Corals reveal a surprising new way to see light

At first glance, corals appear as immobile, silent structures beneath the ocean surface. Yet these organisms hold sensory abilities that scientists are only beginning to appreciate. Though they lack eyes, corals can sense light in surprising ways, adapting to their environment with delicate precision.

Researchers at Osaka Metropolitan University’s Graduate School of Science have recently revealed a unique mechanism that allows corals to detect different types of light.

The research shows that light-sensitive proteins called opsins rely on chloride ions to shift between ultraviolet (UV) and visible light sensitivity depending on pH. This finding broadens our understanding of vision beyond creatures with eyes.

How corals sense light

Opsins are proteins central to animal vision. They bind with retinal, a molecule that naturally absorbs only UV light. To detect visible light, retinal forms a bond called a Schiff base with an opsin.

This bond is positively charged and usually stabilized by a negatively charged amino acid. Without that stabilizer, most opsins would not function.

The absence of stability would prevent the pigment from maintaining its configuration, disrupting the conversion of light into biochemical signals, and ultimately halting the process animals rely on to sense and respond to their visual environment.

Special opsins found in corals

Anthozoans, a group that includes corals and sea anemones, have their own special opsins known as ASO-II. Unlike mammalian opsins, some ASO-II proteins lack the stabilizing amino acids.

“Some ASO-II opsins of reef-building corals lack the usual counterion amino acids found in other animal opsins,” explained Akihisa Terakita, professor at Osaka Metropolitan University’s Graduate School of Science.

The absence of counterion amino acids posed a major question. How could these coral opsins maintain sensitivity to visible light without them?

A molecular switch for light sensing

The research team studied ASO-II opsins in the reef-building coral Acropora tenuis. Using spectroscopy, mutational analysis, and simulations, they discovered a novel process.

Instead of amino acids, the opsins employ chloride ions from their surroundings as counterions. This marks the first known case of an opsin using inorganic ions for stabilization.

“We found that chloride ions stabilize the Schiff base more weakly than amino acids do, so the opsin can reversibly switch between visible-light sensitivity and UV sensitivity depending on the pH,” said Yusuke Sakai, first author of the study.

Light sensing and pH levels

The mechanism hinges on pH conditions. At low pH, protons increase, causing the Schiff base to remain positively charged and absorb longer wavelengths, including visible light.

Chloride ions then provide stability. In higher pH conditions, fewer protons lead to deprotonation, and the opsin shifts back to UV sensitivity. This reversible system allows corals to adapt light detection based on cellular chemistry.

By shifting sensitivity depending on proton balance, corals essentially carry a molecular switch that tunes vision-like processes.

This adaptability ensures they respond effectively to microenvironmental changes, such as metabolic activity, photosynthesis, or stress events, enabling a dynamic balance between UV and visible light sensing crucial for survival and growth.

Symbiosis affects coral light sensing

Corals live closely with algae that generate energy through photosynthesis. The process of photosynthesis changes the internal pH within coral cells, potentially influencing opsin sensitivity.

This suggests corals might tune their light detection to align with the algae’s activity. Such dynamic sensing adds new depth to our knowledge of coral symbiosis.

The study adds further perspective. It highlights how light perception can shape coral responses to both internal and external signals. Opsins do not act in isolation; they interact with cellular pathways involving calcium signaling, gene expression, and metabolic control.

By linking light detection with biochemical responses, corals may adjust processes such as skeletal growth and symbiont regulation. This suggests that vision-like mechanisms contribute to how corals balance energy demands with environmental conditions.

Pushing the boundaries of technology

Professor Mitsumasa Koyanagi suggests that beyond ecology, this discovery opens technological doors.

“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 Professor Koyanagi.

The broader theory of opsin diversity presented in the paper supports this potential. By revealing how proteins can rely on unconventional counterions, the researchers have opened the door to designing light-sensitive tools that adapt to shifting chemical environments.

Such advances could one day enable precise regulation of cellular activity, pushing the boundaries of optogenetics in medicine and biotechnology.

The study is published in the journal eLife.

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