Neon “Sunscreen”? Corals’ Defense Against Bleaching


Some Acropora corals turn bright neon colors in the Pacific Ocean, a response that can help them to recover from coral bleaching. Photo Courtesy of Richard Vevers/The Ocean Agency, XL CATLIN SEAVIEW SURVEY

Hannah Choy, Editor

Rays of sunlight penetrate through the turquoise water, illuminating an expanse of textures. Fish skillfully weave through the extruding branches of the coral, looking for their next meal. While many of these intricate structures have turned a ghostly white color due to bleaching, some coral seem to remain brightly colored. How does this happen? Through the use of neon “sunscreen”!

Coral bleaching occurs as a result of warmer water temperatures caused by global warming and human activities. When the water temperatures are too extreme, corals expel the zooxanthellae algae that inhabit their tissues, making the coral appear bleached. While bleached coral can survive, they are subject to mortality and negative effects due to increased stress, similar to cracked or swollen skin in response to continuously being exposed to ultraviolet (UV) rays. Research suggests that some coral are able to use neon “sunscreen” as a means of recovery from bleaching, similar to how we use sunscreen to protect our skin from the sun’s UV.

When coral loses the beneficial photosynthetic algae when bleached, it often loses color and nutrients provided by the zooxanthellae, which acts as a food source for the corals. This is like our skin being depleted of vitamins from UV radiation. However, in 2017, researchers and underwater photographers featured in the documentary Chasing Coral discovered something surprising during an underwater heatwave in New Caledonia: the corals weren’t turning white as expected, but rather glowing in neon colors! This isn’t a new phenomenon, as a similar experience was documented in the 1990s off French Polynesia’s Society Islands.

The reason for this glow isn’t certain, but research by scientists like Dr. Elena Bollati of the U.K. Coral Reef Laboratory suggests that the coral may use this increased pigmentation as sunscreen to “invite their zooxanthellae back home.” Her studies found certain wavelengths of light to trigger an increase in pigment production, creating a more suitable environment for the algae. This is like the adaptation of people in areas that receive more UV radiation to have more pigmented skin, providing increased protection.

Corals’ neon “sunscreen” seems to correspond with mild heat stress, like warmer waters or brief temperature spikes, as Bollati also found most colors and increased fluorescent compound levels to appear two to three weeks after such events. Additionally, scientists have observed coral with colored areas to regain their zooxanthellae more quickly than less pigmented areas, a feat that will help the coral to survive.

While further research is being done to examine the exact causes of this neon coloration in some corals and not others, one thing is clear: this adaptation isn’t a permanent solution to increased bleaching events. As marine biologist Jörg Widenmann, part of Bollati’s research team states, “Corals have this capacity to fight back…but their long term survival depends on people acting to limit coral change so that corals don’t experience more stress than they can handle.” We can do this by making changes, like using reef safe sunscreen to help coral to continue painting our oceans with beauty.

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