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Warming seas threaten to turn coral reefs from kaleidoscopes of color into bleached fields of rubble. To stop this degradation, some scientists are exploring a surprising salve: probiotics.

Dosing corals with a mix of beneficial bacteria staved off death in a heat wave simulated in an aquarium, researchers report August 13 in Science Advances. In comparison, nearly half of corals given a benign saline solution instead did not survive those same conditions. The research offers a proof of concept that probiotics could help some corals survive heat stress.

“The results are incredibly promising,” says Blake Ushijima, a microbiologist at the University of North Carolina Wilmington who wasn’t involved in the research. The work lends legitimacy to using probiotics as coral medicine, he says, “but we’re just scratching the surface. We don’t understand how a lot of these beneficial microbes work.”

Corals are not singular entities, but coalitions of cooperative players. Center stage are the photosynthetic algae that harness the power of the sun, providing energy to their animal host, the coral polyp. Scores of bacteria live in the coral too, many supporting their host by cycling nutrients or fighting pathogens. Collectively referred to as the coral “holobiont,” corals and their microbial partners form the bedrock of one of the most biodiverse ecosystems on the planet.

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Worsening marine heat waves are testing the integrity of healthy holobionts (SN: 4/10/18). Under heat stress, corals’ algae spew toxic chemicals, prompting polyps to kick them out. This process, known as bleaching, can kill corals (SN: 10/18/16). For example, bleaching from a 2016 heat wave wiped out 29 percent of shallow water corals in the northern Great Barrier Reef. Bacterial communities shift under heat stress too, disrupting the benefits some bacteria provide.

“Overall, we see a breakdown of symbiotic relationships, and all the microorganisms start to struggle,” says Raquel Peixoto, a marine ecologist at King Abdullah University of Science and Technology in Thuwal, Saudi Arabia. She and her colleagues previously showed that treating corals with carefully concocted probiotic cocktails could mitigate coral bleaching in lab experiments. That’s good, she says, “but we wanted to find out if we can protect them against mortality.”

Raquel Peixoto scuba diving and collecting corals
Marine ecologist Raquel Peixoto collects microbes from wild corals in the Red Sea. Her research focuses on treating heat-stressed corals with probiotics.Helena Villela

In the most recent setup, the researchers simulated a marine heat wave across 10 aquaria, each with four fragments of Mussismilia hispida stony corals, turning up the temperature to 30° Celsius for 10 days before returning it to 26°C. Half the corals were squirted with six bacterial strains from M. hispida every three days during the heat wave and every five days afterward, while the other half got a saline treatment. Over the course of 75 days, Peixoto and her colleagues measured coral health and changes in the holobiont’s metabolic activity, along with which genes got turned on and off.

Corals in both groups bleached, but the probiotic treatment ultimately worked. While 40 percent of saline-treated corals succumbed to the heat, all corals bathed in bacteria survived. “That was surprising and super exciting,” Peixoto says. The probiotic seems to aid the recovery of corals by inducing genetic and metabolic changes in the host that are associated with tamping down inflammation and allowing damaged cells to rebuild themselves, the researchers found.

“Climate change is affecting corals faster than they can adapt,” but their microbial partners can respond to changes more quickly, says Kimberly Ritchie, a marine biologist at University of South Carolina Beaufort who wasn’t involved in the research.  Such changes, which probiotic treatments could induce, might “buy corals more time,” she says.

Peixoto and her colleagues plan to move beyond the aquarium, starting experiments in mid-August in to see whether probiotics can help wild corals. But some scientists are skeptical of the added bacteria’s ultimate utility, especially for large reefs that boast hundreds of coral species. “Probiotics are sexy right now,” says Ty Roach, a molecular ecologist at the Hawaii Institute of Marine Biology in Kaneohe. “I can imagine scenarios where this would be a great approach… but I don’t think they’re going to save the reefs.”

For example, applying probiotics to large reefs that boast hundreds of coral species seems logistically challenging, Roach says. And there could be unintended consequences. “What’s good for one coral may not be good for other corals or organisms,” Roach says. “For ecosystems as complex as coral reefs, I can’t see doing this on a large scale without having some unforeseen, potentially harmful, effects.”

Peixoto says that the probiotics used here are carefully screened and no strains known to be harmful to life are used. Overall, probiotics “aren’t going to be a silver bullet,” she says. The only thing that will save the reefs is mitigating carbon dioxide emissions to reduce global warming. “But we still need restoration and rehabilitation to cope with the reality that we have now,” she says, and probiotics show promise.

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