New research has revealed a unique mechanism in reef-building corals that allows them to switch their light sensitivity between ultraviolet and visible light using chloride ions from their environment. This groundbreaking discovery offers a new way to understand protein engineering and function.
A recent study from Osaka Metropolitan University’s Graduate School of Science is fundamentally changing the scientific understanding of how opsins—the light-sensing proteins responsible for vision—function. In most animal opsins, a negatively charged amino acid acts as a counterion to enable visible light absorption.
This new research found that certain coral opsins use a chloride ion from their environment for this function, a novel finding for wild-type animal opsins.
This novel mechanism reveals how corals, despite lacking eyes, can sense and respond to light, or in other words, how they adapt to their environment. This new insight is crucial for understanding how corals might respond to threats from climate change, such as ocean acidification, which is marked by changes in pH.
New class of opsins
In a study published in ELife on September 1, 2025, Osaka researchers investigated how the coral Acropora tenuis can detect light and respond to changing ocean conditions. Although corals don’t have eyes or a brain, they are not blind.
Like many organisms, they use proteins called opsins—the same family of light-sensitive proteins found in human retinas—to detect light in their environment. However, researchers discovered a new group of opsins.
These ASO-II opsins have unique properties that set corals apart from mammals. Using mutational experiments, spectroscopy, and targeted advanced simulation, researchers studied the novel light-sensing mechanism, focusing on the Antho2a opsin.
Instead of using amino acids—found in other animal opsins—ASO-II opsins employ chloride ions. According to a press release, this is the first time scientists have reported an opsin that uses inorganic ions in this way.
“We found that chloride ions stabilize the Schiff base more weakly than amino acids do,” said Yusuke Sakai, a postdoctoral researcher in Terakita’s lab and the first author of the study. “So the opsin can reversibly switch between visible-light sensitivity and UV sensitivity depending on the pH.”
Reef-building corals can switch their sensitivity between visible light and ultraviolet light depending on the acidity of their environment. When conditions are more acidic, their opsins detect visible light; when conditions are more basic, they flip back to UV.
This switching ability may be tightly linked to the corals’ relationship with algae. Because the algae living inside coral cells change the internal pH as they photosynthesize, the corals’ light sensitivity may adjust in real time, allowing the partnership to remain finely tuned.
The discovery carries promise for biotechnology
“The ASO-II opsin of Acropora tenuis was shown to regulate calcium ions in a light-dependent way, hinting at potential applications as an optogenetic tool whose wavelength sensitivity changes with pH,” said Mitsumasa Koyanagi, a professor at Osaka Metropolitan University’s Graduate School of Science and one of the lead authors of the study, in the press release.
These coral opsins can regulate calcium ions in a light-dependent way, suggesting they could be developed as optogenetic tools—with a unique advantage: their wavelength sensitivity itself shifts with pH, opening the door to new forms of light-based control in biology and medicine.
The study has been published in ELife.