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Dying coral reefs turn vivid neon colors into an obvious last ditch attempt to survive

For years, coral reef around the world has been destroyed by Mass bleaching events as the seas continue to heat due to climate change. Corals have little chance of bouncing back from these events – but a new study suggests that they have an unusual method of survival: to put on a lively neon color.

When bleaching events occur, prolonged heat spikes cause coral to become a ghostly white, often leading to their death. But “colorful fading” has the opposite effect: the dying corals get more pigment and shine in shades of bright pink, purple and orange.

Scientists first discovered the mysterious neon coral a decade ago, but they had not been able to find out why it happened. This study, published Thursday in the journal Current Biology, suggests that corals change color as a last ditch effort to survive.

Richard Vevers / The Ocean Agency / XL Catlin Seaview Survey
Colorful fading Acropora corals in New Caledonia.

Richard Vever / The Ocean Agency / XL Catlin

Coral animals coexist with small algae and provide them with protection, nutrients and carbon dioxide in exchange for their photosynthetic powers. Also small increases in annual sea temperatures can destroy this relationship, expel the algae from the coral’s tissue and reveal its white skeleton. After the coral has been exposed, it often breaks down and dies, which changes the ecosystem for the manifold amount of life that relies on it.

Researchers at the University of Southampton’s Coral Reef Laboratory studied 15 colorful bleaching events around the world between 2010 and 2019 – including one in the Great Barrier Reef, the world’s largest coral reef system – and recreated these ocean temperatures in one lab. They found that colorful bleaching events occur when corals produce “what is really a sunscreen layer” on their surface to protect against harmful rays and create a glowing screen that scientists believe encourages algae to return.

The Ocean Agency describes the process as a “chilling, beautiful and heartbreaking” final cry for help as the corals try to catch the algae’s attention.

“Our research shows colorful bleaching involves a self-regulating mechanism, a so-called optical feedback loop, involving both parties in the symbiosis,” said lead researcher Professor Jörg Wiedenmann at the University of Southampton in a press release. “The resulting sunscreen layer will then promote the return of the symbionts. When the recovering algal population begins to absorb the light of their photosynthesis, the light levels inside the coral will decrease and the coral cells will lower the production of the colorful pigments to their normal level.”

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Colorful fading Acropora corals in the Philippines.

Ryan Goehrung, University of Washington

Coral reefs support more species per unit area than any other marine environment, including about 4,000 species of fish, 800 hard coral species, and potentially millions of other undetected species, according to the National Oceanic and Atmospheric Administration. Coral reef disturbances have far-reaching consequences for the marine ecosystem.

It is not only the warming of the sea that causes colorful bleaching. Researchers say that changes in nutrient levels in coral reefs due to agricultural fertilizers also lead to bleaching events – a problem that can be addressed at the local level.

Experts only believe corals that have been subjected to mild or brief disturbance rather than extreme Mass bleaching events, can try to save themselves with this process. These corals can still undergo some of their normal functions for a short time because they hope their algae will return – while drastic changes in sea temperature almost always lead to coral death.

Reports of colorful bleaching during the latest mass bleaching event in the Great Barrier Reef in March and April gave researchers hope that stains of the system have a chance to recover.

“Bleaching is not always a death sentence for corals, the coral animal can still live,” says Dr. Cecilia D’Angelo, a molecular coral biology lecturer at the University of Southampton. “If the stress event is mild enough, corals can reestablish the symbiosis with their algae partner.”

The internal changes that cause colorful bleaching can occur.

Jörg Wiedenmann, Elena Bollati & Cecilia D’Angelo / University of Southampton, Palawan Colorful Pale Image by Ryan Goehrung / University of Washington

Bleaching events used to be few and far between, but they occur almost every year. In 2017 alone, nearly half of the corals on the Great Barrier Reef died – and experts say we are running out of time to save them.

“Unfortunately, the recent episodes of global bleaching caused by unusually hot water have resulted in high coral mortality and left the world’s coral reefs struggling for survival,” D’Angelo said.

Researchers emphasized that although colorful bleaching is a good sign, only a significant reduction in greenhouse gases globally – in addition to improving local water quality – can save coral reefs after this century.

“Now that we know that nutritional levels can affect colorful bleaches as well, we can more easily pinpoint cases where heat stress may have been exacerbated by poor water quality,” researchers said. “This can be managed locally, while the ocean’s heat waves caused by climate change will need global leadership. Together, these measures can secure a future for coral reefs.”

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