When Coral’s Colorful Show Is a Sign That It’s Sick

Breaking up is hard to do, and the measures some take to get their partners back can be colorful at times. Coral reefs are that way, too, but for them, it’s a matter of life and death.

When certain species of coral flash a shimmering palette of vibrant pinks, reds, blues, purples and yellows, they aren’t simply showing off. This coral is attempting to recover the algae they cannot live without, according to a study published Thursday in the journal Current Biology.

Coral depends on a remarkable symbiotic relationship with algae, which lives inside the organism’s tissue. When the algae-coral partnership is thriving, many coral display a healthy brown hue.

Sometimes, after environmental stress, such as a spike in seawater temperature, the algae dies, or the coral expels it. Without that brownish internal photosynthetic factory pumping out meals for the coral, the underlying skeleton shines through the translucent coral flesh as bleach white, and the coral is at risk of starving to death.

But the scientists found that in order to get the algae back, some species envelop themselves in bright, sometimes fluorescent colors, which mitigate intense light reflections through the coral and create conditions for the light-sensitive algae to return.

“They produce their own sunscreen, these colorful pigments,” said Jörg Wiedenmann, professor of biological oceanography at Southampton University in England, who led the study. “They do it on a regular basis as a survival technique.”

As the centerpieces of vast marine ecosystems, coral reefs are a critical component of life on Earth. They are said to account for one third of all biodiversity in the sea, and are the source of food and income for an estimated half-billion people.

But over the last few decades, and particularly the last several years, scientists say global warming has threatened the reefs’ existence through widespread bleaching.

ImageWhen under stress, some species of coral envelop themselves in bright, sometimes fluorescent colors to create conditions for light-sensitive algae to return.
Credit…Ryan Goehrung/University of Washington

If the stress is not too severe or prolonged, the technicolor display known as color bleaching can save some coral, Dr. Wiedenmann’s team determined.

Some healthy corals display vivid colors, and many experts wondered if the color bleaching process was just a matter of visibility. Perhaps the brownish algae masked other coral’s pigments. Or, perhaps the coral and the algae were competing over blue light rays and once the algae were gone, the coral flexed its fluorescent muscles under all the extra blue light.

But Dr. Wiedenmann said their study determined that the process is actually an optical feedback loop that helps to restore the symbiotic relationship.

In the first stage of this loop, the algae are lost and the coral turns bleach white. That causes more light to reach and bounce off the reflective coral skeleton.

Within two or three weeks of the original stress incident — a heat wave or some shock to the nutrients available to the coral — the extra light triggers genes in the coral to manufacture the color pigments. The more sunlight they take in, the more pigment they produce. The pigments block certain wavelengths of light, making it possible for the algae to safely recolonize the coral.

“The optical feedback loop is a beautiful example of how nature regulates processes,” Dr. Wiedenmann said. “The corals are changing their physiological setup and are responding to an environmental cue.”

But pigment production can happen only under mild temperatures stress.

Although there is great concern about the long-term health of the reefs globally, Dr. Wiedenmann and his colleagues are encouraged that, in some cases, certain coral species, at least those in shallow waters, have the capacity to recover by reuniting with their old partners.

“It’s fascinating to see a symbiotic association with an animal and a plant,” he said, “and they are collaborating to produce a stress response that is beneficial for the partnership.”

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