For the third time in 5 years, an underwater heat wave has turned vast stretches of coral on Australia’s Great Barrier Reef ghostly white, a desperate survival strategy that is often a prelude to coral death. Now, scientists there have taken a small step toward helping coral survive in a warmer world. For the first time, researchers have grown algae in a lab that can reduce coral bleaching, as it’s known. The results are a notable advance in the growing field of “assisted evolution,” in which scientists are working to alter coral genetics to help them endure hotter water.
It’s a “groundbreaking” study, says Steve Palumbi, an evolutionary biologist at Stanford University who was not involved with the work. But he cautions that the approach is a far cry from something that could be used in the wild.
Coral and their algae are deeply intertwined. The tiny, plantlike organisms live inside the cells of coral polyps, the small, anemonelike single animals that form colonies to create the fantastically shaped skeletons typically called coral. The algae, called zooxanthellae, use coral waste products to help photosynthesize food, while in turn nourishing the coral host.
But that relationship sours during heat waves. Coral polyps eject the algae from their bodies, a phenomenon scientists suspect is a reaction to a flood of tissue-damaging molecules released by the overheated algae. Without their algae, the corals turn white, and, if the bleaching is severe enough, they can starve to death.
Coral geneticist Madeleine van Oppen hoped the algae could be coaxed to evolve into strains that reduced the bleaching response. To do that, van Oppen—a leading promoter of assisted evolution at the University of Melbourne—and colleagues turned to a common coral alga, Cladocopium goreaui. Starting with clones of a single copy of the alga to ensure they were genetically identical, they raised more than 100 generations over 4 years in 31°C water, comparable to a heat wave on the Great Barrier Reef.
Then Patrick Buerger, a postdoctoral researcher at the university, squirted 10 different strains of the hot water algae into separate vials containing coral larvae. The pinhead-size animals absorbed the algae into their cells. He did the same thing to algae raised in more typical 27°C water. The team then stuck the combined larvae and algae into 31°C water for 1 week.
The results were mixed. Some heat-adapted algae that coped with hot water on their own didn’t fare well when paired with coral larvae. The density of algal cells started to fall, a sign of bleaching. Some larvae died. But in three algal strains, the cell density rose by 26%, the researchers report today in Science Advances. “Some of these [algae] can decrease coral thermal bleaching,” van Oppen says. “So that is very exciting.”
There are genetic clues about why some algae stood out. In one breed of the bleach-resistant algae, genes tied to converting carbon into sugars became more active after the hot water exposure, whereas genes related to photosynthesis turned down. It’s possible that the decrease in photosynthesis protected the coral from toxic byproducts called reactive oxygen species that can spike during a heat wave, while the carbon activity helped keep the coral fed, she says.
Although the findings are promising, scientists have to answer a lot of questions before such algae might be used to aid wild coral. It’s not known how these algae will interact with adult coral polyps. Nor is it known how they might compete with wild algae out on a reef.
Another question is whether the heat adaptations come from long-lasting changes to the algal genome, or temporary changes that could fade over several generations in the wild, says Debashish Bhattacharya, an evolutionary genomicist at Rutgers University, New Brunswick. Bhattacharya, who has bred algae for salt tolerance and biofuels, says he suspects that with only hundreds of generations, the changes are likely not permanent. “I don’t think we’re talking about better genetic profiles here.”