Coral bleaching is one of the big concerns of our time. Over 93 percent of the Great Barrier Reef is bleached, with around half of it dead from this process. For much of the coral reefs, it may well be too late. But the more we understand about bleaching, the better we can help to curtail it.
In light of this, researchers have captured coral bleaching on film for the first time. Their findings are published in the journal Coral Reefs.
Coral have a mutually beneficial relationship with the algae that live in their tissues, such the Symbiodinium algae. The coral provide the algae with surface area for photosynthesis, while the algae provide the coral with their surplus sugar. However, the Symbiodinium algae can become stressed and leave the coral’s tissue when they’re subject to disease, pollution, or overheating – not good news when our global sea temperatures are on an "unstoppable" rise. Their absence means the coral lose a major source of food, becoming weak and highly susceptible to disease. It also leaves them looking pale and white, hence the term “bleaching”.
To simulate rising sea surface temperatures, researchers from Queensland University of Technology (QUT) placed corals, Heliofungia actiniformis, into a tank and gently rose the water temperature to 26 to 32°C (78 to 89°F) over a period of eight hours.
"What's really interesting is just how quickly and violently the coral forcefully evicted its resident symbionts," researcher Brett Lewis, from QUT's Science and Engineering Faculty, explained 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."
While the speed of the bleaching came as a surprise to the researchers, the biggest breakthrough was understanding the mechanism of how this coral removes the algae. It has all to do with a weird process of inflation and swelling, then a sudden contraction that "violently" expels the algae.
Dr Luke Nothdurft from QUT's marine facility in the School of Earth, Environmental and Biological Sciences said: "Our H. actiniformis used a pulsed inflation to expel Symbiodinium over time – inflating their bodies to as much as 340 percent of their normal size before suddenly and violently contracting and ejecting Symbiodinium through their oral openings over the four to eight-day duration of the experiments."
