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When Victoria Glynn came to Panama to study the effects of extreme ocean temperatures on coral reefs at the Smithsonian Tropical Research Institute (STRI) as a pre-doctoral fellow in professor Rowan Barrett's lab at McGill University, she drew corals to explain her work to kids. Now, her illustrations help broader audiences reach an "Ah ha!" moment as she explains how corals from more variable ocean environments may be better equipped to survive rising ocean temperatures than corals from more stable environments—in a paper in Current Biology.

Ask someone to draw a coral and they might draw a lump or a deer-antler shape, maybe with some fish or shells to illustrate its sea-floor setting. But Victoria's drawings are much more intricate because corals consist of the coral animal and its skeleton; the symbiotic algae, for energy capture; and a host of tiny bacteria, its microbiome—like we have in our guts, responsible for a lot of other functions. Scientists call this the coral holobiont, from the Greek for 'the whole living thing."

"Most people know that our gut microbiome plays a major role in our health, depending on our diet and the microbes we have. In many ways, corals are not so different," Victoria, now a post-doctoral associate at the University of Vermont, explains.

"Their survival is intricately tied to their microbiomes. When I explain how corals stay healthy as their environment changes, I hope my drawings help people see just how complex they really are, and why it's crucial to consider all the organisms involved: the coral animal, their symbiotic algae and the bacterial microbiome."

To survive, coral and their algae maintain a tight relationship, but when ocean water gets too hot, the algae often jump ship, leaving just the white coral skeleton behind, a phenomenon called coral bleaching.

Victoria did her doctoral work in Panama as part of the Rohr Reef Resilience Project led by Sean Connolly, STRI staff scientist. STRI's location gives researchers easy access to the Tropical Eastern Pacific, an area of ocean extending from Ecuador's Galapagos Islands north to Costa Rica's Cocos Islands. This is a perfect natural laboratory for learning how corals respond to temperature extremes.

Project scientists take advantage of the frigid ocean currents that come to the surface in the Gulf of Panama to ask if corals growing there are more resilient to temperature extremes than corals in other places where temperatures are not so extreme, and why.

In this study, they asked three big questions: How do high ocean temperatures affect the relationship between the coral animal and its algal partner? And what about its bacterial microbiome? And do these relationships explain how some corals are better able to survive at high temperatures?

The group sampled cauliflower corals (Pocillopora spp.) in the Gulf of Panama (where there are yearly temperature fluctuations) and in the Gulf of Chiriquí (nearby but with more stable year-round temperatures) and then ran an experiment to see what happens when they turn up the heat.

"We exposed corals to rapid heat stress in tanks on the yacht and, as the temperature climbed, we took samples so we could extract the DNA of the corals, their algae, and bacteria," said Victoria. "This way, we gained insights into the relationships between the corals and the different members of their microbiome as the temperature rose."

• The corals themselves: Genetically, the corals from the two sites were similar, indicating that they must disperse easily along the coast, mixing different populations; but the few genetic differences between corals at the two sites may be important. The authors think that there could be differential selection on genes previously associated with the ability to resist thermal stress, with more ability to resist stress in the corals from the more variable Gulf of Panama.

• The algae: The dynamics for the algae surprised them. In earlier experiments, at high temperatures, corals shifted to a different genus of algae that was more heat tolerant, but in this experiment, some corals kept their original algal partner.

• The bacteria: The bacterial microbiome from corals at both sites was disrupted by higher temperatures, rapidly entering a disease-like state. But compared to previous studies on Australia's Great Barrier Reef, the corals from the Gulf of Panama had less stable microbiomes at high temperatures.

The Australian experiments lasted longer—from weeks to months—and corals experienced temperatures ~4–5°C above their mean monthly maximum temperature, which can be thought of as the average hottest temperatures experienced.

In the Panama experiment, which were less than 24 hours long, corals were exposed 10.5°C above their mean monthly maximum temperatures, and the microbes associated with the corals changed to a more diseased state at around 7.5°C above the hottest average temperatures.

To disrupt the relationship between corals and their bacteria, it took higher temperatures that the temperatures it took to stress out the coral animal itself, suggesting that for the Pocillopora corals in Panama, it's more likely that a coral will die at high temperatures even before its microbiome is severely affected.

At the highest temperatures, the corals collected from the Gulf of Panama, where temperatures are more variable, handled the heat better. But corals from the stable-temperature environment struggled when they were heated.

The team's findings support the idea that the Tropical Eastern Pacific's naturally variable environments may contribute to these corals' enhanced ability to withstand heat. This may explain why after the catastrophic 1982 El Niño Southern Oscillation event.

"Coral reefs cover just 0.1% of Earth's surface but they support around 25% of all marine life. Reefs also provide critical services to more than a billion people globally, through fisheries, tourism, coastal protection, and cultural significance. As ocean temperatures continue to rise, coral reefs are increasingly under threat," said Victoria.

"Understanding what makes some corals more resilient to warming oceans will be essential for guiding conservation efforts, protecting coastal communities, and safeguarding biodiversity. When we think about these complex organisms, we need to get away from two-partner thinking and view them as an integrated whole," said Victoria.

"My artwork helps me do that, and also lets me share my love for the beauty of nature, and my passion for conserving the underwater world."