Probiotics are a popular dietary supplement, promising to boost people’s health by populating their guts with healthy microbes.
People trying to help coral endure heatwaves might soon be reaching for something similar. Infusions of a special probiotic cocktail helped coral stay healthier as they stewed in overheated water in the Red Sea near Saudi Arabia, scientists reported last week in Cell Reports. Even a mix of dead microbes, a so-called postbiotic, proved beneficial.
While scientists have seen promising lab results that such microbial tweaks might help ailing corals, this is the first report from real world conditions: wild coral in the open ocean during a real underwater heatwave.
“When an innovative idea like this actually works—particularly under field conditions—we are positively surprised,” said Erika Santoro, a postdoctoral researcher at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia. “The most exciting takeaway is that this study opens a new avenue for coral microbial therapies.”
People might picture coral as rigid, brightly colored underwater sculptures. But they are so much more. This “skeleton” is just the mineral home to colonies of thousands of tiny coral polyps, identical clones resembling little sea anemones. Then there are the symbiotic algae that live inside the polyps, providing much of the coral’s energy through photosynthesis. Finally, there’s a stew of microbes living in an around the coral.
Scientists are experimenting with each of these kinds of organisms in a desperate bid to help coral endure the underwater heatwaves proliferating across the world. Without help, most corals are expected to be killed by the end of the century, as the oceans absorb much of the excess heat generated by global warming. That would lay waste to one of the most productive and diverse marine ecosystems on Earth.
Researchers are altering coral genetics in hopes of boosting heat tolerance by measures such as crossbreeding coral from different parts of the world. They are working to find and breed strains of algae that will do the same, in some cases accelerating evolution in overheated laboratory waters. They are also studying the microbial communities surrounding coral, in search of mixtures that might help coral stay healthy.
In this new research, Santoro and colleagues tested two different microbial mixtures collected from Red Sea corals and identified as likely candidates based on their genetic profiles and performance in earlier experiments. In 2022, during a prolonged underwater heatwave in the Red Sea that pushed temperatures to nearly 32° Celsius (90° Fahrenheit), the scientists dosed colonies of spiky Acropora valida in a reef near KAUST with one of five mixtures. Two got different probiotics. Two received dead “postbiotic” combinations of the same blends. The last got a salt water placebo with no microbes.
Over 15 days, the scientists watched for changes in color that indicated the coral might be ejecting their algal companions, a response to heat stress. They also measured how efficiently the algae were using sunlight for photosynthesis.
The results were encouraging. While they saw little difference in color changes, coral treated with the placebo and one postbiotic experienced a big drop in their photosynthetic performance. By contrast, corals that got either probiotic or one of the postbiotics showed no decline in photosynthesis over the 15 days.
A survey of the microbial population around the corals at the end of the experiment showed that the communities in the healthier corals looked different than in the sicklier ones. That difference was also reflected in chemistry tied to their metabolic activity.
The success of one of the postbiotics was “particularly exciting,” said Santoro. That’s because dead microbes could be more practical to use on a large scale, because they can be stored more easily since they aren’t alive.
A single 15-day experiment is hardly evidence of a silver bullet. There likely won’t be a single measure that gives coral the key to surviving, and no one is talking of measures that could rescue coral if climate change continues on its current trajectory.
Take this as a glimmer of good news amid an ocean of bad news for coral.
Santoro, et. al. “Probiotic and postbiotic treatment improves coral health and promotes specific metabolic and microbiome changes in situ during a heatwave.” Cell Reports. Sept. 3, 2026.
Image courtesy of Coral Probiotics Village
