Sometimes, Stressed Corals Become Even More Colorful


Tom Hale

Senior Journalist

clockMay 21 2020, 16:10 UTC

Acropora corals that have undergone "colourful bleaching" in the Philippines in 2010. Courtesy of Ryan Goehrung/University of Washington

Ghostly white coral reefs are one of the defining images of our drastically changing natural world, but a new study has shown that not all suffering corals turn pale when they are bombarded by pollution and rising ocean temperatures – some glow with stunning colors and vivid vibrancy. 

New research by the University of Southampton in the UK has explained an unusual process known as "colorful bleaching" that corals use as an emergency recovery strategy to fight against environmental changes. Like a corny motivational tale, adversity and woe help them glow even brighter. 


“It was as if the corals were screaming for attention in vivid color, trying to protect themselves from ocean heatwaves. We’d witnessed the ultimate warning that the ocean is in trouble,” Richard Vevers, CEO and founder of The Ocean Agency, who was involved in the documentation of the 2016 colorful bleaching event in New Caledonia, commented in a statement given to IFLScience. 

Corals get their dashing colors through a symbiotic relationship with tiny algae and single-celled zooxanthellae that live within their tissues. The microorganisms provide the coral with food (and color through their photosynthetic pigments), while the coral provides them with shelter. Unfortunately, this happy relationship can become stressed if the water becomes polluted or too warm, forcing the microalgae and zooxanthellae to evade the coral tissues. Not only do the corals often lose their pigments, giving them the distinct white "bleached" look, they also lose their food supply, making them become weak and vulnerable to disease. 

However, coral reefs don’t always become drained of color at the first sign of danger. Reported in the journal Current Biology today, a new study shows coral reefs that endure mild or brief episodes of ocean warming can undergo a process, known as an “optical feedback loop,” which causes the coral to become more vivid with color. 

The absence of the coral’s microscopic partners allows more light to flood into the corals' inner tissues, boosting the production of colorful and photoprotective pigments. This creates a colorful layer that acts like sunscreen, blocking some UV light and, in turn, providing conditions that are favorable for the return of zooxanthellae. 

"In some cases, very colorful corals may have been mistaken as a particularly health corals instead of stressed ones. However, in most cases there are some corals that bleach white due to the involved color polymorphisms, so quite likely, observers will realize that something is wrong," Professor Jörg Wiedenmann, head of the University of Southampton's Coral Reef Laboratory, told IFLScience.

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

"It has been long known that not all bleaching events are the same and a 'patchiness' of the bleaching response has been observed frequently," he added.


While this study further highlights how corals hold the power to recover from slight environmental changes, the overall picture is fairly bleak. 

Just last month, it was reported that Australia's Great Barrier Reef had experienced its third coral bleaching event in five years. Things are only pitched to get worse, too. Due to greenhouse gas emissions, the global temperature has increased by 1°C over the past 150 or so years. Even if the world limits global warming to 1.5°C above pre-industrial levels, we can expect to see a further 70 to 90 percent loss of coral reef cover. A rise of 2°C, however, is game over for over 99 percent of the world’s reefs.

If there's no action to mitigate climate change in the coming decades, there’s a strong chance most coral reefs on Earth will be gone by the end of this century.

Acropora corals undergoing colorful bleaching in New Caledonia. Courtesy of Richard Vevers/The Ocean Agency/XL Catlin Seaview Survey 

  • tag
  • climate change,

  • coral reef,

  • pollution,

  • coral,

  • bleaching,

  • ecosystem,

  • ocean warming,

  • marine environment,

  • bleaching event