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A Beacon of Hope for Battered Reefs: Bleaching-Resistant Corals

Biologists from the University of Pennsylvania have discovered that corals resistant to bleaching can maintain their resilient characteristics in new environments, laying the foundations for future reef restoration through coral transplantations.

The massive destruction of coral reefs has been one of the most prominent environmental concerns of the last five decades. Approximately 60 major episodes of coral bleaching occurred between 1979 and 1990, but the most devastating event was reported to take place in 2015, affecting almost half of all Hawaiian coral reefs.

Coral bleaching takes place when warm ocean temperatures trigger corals to release the algae with which corals share a symbiotic relationship. While corals offer shelter for the algae, the algae provide food for corals. As such, the loss of the algae would mean the loss of their food source, more stress, and potentially death. This disastrous chain of events has been attributed to climate change and human activity. However, all is not lost. Some corals have been observed to recover, while others can resist bleaching altogether.

Prompted by this interesting observation, a new study spearheaded by Katie Barott of the University of Pennsylvania has discovered that battle-tested, resilient corals are capable of thriving even when exposed to harsh environments and additional heat stress, presenting new hope that these hardy corals can be transplanted to restore reefs in the future.

“The big thing that we were really interested in here was trying to experimentally test whether you can take a coral that seems to be resistant to climate change and use that as the seed stock to propagate and put out on a different reef that might be degraded,” Barott says. “The cool thing was we didn’t see any differences in their bleaching response after this transplant.”

The team has been pushing forward for coral transplantation, as has been done for the Great Barrier Reef. Coral transplantation involves relocating hardy corals that have previously survived past bleaching events to new reef sites for propagation in affected regions. However, the success of this effort is hinged entirely on coral resilience. They need to continue exhibiting sturdy characteristics while simultaneously overcoming the stress of being relocated to new environments.

For that reason, the researchers decided to investigate the efficacy of coral transplantation using corals from two reefs in Hawaii’s Kane‘ohe Bay. The two reefs differed in flow rate: one being closer to shore with more stagnant waters and the other farther from shore with a higher flow. From each reef, they collected samples from corals that had resisted bleaching during the 2015 bleaching disaster and attempted to regrow them in the other.

After six months of inhabiting a new reef, both groups of corals were then subjected to a simulated bleaching event for several days in the research lab tanks, where they raised the water temperature to induce bleaching. They carefully monitored coral health and the environmental surroundings, gathering data on photosynthetic rates, metabolism, calcification rates, and the health of the symbiotic algae. What they found was that the coral samples not only successfully adapted to their new environments – the new reef and lab tanks – but also sustained their sturdy characteristics.

“What was really novel is that we had this highly replicated experiment,” said Barott, “and we saw no change in the coral’s bleaching response.”

Additionally, it was discovered that corals grown in the outer lagoon had superior reproductive fitness. It was reasoned that these areas had higher water flow rates, which is crucial to allow the removal of wastes and replenish food sources.

“The corals from the ‘happy’ site–the outer lagoon that had higher growth rates prior to the bleaching event–generally seemed a little happier and their fitness was higher,” Barott explained, “That tells us that, if you’re going to have a coral nursery, you should pick a site with good conditions because there seems to be some carryover benefit of spending time at a nicer site even after the corals are out planted to a less ‘happy’ site.”

Nevertheless, the success of coral transplantation can only provide a temporary solution to the looming threat of climate change. If the effects of climate change continue to accelerate, it would only be a matter of time before the adaptive processes of these corals are overtaken by the rapid increase in temperatures.

“I think techniques like this can buy us a little bit of time, but there isn’t a substitute for capping carbon emissions,” she says. “We need global action on climate change because even bleaching-resistant corals aren’t going to survive forever if ocean warming keeps increasing as fast as it is today.” [APBN]

Source: Barott et al. (2021). Coral bleaching response is unaltered following acclimatization to reefs with distinct environmental conditions. Proceedings of the National Academy of Sciences, 118(22).