Coral Reef Genes

In a rare piece of good news for coral reef resilience and sustainability, a team of scientists from Stanford University have confirmed the genetic basis for at least one species of coral, located in American Samoa, to adapt and thrive in an ecosystem characterized by extremely warm temperatures of water.

Corals that build coral reefs are living animals that secrete skeletons of calcium carbonate. One-celled algae called zooxanthellae live inside the coral polyps. Unlike a parasite, the symbiotic algae has a mutually beneficial relationship with the coral.

But excess heat has a deleterious effect on both algae and coral. When the ocean heats up, the algae stops making sugar and the coral flushes its partner out of the polyp. Lacking oxygen and other nutrients, the coral loses color. Such bleaching events, along with ocean acidification, pollution, coastal development, and overfishing, are factors that have led scientists to warn that 25% to 30% of the world’s coral reefs are already severely degraded. And that’s not taking into account the impact of rising sea surface temperatures in the next 50 years.

Thus, the environmental community expressed cautious optimism regarding Stephen Palumbi’s research results. The Stanford professor and director of the Hopkins Marine Station knew he’d found something unusual when he began studying a colony of Acropora hyacinthus corals off the Ofu Island in American Samoa in 2004—in a spot where the coral thrived in shallow pools of water less than five feet deep and temperatures soared over 94 degrees in the summer.

Palumbi and his team zeroed in on the comparative genomics of the coral located in the hot location, compared with snippets of the same species living in cooler waters sometimes only yards away.

The researchers found evidence of what Gary Williams, Academy curator in Invertebrate Zoology and Geology, calls preadaptation. In such cases, a species evolves to use a preexisting structure or trait inherited from an ancestor for a potentially unrelated function. In Ofu, the Stanford team discovered that the ongoing, seasonal exposure to warmer waters activated 60 genes that enable the symbiotic algae to combat heat-related stress. Palumbi’s team also pinpointed high levels of the protein ubiquitin that allow the algae to recycle molecules damaged by hot water.

“That’s interesting that they linked the cause of coral death by bleaching to a function of the algae’s immune system,” Williams said.

However, Williams cautions, “Bleaching does not automatically mean the coral has died.” It depends on how long or severe the bleaching event is. “There is always the potential that the algae can return and reinfect the host. Coral is more resilient than we realize.”

According to Williams, Academy scientists often see examples of this “on just about every expedition we take.” Referring to a 2006 dive in the Palmyra Atoll, 1200 miles south of Hawaii near the equator, Williams describes seeing patches of bleached coral, as well as some healthy and flourishing corals living very close to the surface in warm, brightly-lit water. “Coral bleaching is not usually permanent,” he explains. “The culprit of bleaching is probably an El Niño event that heats up parts of the western Pacific. But when the cooler waters—a.k.a. La Niña—comes back, the algae return and coral reef rebuilds and thrives.”

The larger implication “is that similar adaptations may also occur in other coral species,” says Williams. “In the future, we can hope that other types of coral will have the ability to respond to changing conditions in the same way.”

If not, Williams takes the long view. He pulls out a chart to show there are approximately 5,350 species of corals in the world. The greatest percentage—64%—is octocorals, which thrive in regions from shallow intertidal zones to depths as great as 6,000 meters or more.

“People say we’re losing coral at an unprecedented rate, but they’re only talking about the 750 hermatypic, reef-building species—14% of the total. When we speak of losing coral, we’re only speaking about this subgroup. Even in a worst-case scenario—mass extinctions of a majority of hermatypic corals—other successional, pioneer species will come in to fill the empty niche. Nature has a way of taking care of itself if you solve the underlying problem.”

Barbara Tannenbaum is a science writer working with the Academy's Digital Engagement Studio. Her work has appeared in the New York Times, San Francisco Magazine and many other publications.