Understanding how corals may resist climate change

Coral reefs cover just one tenth of one percent of the world’s ocean surface, and yet the so-called “rainforests of the sea” represent some of the most diverse ecosystems on our planet. They host some 25% of all marine species, and provide the world’s economy with billions of dollars through tourism and through supporting various commercial fisheries.

Reefs are constructed from calcium carbonate, a secretion of small animals called corals which gather in colonies, building the massive structures that make for some of the world’s most breathtaking underwater vistas. But corals are finicky little creatures and even minute changes in water temperature can wreak havoc on them, resulting in coral reef “bleaching.” They’re the Goldilockses of the shallow oceans, requiring that the water be neither too hot nor too cold, but just right.

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Coral gardens in the U.S. National Park of American Samoa.

At least, that’s the common understanding of coral preferences. But there’s a curious phenomenon in which corals in naturally warm waters can survive levels of heat that would bleach corals of the very same species that inhabit cooler waters. That suggests that corals must somehow acclimate to local conditions, at least in some situations. Indeed, recent research has reveals populations of corals that are resistant even to acidification, which suggests that they must have either immediate physiological mechanisms to protect them from extreme environmental shifts or longer-term evolutionary mechanisms for doing so.

Biologist Stephen R. Palumbi and colleagues of Stanford University wanted to understand what the mechanisms were through which some corals seem able to withstand environments that, according to common knowledge, ought to be fatal. They conducted a very clever experiment at the reefs found in the U.S. National Park of American Samoa on Ofu Island, and they published their findings yesterday in the journal Science.

The “critical bleaching temperature” in those waters is 30°C, or 86°F. But in one pool, water temperatures reach 35°C (95°F) during mid-day low tides. In another pool, the temperature rarely rises above 32°C (89.6°F).

The researchers focused on a single coral species found in both pools, Acropora hyacinthus. They started by seeing how they handled environmental stress. Hunks of coral from both spots were removed and brought inside a lab where they were plunged into a bath that increased from 29°C (just under the critical bleaching point) to 34°C, and they were left at that toasty temperature for three hours. That pattern, the researchers say, imitates the natural variation in temperature during a single tidal cycle in the warmer pool.

One way to see just how well the corals can suffer their warm bath is being seeing how much chlorophyll is retained by symbiotic algae that coexist with the corals on the reef. As expected, the chlorophyll retention after the “experimental heat stress” was 80% for corals from the warmer pool, but only 45% for those from the cooler pool. Those values provided an important baseline reference for the second part of the study.

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Graduate student Rachael Bay tagging corals for growth studies in Ofu lagoon, U.S. National Park of American Samoa.

Palumbi’s team went back to the reefs and transplanted several coral colonies from the warm pool into the cooler one, and vice versa. Once the transplanted corals had made themselves at home, the researchers returned 12, 19, and 27 months later to see how they liked their new digs. They harvested samples from each of the transplanted colonies and subjected them to the same warm bath experiment as in the first part of the study. Would the transplanted corals from the cooler pool acquire the same heat resistance as those native to the warmer pool?

The short answer is: yes, almost. In 22 of 23 experiments, corals transplanted into the warmer pool shower higher chlorophyll retention during the warm bath experiment than did those native to the warmer pool but transplanted into the cooler one. The transplants didn’t retain quite as much as the natives – 67.5% compared to the 80% measured in the baseline experiment – but they still did better than the corals from their home neighborhood.

On the other hand, the warmer pool coral transplants reduced their retention to match their new neighbors in the cooler pool: 47% compared to 45%.

The transplant experiment demonstrates that corals can adapt fairly rapidly to new environmental stressors by adjusting its physiology. But that’s only half the story.

The researchers also looked the way genes were expressed in the corals in either pool. Despite belonging to the same species, the corals differed in 74 different genes according to their “pool of origin,” suggesting that there is a genetic component to their localized temperature adaptability as well. The symbiotic algae, however, were identical. It wasn’t the algae that determined how well the reefs could adapt to temperature changes; it was the corals themselves.

The researchers are guarded in their optimism. On one hand, they write, “these experiments showed that some corals are capable of broad acclimatization to microclimate and developed enhanced resistance to bleaching.” On the other hand, they caution, “we do not yet know how many coral species can acclimate or evolve.” They point out that while several dozen species of coral live in the over-heated pools of Ofu Island, it is unknown whether they are all similarly able to acclimate to warmer waters, or if there is an upper limit on that sort of adaptation. And heat is only one stressor. Add ocean acidification into the mix, and corals might have a harder time responding to environmental change. Still, the data generated from this experiment will provide important insight moving forward for studying how coral reefs might respond to a warming world. – Jason G. Goldman | 25 April 2014

Source: S.R. Palumbi, D.J. Barshis, N. Traylor-Knowles, and R.A. Bay. (2014). Mechanisms of Reef Coral Resistance to Future Climate Change. Science.

Images courtesy Steve Palumbi. Video by GG Films. Used with permission.

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