In a scientific first, researchers at the Queensland University of Technology (QUT) in Australia have captured a time-lapse video of how corals typically behave during a bleaching event.
With widespread bleaching continuing to threaten reef systems around the world, Luke Nothdurft and his colleagues at QUT examined the impact of climate change on the overall health and well-being of corals.
For their work, the researchers exposed a single polyp mushroom coral known as Heliofungia actiniformis to environmental stress brought on by an increase in water temperature. They placed samples of the coral in aquariums and then raised the temperature to about 26 to 32 degrees Celsius (79 to 90 degrees Fahrenheit). The water was kept at the highest temperature for eight days.
Nothdurft and his colleagues found that when the corals experienced pressure from their environment, they became less capable of sustaining the colorful algae called zooxanthellae that inhabited their body.
The H. actiniformis ended up ejecting the zooxanthellae that they housed when the water became too warm for them to handle. The researchers were able to photograph how one of the corals belched the Symbiodinium algae from its body, causing it to lose much of its colorful veil.
While coral bleaching has long been known to scientists, the exact details regarding how coral polyps eject their tiny tenants weren't quite understood until now.
"What's really interesting is just how quickly and violently the coral forcefully evicted its resident symbionts," lead author Brett Lewis said.
"The H. actiniformis began ejecting the symbionts within the first two hours of us raising the water temperature of the system."
According to Lewis, the H. actiniformis has been shown in previous studies to be one of the corals in the Great Barrier Reef relatively resilient to bleaching, even though much of the surrounding species have already succumbed to the event's full effect.
The study's findings suggest that the coral polyp's resilience may have something to do with how fast it is able to eject the zooxanthellae algae from its body as it experiences thermal stress. The action may even help increase the organism's likelihood of surviving abnormally high water temperatures.
During the experiment, the researchers saw the H. actiniformis employ a process known as pulsed inflation. The size of the coral polyp's body increased by as much as 340 percent just before it forcefully ejected the algae from its body through an oral opening. This action went on for four to eight days.
Symbiosis Between Coral And Algae
Nothdurft explained that certain species of corals capable of rebuilding reefs have developed a mutually beneficial relationship with the Symbiodinium algae living in their bodies.
The Symbiodinium provides the corals with the necessary nutrients to help keep them going, while the corals provide the algae with protection and enough surface area in order to carry out photosynthesis.
Nothdurft pointed out that if environmental conditions are able to recover quickly following a bleaching event, then certain species of corals may be able to regain the algae expelled, as well as their color.
The findings of the Queensland University of Technology study are featured in the journal Coral Reefs.
