Glowing to Survive: Reefs’ Neon Color Show a Defense Against Coral Bleaching

Acropora corals with colorful bleaching in New Caledonia. (Photo courtesy The Ocean Agency / XL Catlin Seaview Survey)

(CN) — Some coral reefs are adapting to warming ocean temperatures by making their own sunscreen in the form of bright neon colors — a strategy that invites coral animals to return to reefs and is seen as a critical adaptation to maintain healthy coral reefs around the world.

In a study published Thursday in the journal Current Biology, researchers at the University of Southampton detail a series of controlled laboratory experiments they conducted at their coral aquarium facility.

In the experiments, “colorful” coral bleaching events cause coral to produce a layer of vibrant sunscreen, which encourages the coral animals vital to a mutually beneficial “symbiosis” relationship to return to coral habitats they abandon due to the effects of warming oceans.

Many coral animals live with tiny algae embedded in their cells — it’s a fragile symbiotic relationship where the algae gain shelter, carbon dioxide and nutrients while the coral receives photosynthetic products to fulfill their energy needs.

When the ocean’s temperature rises, the symbiosis breaks down and the algae are lost, causing the coral’s white limestone skeleton to shine through and be damaged via coral bleaching. The condition can be fatal to coral — when its live tissue is gone and its skeleton is exposed to erosion, an entire coral reef can break down in a few years, causing the biodiversity that relies on its complex structure to fall apart in the process.

But some coral undergoes colorful coral bleaching — emitting a range of different bright neon colors — derived from photoprotective green fluorescent protein-like pigments produced by the coral host.

The colorful adaptation could prove vital for overcoming the fatal coral bleaching incidents that have threatened coral reefs worldwide. But the colorful coral bleaching — rather than the whitskeleton exposure of common coral bleaching events — is believed to take place due to mild ocean warming or disturbances in their nutrient environment, rather than extreme events.

Colorful bleaching occurred between this past March and April in some areas of the Great Barrier Reef, suggesting some patches of the world’s largest reef system may have better recovery prospects than others.

“Bleaching is not always a death sentence for corals, the coral animal can still be alive. If the stress event is mild enough, corals can re-establish the symbiosis with their algal partner. Unfortunately, recent episodes of global bleaching caused by unusually warm water have resulted in high coral mortality, leaving the world’s coral reefs struggling for survival,” Dr. Cecilia D’Angelo, lecturer of molecular coral biology at Southampton, said in a statement.

Colorful coral bleaching involves a self-regulating mechanism, or so-called optical feedback loop, involving both the coral and the algae animals, according to Jorg Wiedenmann, head of Southampton’s Coral Reef Laboratory.

In healthy coral, much of the sunlight is taken up by the photosynthetic pigments of the algae. But when coral loses those algae, excess sunlight travels back and forth, resulting in the white coral skeleton.

The increased internal light in coral is “stressful” for the algae and can delay their return.

“However, if the coral cells can still carry out at least some of their normal functions, despite the environmental stress that caused bleaching, the increased internal light levels will boost the production of colorful, photoprotective pigments,” Wiedenmann said in a statement accompanying the study.

“The resulting sunscreen layer will subsequently promote the return of the symbionts. As the recovering algal population starts taking up the light for their photosynthesis again, the light levels inside the coral will drop and the coral cells will lower the production of the colorful pigments to their normal level,” Wiedenmann added.

Researchers also reconstructed the temperature history of 15 colorful bleaching events around the world using satellite imagery. The data was consistent with the results of the controlled laboratory experiments suggesting colorful bleaching occurs due to brief or mild episodes of heat stress.

Acropora corals with colorful bleaching in New Caledonia. (Photo courtesy The Ocean Agency / XL Catlin Seaview Survey)
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