep understanding of the plant's light-harvesting structures is vital for such future applications.

Uncovering one such structure, a team of researchers led by Assistant Professor Romain La Rocca, along with Associate Professor Fusamichi Akita, and Professor Jian-Ren Shen from Okayama University, Japan, conducted a high-resolution analysis of a photosynthetic complex found in a marine alga, Chrysotila roscoffensis. This marine alga belongs to the coccolithophore species known for producing calcium carbonate plates and fixing carbon at the ocean surface.

Published online on May 5, 2025, in Nature Communications, the study describes the unique photosynthetic machinery of the haptophyte species and gives a deeper insight into its unique light-capturing and energy transfer mechanism.

Marine algae, especially haptophytes, are vital for marine life, contributing up to 50% of the ocean’s biomass and playing a major role in the global carbon cycle. However, despite their importance, the molecular details of photosynthesis in these species have remained underexplored.

The process of photosynthesis mainly involves two protein-pigment complexes, which are photosystem I (PSI) and photosystem II (PSII). PSII is responsible for initiating the process of photosynthesis using light to split water into oxygen, protons, and electrons, while PSI uses the electrons from PSII and excites them to a higher energy level so they can be used in the process of synthesizing sugars.

PSII is found in the thylakoid membrane of chloroplasts and is composed of antenna proteins, which are light-harvesting complexes that capture sunlight, and a reaction center called the photosynthetic core (with special chlorophylls (P680) and a water-splitting complex). Using advanced imaging techniques with cryo-electron microscopy (cryo-EM) at an impressive 2.2 Å resolution, the researchers mapped the PSII-fucoxanthin chlorophyll c-binding protein (FCPII) supercomplex in the haplophyte species.

“This study analyzed the first structural model of a PSII-FCPII complex in haptophytes,” explains Dr. La Rocca. “Surprisingly, unlike other PSII systems, the complex in the haptophyte shows a unique arrangement of the antenna proteins around the photosystem core.”

The structure showed a characteristic arrangement and structure of the antenna proteins, which are made up of six FCPII antenna protein units per monomer of PSII. The arrangement of these proteins was quite different from those seen in diatoms and green algae, indicating the adaptation of this marine alga to its living environment.

The FCPII units are responsible for gathering light and transferring energy to the core of the photosystem. According to the cryo-EM structure, one antenna protein, FCPII-2, stands out as a central hub in this process. It is positioned in such a way that it receives energy from its surrounding antennas and passes it directly to the PSII core subunit CP47. The FCPII-2 protein is also rich in fucoxanthin pigments, which can effectively absorb light while dissipating excess light energy, preventing cell damage from strong light.

The researchers also identified and sequenced Psb36, a previously uncharacterized PSII subunit that is found at the interface between the core and the antenna system. While this structure has been seen in earlier studies of diatoms and red algae, its sequence wasn’t determined until now.

This study reveals significant details to deepen our knowledge of photosynthetic systems and is also expected to contribute to the development of artificial photosynthesis systems, as the unique arrangement of the light-harvesting system indicates the efficiency of light-harvesting under some light conditions.

“These algae are extremely efficient at harnessing sunlight for energy; by understanding the structure of their photosystems, we get one step closer to mimicking these natural systems for artificial light energy harvesting,” concludes Dr. Shen.

About Okayama University, Japan

As one of the leading universities in Japan, Okayama University aims to create and establish a new paradigm for the sustainable development of the world. Okayama University offers a wide range of academic fields, which become the basis of the integrated graduate schools. This not only allows us to conduct the most advanced and up-to-date research, but also provides an enriching educational experience.

Website: https://www.okayama-u.ac.jp/index_e.html

About Romain La Rocca from Okayama University, Japan

Dr. Romain La Rocca is an Assistant Professor at Okayama University's Research Institute for Interdisciplinary Science. He holds a PhD in structural biology and specializes in protein crystallography and biophysical techniques. He has five publications to his credit, with over 64 citations. His notable works include studies on tau protein aggregation and microtubule-targeting agents. He is currently working on the structures of photosynthetic protein complexes using protein purification and cryo-electron microscopy techniques.