To survive in areas where it is difficult to photosynthesize, some organisms adopt unique strategies. Osaka Metropolitan University researchers have found that a freshwater alga captures far-red light as an additional energy source by arranging ordinary chlorophyll in an extraordinary way.

Far-red light lies beyond the optimal range for photosynthesis for many organisms. Yet in shaded forests and murky waters, where this light dominates, plants and algae still pull off photosynthesis, making something out of almost nothing.

“Whilst certain cyanobacteria use specialized chlorophylls to absorb far-red light, many plants and algae achieve the same effect by reorganizing ordinary chlorophyll a into cooperative assemblies within their photosynthetic antennas,” said Ritsuko Fujii, lead author and associate professor at the Graduate School of Science and Research Center for Artificial Photosynthesis at Osaka Metropolitan University.

Chlorophyll a is a pigment that cannot absorb far-red light on its own. So, how exactly do these organisms achieve photosynthesis?

The team looked for an answer in the freshwater eustigmatophyte alga, Trachydiscus minutus, an organism that accumulates large amounts of a light-harvesting protein that can utilize far-red light. Although the alga can perform photosynthesis under normal light conditions, the high levels of the light-harvesting protein are especially useful for surviving in low-light conditions.

“The organism produces a specialized photosynthetic antenna called a red-shifted violaxanthin–chlorophyll protein (rVCP), which absorbs far-red light even though it contains only chlorophyll a,” Fujii said.

Using cryo-electron microscopy, the researchers determined the structure of rVCP at a high resolution of 2.4 Å. They found that the protein forms a previously unreported architecture: a tetramer composed of two different heterodimers. This unique assembly brings chlorophyll a molecules into close proximity, allowing them to form unusually large pigment clusters.

To understand how this structure affects light absorption, the team combined the structural data with multiscale quantum chemical calculations.

“Our analysis showed that three chlorophyll clusters within each heterodimer play a major role in absorbing far-red light,” Fujii said. “Importantly, this absorption arises purely from energy delocalization across multiple chlorophyll molecules, independently of the charge-transfer effects that are thought to drive similar red-shifted systems.”

These findings reveal a fundamentally different mechanism for tuning the color of absorbed light, one in which the protein scaffold precisely controls interactions between identical chlorophyll molecules, without chemically modifying the pigment. This explains the resilience of these organisms in tough environments.

The discovery also has practical implications. Some eustigmatophytes are known for their ability to store oils, making them promising candidates for sustainable bioenergy production. Harnessing organisms that can photosynthesize efficiently under far-red light could enable oil production in conventionally unsuitable environments.

The unusual tetrameric structure of rVCP may also offer a new blueprint for protein design. Because pigment arrangement is dictated by protein sequence, this framework could help guide the engineering of artificial or enhanced photosynthetic systems.

“As interest grows in expanding photosynthesis into the far-red region to boost overall photosynthetic productivity on Earth, our next goal is to reveal how this complex delivers captures energy to the photosystem and how that mechanism could be optimized,” Fujii said.

The findings were published in the Journal of the American Chemistry Society.

Competing Interests
The authors declare no competing financial interest.

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