When certain species of coral flash a shimmering palette of vibrant pinks, reds, blues, purples and yellows, they are not simply showing off. This coral is attempting to recover the algae they cannot live without, according to a study published May 21 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.”
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 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 Wiedenmann said their study determined that the process is actually an optical feedback loop that helps 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, 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,” Wiedenmann said. “The corals are changing their physiological setup and are responding to an environmental cue.”