Coral reefs are vibrant underwater ecosystems. Many corals possess a remarkable ability to “glow,” emitting light in dazzling hues. Understanding the science behind this glow offers insights into coral health and their interactions with the marine environment.
Understanding Coral Fluorescence
The “glowing” observed in corals is primarily a process called fluorescence. This differs from bioluminescence, which is light generated by a chemical reaction within an organism, such as fireflies. Corals do not produce their own light; instead, they absorb light at one wavelength and then re-emit it at a different, longer wavelength. For instance, corals might absorb blue or ultraviolet (UV) light and then glow green, red, or yellow.
Fluorescence involves specialized proteins within the coral tissue. When these proteins absorb higher-energy light, their electrons become excited and jump to a higher energy state. As these electrons return to their original, lower energy state, they release the absorbed energy as light. This emitted light has a longer wavelength and therefore a different color than the absorbed light.
The Proteins Behind the Colors
Fluorescent proteins (FPs) are the biological components responsible for coral fluorescence. Corals synthesize a variety of these proteins, which are embedded within their tissues and determine the diverse array of colors they can display. Different FPs absorb and emit light at distinct wavelengths, leading to the wide spectrum of hues observed in glowing corals.
The study of these proteins, particularly green fluorescent protein (GFP) from jellyfish, revolutionized molecular biology. Scientists have since identified numerous GFP-like proteins in corals, with different types responsible for blue, cyan, green, and red fluorescence. These proteins are not only fascinating for their role in coral coloration but have also become invaluable tools in scientific research.
Functions of Coral Glow
Scientists have proposed several hypotheses for the biological reasons behind corals producing fluorescent proteins. One prominent idea is that FPs provide photoprotection, acting as a natural sunscreen for the coral and its symbiotic algae, zooxanthellae. In shallow, sunlit waters, these proteins can absorb harmful UV radiation and excess visible light, converting it into less damaging wavelengths or dissipating the energy.
Fluorescent proteins may also play a role in photosynthesis enhancement, particularly in deeper or low-light environments. Some FPs can convert the available blue light, which penetrates deepest into the water, into wavelengths that are more efficiently used by the zooxanthellae for photosynthesis. While this idea is still being investigated, it suggests that glowing could optimize light conditions for the algae. Additionally, FPs may possess antioxidant properties, helping to neutralize reactive oxygen species that can form during periods of high light exposure or stress. Some research also suggests that the glow might attract beneficial symbiotic algae or deter predators, though these functions are less understood.
Glow as a Sign of Coral Stress
While vibrant fluorescence is often associated with healthy corals, an unusual increase in glowing can sometimes indicate stress, particularly during the early stages of coral bleaching. This phenomenon is often referred to as “fluorescent bleaching” or “colorful bleaching.” When corals experience thermal stress or other environmental disturbances, they may expel their symbiotic zooxanthellae, which provide much of their natural color.
As the algae are lost, the coral tissue becomes more transparent, revealing the underlying fluorescent proteins more vividly. This heightened glow can be an early warning sign that the coral is under duress, even before it turns completely white. Research suggests that corals may increase FP production as a protective response to stress, attempting to mitigate light damage after losing their algal symbionts. Scientists are researching how changes in fluorescence intensity can be used as a non-invasive indicator of coral health and resilience.