Watch how corals ‘violently’ bleach as sea temperatures rise

by Shreya Dasgupta on 22 August 2016

Researchers placed individuals of the solitary coral, Heliofungia actiniformis, into a 10-litre aquatic tank and began heating the water up.
Within the first two hours of raising the water temperature, the H. actiniformis began expelling Symbiodinium, the tiny algae that lives inside its tissues, in a process called pulsed inflation.
The intensity and magnitude of the expansion-contraction pulses increased with time, with the coral inflating to up to 340 percent of its original state.

Corals around the world are bleaching.

Australia’s Great Barrier Reef, for example, suffered widespread bleaching this year due to rising sea surface temperatures caused by a combination of global warming and a strong El Niño.

Bleaching occurs because warmer-than-normal sea waters disrupt the mutual relationship between corals and the algae that reside in their tissues. These algae give the corals food and their distinct vibrant colors. So when sea surface temperatures rise above normal average, the heat-stressed corals spit out the algae, turning pale or ghostly white. If elevated sea temperatures continue for prolonged periods, the heat-stressed corals can starve to death.

Now, for the first time, scientists have caught a coral’s bleaching on camera.

Researchers from Queensland University of Technology (QUT) placed individuals of the solitary coral, Heliofungia actiniformis, into a 10-litre aquatic tank, and began heating the water up from 26^oC to 32^oC over 12 hours to see the effect of elevated water temperature on the corals. The corals remained at 32^oC for up to eight days.

Then by using microscopes, digital camera and smart tablets, the team created detailed time-lapse videos of the coral’s reaction to the heat-stress, according to the study published in Coral Reefs.

Within the first two hours of raising the water temperature, the H. actiniformis began expelling Symbiodinium, the tiny algae that lives inside its tissues, in a process called pulsed inflation.

In the video, the coral first rapidly expands, then contracts to release a green plume of algae, becoming paler over time. The intensity and magnitude of the expansion-contraction pulses also increased with time, with the coral inflating to up to 340 percent of its original state, the authors write in the paper.

Under the raised temperature conditions, the corals survived between two and eight days, the team found.

“What’s really interesting is just how quickly and violently the coral forcefully evicted its resident symbionts,” Brett Lewis, from QUT’s Science and Engineering Faculty, said in a statement.

This rapid expulsion of the algae during heat stress could actually increase H. actiniformis’s chance of survival during abnormally high sea temperatures, Lewis added.

This is because over-exposure to high sea surface temperature can lead to elevated levels of photosynthesis by the algae, resulting in toxic amounts of oxygen in the corals’ tissues. So by expelling the algae quickly, the corals could be protecting themselves in the short-term. But if the warmer conditions continue for longer durations, then algae can fail to re-colonize the corals, leading to the corals’ death.

“If the Symbiodinium is removed from the host and does not recolonise quickly, the corals can die,” co-author Luke Nothdurft of QUT said in the statement. “Mass coral bleaching events are a concern for scientists globally with recent events on the Great Barrier Reef highlighting the threat of elevated water temperatures to the heath of reef ecosystems.”