Coral doesn't really like warming water, something scientists have know for quite some time. But by recording a close-range, time-lapse video of coral's response to rising sea temperatures, researchers have gained some new insights into how it expels algae from its tissue, a risky defense mechanism known as coral bleaching.
This year, Australia's Great Barrier Reef suffered through the worst coral bleaching event in its history. These events are triggered when sea temperatures venture outside of their normal range, which places stress on the algae living inside the coral that not only gives the corals their vibrant colors, but the nutrients they need to survive as well. The algae becomes toxic to the coral at these higher-than-normal temperatures, so it is ejected from the tissue as a coping mechanism. But this leaves the coral white, withering and — if temperatures don't soon drop to allow the return of the algae — in danger of death.
Mass bleaching events have occurred in the Great Barrier Reef before, in 2002 and 2008, but a survey in April revealed this year's to be more severe and widespread than ever before, with 93 percent of the reef affected by bleaching.
To learn more about how this process plays out, scientists from Australia's Queensland University of Technology placed a coral species called Heliofungia actiniformis into a 10-liter (2.64 gal) tank and used a combination of a microscope, digital camera and tablet to film its behavior as they turned up the heat.
With the temperature raised from 26° C to 32° C (78.8° F to 89.6° F) over twelve hours, and then left there for up to eight days, the team was able to observe in new detail the tricks the coral uses to endure higher temperatures.
"Our H. actiniformis used a pulsed inflation to expel Symbiodinium (the algae cells) over time (seen as greenish plumes in the video) – 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," says Dr Luke Nothdurft, from the university's School of Earth, Environmental and Biological Sciences.
H. actiniformis happens to be a coral species that is particularly resilient to coral bleaching, compared to neighboring species that suffer more dramatically from the changing temperatures. The scientists say that their research has revealed one of the possible reasons why, with the H. actiniformis violently ejecting the coral within the first two hours of the change in water temperature.
"Our observations suggest this resilience could be due to the rapid expulsion of the coral's algal symbionts during thermal stress, and could very well increase H. actiniformis' chance of survival during abnormally high sea temperatures," says Mr Lewis.
The time-lapse video can be seen below.