What Is Coral Bleaching? How Some Reefs Glow Before They Die

Bleached coral are not always white.

Reef Check France

The Netflix documentary Chasing Coral documents coral bleaching in the Great Barrier Reef and around the world like it has never been seen before. Using custom, high-tech underwater gear that only sometimes worked, the crew managed to capture for the first time what a global mass die-off of coral actually looks like.

It’s as exciting as it is upsetting. High ocean temperatures between 2014 and 2017 impacted 70 percent of the world’s coral reefs. On some sections of the Great Barrier Reef 83 percent or more of the coral died. Worldwide, coral covering an estimated 4,633 square miles died.

This isn’t normal. Before the 1980s, mass die-offs of coral was unheard of. But global warming, caused mainly by fossil fuel burning, has turned up the heat in the oceans, making it just a little too hot to handle. A couple of degrees Fahrenheit might not seem like a lot, but think of it like a fever — like humans, reefs don’t deal with temperatures outside a range of normal very well. A fever that lasts a day won’t kill you, but one that gets too hot or lasts too long will. It’s the same with coral bleaching.

Bleaching events will be more frequent and severe in the future.

The Ocean Agency / XL Catlin Seaview Survey

What is coral bleaching?

Coral are animals most closely related to a sea anemone or jellyfish. They take the form of polyps — a tentacled fleshy mouth. Many coral live in colonies that work together to build hard skeletons of calcium carbonate — this is the structure that builds up over time to become a reef.

Coral get most of their food from a symbiotic relationship with single-celled algae called zooxanthellae. These live within the polyp flesh, where they photosynthesize and produce energy, which the coral feeds on. These plant cells also give coral their color, typically an earthy brown.

But this relationship becomes mutually destructive when things heat up. In temperatures just a couple degrees outside the range of normal, zooxanthellae begin to release toxins into the flesh of the coral, and the coral promptly gives them the boot.

This image shows the same reef in American Samoa before, during, and after a coral bleaching event.

Ocean Agency / XL Catlin Seaview Survey / Richard Vevers

This is bleaching. Without plant pigments, the flesh of the polyps is totally clear, like a jellyfish. The bright white calcium carbonate skeleton shines through.

A coral can survive a while without its algae friends, but if the water temperature does not return to normal within a few weeks, it will die. The flesh rots away and the skeleton is overtaken with stringy brown algae.

What’s up with coral that glow in bright colors?

Algae is not the only source of pigmentation in coral. Some also fluorescent pigments that glow bright colors, including pink, purple, blue, green, and red.

Scientists have recently discovered that these pigments, produced not by the algae but by the coral polyps themselves, are a type of chemical sunscreen that protects it from the harshest direct rays. In a healthy coral, this fluorescence is stunning but mostly subtle. Parts of the organism most exposed to the sun, especially tips, will glow, but the rest mostly retains its brownish green hue.

Not all coral do this, and even within the same species on the same reef you might see one glowing coral next to one that’s dull. It’s an evolutionary genetic adaptation that is seen more often, unsurprisingly, in regions where the direct sun gets most intense.

A bleached coral protects itself from the sun with a glowing, blue pigment. 

The Ocean Agency / XL Catlin Seaview Survey

If a coral with this special ability is bleached, something spectacular and tragic happens. The zooxanthellae are expelled as expected, and the coral then uses this chemical sunscreen to try to protect its now-naked flesh. The result is a sea of coral glowing bright in a rainbow of color.

“When you see it underwater, it’s the most spectacular site in nature that you will ever come across,” says Richard Vevers, whose mission to save the reefs is documented in Chasing Coral, tells Inverse.

The film was mid-production when he got the call that extraordinary event was happening on reefs off of New Caledonia, an island territory east of Australia. “I jumped on a plane immediately to get out there, saw what was happening and brought a film team in, because I don’t think anyone’s ever photographed or filmed a scene like it,” says Vevers. “What you find is just coral as far as the eye can see, all glowing in different colors. It’s just an unbelievable sight.”

The sad truth that these animals are starving, and if the water does not quickly cool, they die.

There’s still a lot we don’t know about these chemical sunscreens, but it could be that they do offer a measure of protection in a bleaching event. Vevers says the coral of New Caledonia weathered the bleaching event better than anticipated, which fewer casualties.

If these glowing coral are more resistant to the effects a warming ocean, it could be that they will make up a larger percentage of coral overall in the future. Although, it should be said, that survivors of this assault on coral reefs will be few. Best case scenario, 90 percent of reefs that were on this Earth a few decades ago are gone by 2050, and we rebuild from there.

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