Diving Deep to Save Coral Reefs

An interdisciplinary Northwestern research team developed the first global index of susceptibility to coral bleaching.

Luisa Marcelino had just started a postdoctoral program in molecular biology at the Massachusetts Institute of Technology when she started scuba diving in the waters off of New England. After she went on a dive in the Bahamas to collect species for the New England Aquarium, “I was sold,” says Marcelino, a native of Portugal. “The underwater life, it’s so rich. There’s so much to see. It feels like you go onto a different planet.”

Marcelino, a research assistant professor of environmental microbiology, has been diving ever since, and she’s concerned about the health of one of the world’s primary underwater ecosystems — coral reefs, which are at severe risk due to climate change.

Reef-building corals have a symbiotic relationship with photosynthetic algae, called zooxanthellae, which live in their tissues. “The corals are like greenhouses growing their algae,” Marcelino says. The coral provides the algae with protection, while the algae provides the coral — a living animal — with oxygen and food.

Rising water temperatures and ocean acidification, two results of climate change, are taking a toll on coral reefs, Marcelino says. When ocean temperatures rise, even as little as 2 degrees Fahrenheit, the coral expels the colorful algae, leaving their stark-white skeletons exposed. Some corals bounce back; many don’t.

Up to 93 percent of Australia’s Great Barrier Reef is bleaching in what Marcelino says is “quite probably the worst mass bleaching ever. We’re seeing this year that the mortality is huge, as in 50 to 100 percent. It’s been so hot for so extraordinarily long. [Thanks in part to El Niño], this has been the longest warm-water episode ever.”

Through a series of studies, Marcelino, Timothy Swain, her postdoctoral fellow, and Vadim Backman, the Walter Dill Scott Professor of Biomedical Engineering, have been quantifying the impacts of global climate change on coral reefs to see why different corals bleach differently. They developed the first “global index” detailing which corals are most susceptible to coral bleaching and most likely to die. Using a meta-analysis of all available historical records on coral bleaching from 1982 through 2006, the coral bleaching response index can be used to compare the bleaching responses of corals throughout the world and predict which corals may be most affected by future bleaching events. The index provides a standardized measure of vulnerability, by species of coral, to thermal stress.

“If we have a way of ranking the corals based on how susceptible they are to bleaching, we may be better equipped to conserve them,” Marcelino says.

The team’s most recent study explored the susceptibility of the algae to thermal stress with another future study in the works that puts together the research on thermal stress on both partners in the process of reef building.

Backman and Marcelino collaborated on earlier research that used his cancer detection tools to look at the optical properties of the coral skeletons. They found that corals that scatter light quickly to their algae —to get more food under the typically low-light conditions underwater  — are at higher risk of bleaching.

Not only are coral reefs beautiful, says Marcelino, but they also slow down coastal erosion, serve as a sink for carbon dioxide (helping to regulate global climate) and host incredible biodiversity. Because coral reefs support up to a quarter of all fish species, the livelihoods of
500 million people and income worth more than
$30 billion are at risk from coral bleaching.

“People ask me, ‘What does it matter to us here in Chicago?’ ” Marcelino says. “It’s not so much that the coral reefs are beautiful, and they are, of course, but it’s that the whole ecosystem is changing too, and we cannot perform all the services the original ecosystem can,  no matter how good we are at engineering complex systems.

“Corals have been around for something like 50 million years,” she adds. “We can’t do the same job. We can’t substitute a lot of these services that these ecosystems provide. I always tell my students in public health that the first anti-retroviral drug for HIV came out of a sponge from the Caribbean. We should still be tapping into and exploring the potential of coral reefs.”