Photosynthesis without the burn

Too much sun can ruin a day at the beach. It can also ruin photosynthesis, scorching plants and other organisms that depend on capturing sunlight for energy. Beneath the waves, though, algae have found a clever shield. Osaka Metropolitan University researchers and their colleagues discovered that a pigment called siphonein helps marine green algae keep photosynthesis humming, without the burn.

Photosynthetic organisms rely on delicate light-harvesting complexes (LHCs) to capture sunlight for energy. During photosynthesis, chlorophyll absorbs light and enters an excited singlet state. Under normal light conditions, this energy is efficiently transferred to the photosynthetic reaction center to drive chemical reactions. But excessive light can push chlorophyll molecules into a dangerous “triplet” state, which is a source of reactive oxygen species capable of causing oxidative damage.

“Organisms use carotenoids to quickly dissipate excess energy, or quench these triplet states, through a process called triplet-triplet energy transfer (TTET),” said Ritsuko Fujii, lead author and associate professor at the Graduate School of Science and Research Center for Artificial Photosynthesis at Osaka Metropolitan University.

Until now, however, the rules governing this photoprotection remained largely unknown.

The research team looked for an answer in Codium fragile, a marine green alga. Similar to terrestrial plants, it possesses a light-harvesting antenna called LHCII, but with a twist: it contains unusual carotenoids such as siphonein and siphonaxanthin, which allow the alga to use green light for photosynthesis.

“The key to the quenching mechanism lies in how quickly and efficiently the triplet states can be deactivated,” said Alessandro Agostini, researcher at the University of Padua, Italy and co-lead author of the study.

Using advanced electron paramagnetic resonance (EPR) spectroscopy, which detects triplet excited states directly, the team compared spinach plants with Codium fragile. In spinach, weak signals of chlorophyll triplet states remained detectable. In contrast, in Codium fragile, these harmful states vanished entirely, clear evidence that carotenoids in the algal system quench them completely.

“Our research has revealed that the antenna structure of photosynthetic green algae has an excellent photoprotective function,” Agostini said.

Combining EPR with quantum chemical simulations, the team pinpointed siphonein, located at a key binding site in LHCII, as the primary driver of this remarkable protective effect. Their work also clarified the electronic and structural principles underlying efficient TTET, showing how the peculiar electronic structure of siphonein and its position in the LHCII complex strengthen its ability to dissipate excess energy.

The findings demonstrate that marine algae have evolved unique pigments not only to capture the green-blue light available underwater but also to enhance their resilience against excessive sunlight.

Beyond advancing our understanding of photosynthesis, the study results open the door to developing bio-inspired solar technologies with built-in protective mechanisms, and more durable and efficient renewable energy systems.

“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,” Fujii said.

The findings were published in Cell Reports Physical Science.

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