Climate change is expected to devastate coral reefs, as warmer oceans are believed to be inhospitable to corals. But corals may be more robust than commonly thought. A number of studies have found coral colonies that endure high water temperatures. Now, a team of scientists has taken a step toward identifying the genetic mechanisms that might be giving some corals a natural resilience to thermal stress.
Coral reef ecologist Daniel Barshis and colleagues at Stanford University in Palo Alto, California, took advantage of markedly different environmental conditions in two nearby but separate pools on a reef at Ofu Island, American Samoa. Because of local factors that isolate some areas of the reef from winds and waves that might mitigate temperature extremes, some pools in the reef are highly variable in temperature, with summertime water temperatures topping 34°C, which, depending on other factors, can trigger bleaching, or a damaging loss of the symbiotic algae that corals depend on. Yet Acropora hyacinthus, a common reef-building coral found in these pools, grows faster and is more thermally tolerant than corals of the same species in nearby pools that do not get as hot. The team took samples of corals from both the highly variable and the moderately variable pools and subjected them to thermal stress experiments under laboratory conditions while monitoring the levels of expression, or activity, of a wide range of genes.
The researchers identified 60 genes with an unusual expression pattern. Under normal temperatures, these genes were more active in the corals from the highly variable pool. But when water temperatures rose, they were more active in the corals from the moderately variable pool. "We're not really sure if the tolerance is a direct result of the activity of these genes or an associated factor," Barshis says. But perhaps the higher gene expression under normal conditions—which the team calls "frontloading"—prepares these resilient corals for periodic hot water, the team reports online today in the Proceedings of the National Academy of Sciences.
Barshis calls the findings preliminary because of the complexity of coral biology. The experiment did not examine the potential contribution of the Symbiodinium, a group of unicellular algae commonly called zooxanthellae, that symbiotically provide the energy and nutrients of photosynthesis to the coral while gaining a home within the coral tissue. Some types of Symbiodinium are associated with greater coral heat resistance. It's also not yet clear if the observed resilience reflects short-term acclimatization to higher water temperatures or adaptation that might be passed on to future generations within a coral population. Ongoing and planned experiments seek to address these questions.
Still, Barshis says, the study "somewhat paints a hopeful picture" for coral reefs. He suspects that, like the Ofu reef, most major reefs harbor high-temperature habitats that may be fostering the diversity needed for corals to survive extreme temperatures. "If we protect reefs from overfishing, anchor damage, coastal development, and other more manageable effects, there is diversity out there that will help [corals] hold on," he says.
"It is a really interesting study," says James Guest, a coral reef ecologist at Nanyang Technological University in Singapore. He says the Stanford team's use of the reef at Ofu Island provides a very valuable model system for comparing the behavior of corals under differing environmental conditions. It is clear that the world will undergo a certain amount of climate change, and a better understanding is needed of how corals might adapt or acclimatize to changing temperatures, he says. "Those are the important studies to do now."