Time-Lapse Video Reveals the Process of Coral Bleaching

A time-lapse video filmed by scientists at the Queensland University of Technology (QUT) in Australia reportedly provides viewers with a first-ever look at coral bleaching in progress, The Telegraph reports. Their observations, which they recently published in the journal Coral Reefs, explore the underlying mechanisms behind the harmful phenomenon.

Coral gets its brilliant colors from special, symbiotic algae called zooxanthellae, which live inside its tissues and consume its metabolic waste. In turn, the algae produce sugars and amino acids, which the coral polyp eats. But when corals become stressed from changing water temperatures, pollution, changes in salinity, and other external factors, they undergo a destructive process called coral bleaching. The algae leach out of the coral, and the coral turns a stark white as its calcium-carbonate skeleton is exposed. The coral is still alive at this point, but it’s now only receiving 20 percent of its needed food intake. If bleaching continues, the organism will eventually die.

Since coral reefs shelter 25 percent of marine species, the consequences of bleaching are dire. Scientists already knew that factors like climate change and El Niño, which heat up oceans, are major culprits. (Conversely, too-cold water can also cause coral bleaching.) However, they had never watched the process unfold.

To simulate the effects of coral bleaching, QUT scientists took a 2.6-gallon aquarium filled with Heliofungia actiniformis coral specimens. Over the course of 12 hours, they gradually increased the heat from around 79°F to nearly 90°F. The researchers kept the heat up for eight days, filming continuously as the coral gradually expelled the algae cells.

The footage gave the researchers an invaluable look at what happens when coral reefs bleach. First, the corals swelled—sometimes up to nearly 340 percent of their normal size—during a process called pulsed inflation. Then, they started to violently shudder as they spewed out their color-giving zooxanthellae. With each spasm, they grew whiter and whiter.

“What’s really interesting is just how quickly and violently the coral forcefully evicted its resident symbionts ,” QUT researcher Brett Lewis said in a press release.  “The H. actiniformis began ejecting the symbionts within the first two hours of us raising the water temperature of the system.”

According to the scientists, coral bleaching isn’t all bad—at least in the short-term scheme of things, National Geographic points out. Past studies have shown that H. actiniformis is relatively resilient to coral bleaching. This might be because the coral expels the zooxanthellae so quickly: When waters heat up, the algae becomes toxic, and the coral increases its chance of survival by getting rid of it. But if the water temperature doesn’t lower quickly enough for new algae to re-inhabit the coral’s tissues, it will likely die.

Coral bleaching is a global problem. Surveys of the Great Barrier Reef indicate that more than 90 percent of it is damaged by bleaching, and as temperatures continue to soar worldwide, reefs in the Caribbean, Atlantic, and Pacific are expected to suffer as well. There are a few bright spots where corals are doing well, despite the environmental pressures. Scientists are studying them to find out why.