Marine green algae have evolved a pigment called siphonein that shields their photosynthetic machinery from intense sunlight. Researchers from Osaka Metropolitan University discovered how this carotenoid efficiently quenches harmful energy states. The finding could inspire more durable solar technologies.
Under intense sunlight, photosynthetic organisms risk damage from excess light energy that can produce reactive oxygen molecules harming cells. Certain marine green algae, however, have adapted unique protections. A study by researchers from Osaka Metropolitan University and collaborators revealed that the pigment siphonein enables efficient photoprotection in these algae.
The team focused on Codium fragile, a marine green alga that possesses a light-harvesting complex called LHCII, similar to land plants, but enriched with rare carotenoids like siphonein and siphonaxanthin. These pigments allow the algae to utilize green light prevalent underwater for photosynthesis.
"Organisms use carotenoids to quickly dissipate excess energy, or quench these triplet states, through a process called triplet-triplet energy transfer (TTET)," explained Ritsuko Fujii, lead author and associate professor at Osaka Metropolitan University's Graduate School of Science and Research Center for Artificial Photosynthesis.
Using electron paramagnetic resonance (EPR) spectroscopy, the researchers compared Codium fragile to spinach. In spinach, traces of harmful chlorophyll triplet states persisted, but in the alga, these signals vanished completely, indicating successful neutralization by its carotenoids.
"The key to the quenching mechanism lies in how quickly and efficiently the triplet states can be deactivated," said Alessandro Agostini, co-lead author and researcher at the University of Padua, Italy.
Combining EPR data with quantum chemical simulations, the study identified siphonein at a critical binding site in LHCII as the primary defender, owing to its molecular structure and positioning that disperses excess energy effectively.
"Our research has revealed that the antenna structure of photosynthetic green algae has an excellent photoprotective function," Agostini added.
Published in Cell Reports Physical Science on October 22, 2025, the work highlights how marine algae evolved pigments to absorb underwater light while enduring sunlight intensity. Fujii noted, "We hope to further clarify the structural characteristics of carotenoids that increase quenching efficiency, ultimately enabling the molecular design of pigments that optimize photosynthetic antennae." This could advance bio-inspired solar systems resistant to light damage.