As climate change makes the ocean hotter, marine heat waves are causing coral bleaching—when a healthy, colorful reef suddenly expels algae. The reef turns white, and tiny coral polyps, which had been living symbiotically with the algae, risk disease and starvation. If the stress continues too long, the corals die.

Probiotics designed for coral reefs might be able to help. In a new study, scientists simulated a heat wave in aquariums, pushing the temperature of the water up to 86 degrees for 10 days. In some of the tanks, fragments of coral reefs were dosed with a mix of beneficial bacteria. Others were sprayed with a placebo dose of saline. All of the corals that got the probiotics survived. Forty percent of the corals in the other group died.

“We are using native bacteria that are very abundant in the reef,” says Raquel Peixoto, a marine science professor at Saudi Arabia’s King Abdullah University of Science and Technology and one of the authors of the study. Climate change can disrupt that balance. She compares the process of rebuilding the ecosystem of microorganisms to reforestation. “We are trying to reboot this microbiome by replacing these bacteria,” she says.

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