Application of environmental genomics to coral conservation

The oceans are a key factor in the health of the planet, absorbing over 90% of the solar energy. They demonstrate the extent to which rising temperatures threaten coral reefs and other vital ecosystems that support biodiversity. In 2016 and 2017, a sharp rise in surface temperatures in the Pacific Ocean caused mass bleaching on an unprecedented scale. Australia’s Great Barrier Reef has been particularly hard hit.

Bleaching occurs when heat stress disrupts the symbiotic relationship between corals and the tiny algae that live within them, providing a source of nutrients for the corals and giving them their color. Persistent bleaching can lead to coral death. Over the past two decades, anomalous heat waves have caused entire sections of coral reefs off the Australian coast, several kilometers long, to turn white.

Scientists have already found that some coral reefs are better equipped to cope with recurring heat stress than others. For his thesis research, Oliver Selmoni, PhD assistant at the Laboratory of Geographic Information Systems (LASIG) of EPFL, applied the principles of environmental genomics to characterize this adaptability. Selmoni cross-referenced the results of genetic analyzes of coral samples with ocean temperature data captured by satellites to determine what made some corals more able to withstand rising temperatures.

Building a studio from scratch

After applying his method to pre-existing data on a coral species in Japan, Selmoni traveled to New Caledonia to build a new study from scratch. He collected his coral samples with the help of IRD scientists based in Nouméa. The results were published in Nature Scientific reports on November 12. “New Caledonia is home to the second longest coral reef in the world, extending to over 1,000km,” says Selmoni. “This relatively compact ecosystem is exposed to dramatic contrasts in environmental conditions, making it an ideal candidate for studying climate adaptation.”

The study aimed to test two hypotheses. The first is that coral populations learn to adapt to warmer seas after experiencing prolonged heat stress for many years. “The longer the higher temperatures persist, the more likely it is that climate resilience characteristics will develop and be transmitted from generation to generation,” says Selmoni. The second hypothesis concerns connectivity: corals reproduce by releasing larvae into the water, which are then transported into ocean currents. “Corals rely on nearby populations to survive. When a coral reef is destroyed by environmental stressors or human activity, larvae from elsewhere are needed to initiate restocking,” he adds.

Creation of marine protected areas

Selmoni’s first task was to assess the composition of the marine environment in New Caledonia, using satellite data dating back 30 years. After selecting 20 sites with the greatest temperature contrasts, he headed to the field to collect samples. “We focused on three flagship Pacific coral species that are susceptible to bleaching and relatively easy to find,” he recalls. “It was a huge undertaking: 3,000 km by road and another 1,000 km by boat!” Selmoni shared details of his experience on the EPFL Out There blog.

Using environmental genomics methods at LASIG, he found that field observations support his hypotheses of connectivity and adaptation. “As expected, we observed a correlation between the likelihood of adaptation and prolonged exposure to high heat stress. In contrast, corals in locations that had never experienced heat stress did not show traits of climate adaptation,” Selmoni explains. .

Looking ahead, the maps developed in the study could be used to establish new marine protected areas (MPAs) – areas where fishing, tourism, industry and other human activities are limited – in places where, through connectivity, heat-resistant coral strains could populate. cliffs around the archipelago. Another option would be to select and grow climate-adaptive corals, then transplant them to nearby coral reefs that are less able to withstand rising temperatures, thereby accelerating the natural selection process. “Over time, these more resistant strains can help rebuild damaged coral reefs or make existing coral populations more resistant to bleaching,” adds Selmoni.