What Is Coral Bleaching?

Pedro_P

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In "bleached" reefs, the corals' white skeletons are visible under their transparent flesh.
Credit: Chasing Coral
▼ Once vibrantly colored and teeming with life, many coral reefsaround the planet are now bleached and barren, thanks to a condition called coral bleaching. Their color drained, bleached reefs stand like skeletons along the world's coastlines, from Australia and Madagascar to the Persian Gulf and the Caribbean Sea.
But coral bleaching is much more than an aesthetic loss. It is an environmental indicator: an omen of starving animals, a failing ocean ecosystem and a devastating change in global climate. Rising ocean temperatures are the fundamental cause. But before we can understand why these beautiful coral ecosystemsare now at risk, we have to understand how they got their radiant color in the first place.
How do corals get their color?
Coral reefs are made up of polyps, small, colorless animals that have a sac-like body with a mouth-like opening and a crown of stinging tentacles. A coral reef consists of many individual polyps functioning together as one unit.
The polyps themselves are transparent. Coral reefs get their color from the tiny creatures living inside the polyps: algae called zooxanthellae.
Coral and zooxanthellae enjoy a mutually beneficial partnership, known as symbiosis. Coral provides the algae with shelter, access to sunlight and other resources needed for photosynthesis. The algae, in turn, share the nutrients produced by photosynthesiswith the coral. As much as 90 percent of the nutrients that algae produce are transferred to their coral hosts, according to the National  Oceanic and Atmospheric Administration  (NOAA).
Why do corals bleach?
Under environmental stress, the intricate algae-coral partnership becomes unhinged. Factors such as temperature changes, pollution and overfishing can destabilize the relationship and cause the coral to expel the algae. Once the algae are gone, the coral's bright white calcium-carbonate exoskeleton is visible through its transparent tissue, hence the name coral bleaching.
Rising sea temperatures brought on by global warming have become the greatest danger to coral reefs, according to NOAA. Temperature spikes of only 1.8 to 3.6 degrees Fahrenheit (1-2 degrees Celsius) can trigger mass bleaching eventsthat affect tens to hundreds of miles of coral reef. This type of heat stress affected 70 percent of the world's coral reefs between 2014 and 2017.
Coral bleaching happens gradually, said Ruben Torres, a marine scientist and the founder of Reef Check Dominican Republic, a nonprofit ocean-conservation group. As water temperature rises above the coral's comfort zone, the algae begin to leave, and the coral grows paler until all of the algae are gone.
"Once algae are gone, they [the corals] lose their source of energy," Torres said. "They are basically starving to death."
Bleached coral are still alive, but without the algae, the coral are vulnerable.  (▪ ▪ ▪)

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Rhett_Bassard

You provide an eye-opening contribution that I want to supplement with a second and more invasive form of 'coral bleaching' I observed working as a scuba diver in the Caribbean:

Less-enlightened and criminal lobster hunters will use squirt-bottles of laundry bleach [sodium hypochlorite] to drive the crustaceans from underneath coral head hiding spots. All in the name of revenue.

If you see bleach bottles in the boats or on the beaches of your favorite tropical paradise, boycott the local lobster trade.

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