For over 10 years, I have had the chance to photograph the coral reef, at night, in fluorescence. It is a memorable experience and if you have some time after reading this article, do not hesitate to visit my portfolio to discover my latest fluorescence photographs. I also allowed myself to add some throughout this article to complete the scientific information. You will also find all the sources at the bottom of the article.
When diving into the heart of a coral reef, a fascinating light show can be offered to us: some corals emit a bright light, often green, blue or red. This phenomenon, called coral fluorescence, has intrigued scientists for decades. But why do these marine organisms develop this ability? Is it simply an aesthetic characteristic or a fundamental mechanism of protection and adaptation?
Behind this glow lies a sophisticated biochemical process involving fluorescent proteins that can absorb and re-emit light at a different wavelength. This mechanism plays a key role in protecting corals from environmental stresses, including UV rays and global warming.
In this article, we will explore the scientific reasons for this fluorescence, its role in coral survival, and its interaction with other marine organisms. We will also see how scientists use fluorescence as a tool to monitor the health of coral reefs and where it is possible to observe this phenomenon while diving.
What is fluorescence in corals?
Coral fluorescence is a natural phenomenon that intrigues scientists and divers around the world. Some corals emit a bright light, often green, blue or red, when exposed to a light source, particularly under the effect of ultraviolet rays. This phenomenon is due to the presence of fluorescent pigments that absorb part of the light and re-emit it at a different wavelength.
A unique biochemical reaction
This fluorescence process is linked to fluorescent proteins present in coral tissues. These proteins, called GFP (Green Fluorescent Proteins) and their derivatives, are capable of transforming blue light into another color of the visible spectrum. Initially discovered in jellyfish, these proteins have been identified in many species of tropical corals. They play a key role in regulating the light that reaches the microalgae living in symbiosis with corals.
Fluorescent pigments: a natural shield
These pigments aren’t just for show. They play a vital protective role. By modulating the intensity and quality of light entering coral tissue, they prevent excessive stress on the symbiotic microalgae. These algae, called zooxanthellae, are responsible for the photosynthesis that feeds the coral. Too much light can cause them to be expelled, a phenomenon known as coral bleaching.
Why do some corals become fluorescent?
Not all corals fluoresce, and those that do do not always emit the same colors. This diversity of fluorescence is linked to several environmental and biological factors.
Protection against UV rays and ocean warming
In environments with intense light, especially in shallow waters, fluorescence helps filter out UV rays. These rays can be harmful to coral cells and cause oxidative stress. By re-emitting light at a different wavelength, fluorescent pigments protect sensitive coral cells.
With global warming, some corals tend to intensify their fluorescence. This phenomenon could be a response to thermal stress, particularly during bleaching events. Studies have shown that some species develop increased fluorescence just before or after a bleaching event, which could be an attempt at light regulation to compensate for the loss of zooxanthellae.
Secret communication between marine organisms
Fluorescence may also play a role in communication between marine organisms. Some species of fish and invertebrates are sensitive to the wavelengths emitted by fluorescent corals, which could influence their behavior. Some researchers suggest that fluorescence attracts cleaner fish or specific prey, establishing a beneficial interaction between marine species.
What is the impact of climate change on coral fluorescence?
With rising temperatures and ocean acidification, coral reefs are under pressure. Fluorescence is directly affected by these environmental changes.
An adaptation or a warning signal?
Some corals increase their fluorescence in response to heat stress, but this phenomenon is not necessarily a sign of successful adaptation. Rather, it may be a last resort before irreversible bleaching. When conditions become too extreme, even fluorescent corals can no longer survive, leading to the death of reefs.
How Scientists Use Fluorescence to Monitor Reefs
Researchers are using coral fluorescence as an indicator of reef health. Using special cameras and sensors, they can identify corals in distress before bleaching is visible to the naked eye. This approach helps anticipate stress episodes and better understand the mechanisms of reef resilience in the face of climate change.
Can you see these fluorescent corals while diving?
Observing fluorescent corals is a unique experience for divers and underwater photographers.
Where to observe these underwater wonders
Fluorescent corals are found in many parts of the world, including reefs in the Red Sea, Indonesia, Australia's Great Barrier Reef, and the Caribbean. Night dives provide the best spectacle, as the fluorescence is more visible in the absence of ambient light.
The best techniques to capture their light
To observe and photograph coral fluorescence, divers use blue light lamps and yellow filters on their masks or camera lenses. This equipment allows the fluorescence to be revealed with maximum intensity, offering a fascinating spectacle.
Conclusion: Fluorescence, a miracle of evolution
Coral fluorescence is more than just an aesthetic phenomenon. It is a testament to nature’s ingenuity in adapting to extreme conditions. This protective mechanism may help some corals withstand climate change, but it remains a fragile indicator of reef health.
Understanding and protecting these natural wonders is essential to ensure the survival of marine ecosystems. Coral reefs play a fundamental role for marine biodiversity and for millions of people who depend on them. By exploring coral fluorescence, we delve into a fascinating world where science, ecology and beauty meet.
Scientific references
Dove, S., & Hoegh-Guldberg, O. (2006). The role of fluorescence in the photobiology of reef-building corals . Marine Biology, 149 (5), 979-993. https://doi.org/10.1007/s00227-006-0229-2→ This article explores the function of fluorescent pigments in light regulation and protection of corals from oxidative stress.
Salih, A., Larkum, A., Cox, G., Kühl, M., & Hoegh-Guldberg, O. (2000). Fluorescent pigments in corals are photoprotective . Nature, 408 (6814), 850-853. https://doi.org/10.1038/35048564→ This study highlights the role of fluorescent proteins as a natural shield against UV radiation.
Mazel, CH, & Fuchs, E. (2003). Contribution of fluorescence to the spectral signature and perceived color of corals . Limnology and Oceanography, 48 (1), 390-401. https://doi.org/10.4319/lo.2003.48.1_part_2.0390→ Analysis of fluorescence light spectra in different coral species and their impact on underwater visual perception.
Gittins, JR, D'Angelo, C., Oswald, F., Edwards, RJ, & Wiedenmann, J. (2015). Fluorescent protein-mediated colour polymorphism in reef corals: Multilevel regulation underpins phenotypic diversity . Scientific Reports, 5 , 11547. https://doi.org/10.1038/srep11547→ This research shows how coral fluorescence varies depending on environmental and genetic conditions.
Smith, E.G., D'Angelo, C., Sharon, Y., Tchernov, D., & Wiedenmann, J. (2017). Acclimatization of symbiotic corals to mesophotic light environments through wavelength transformation by fluorescent protein pigments . Proceedings of the Royal Society B, 284 (1857), 20170320. https://doi.org/10.1098/rspb.2017.0320→ This article explores how corals use fluorescence to maximize light efficiency at different depths.
