Scientists identify first-ever probiotic treatment for a devastating coral disease
Stony coral tissue loss disease has hammered Caribbean reefs since 2014. Beneficial microbes may one day help corals fight back.
A bunch of bright yellow bacteria may prove to be an ally in the battle against a catastrophic marine plague, according to a new study in Communications Biology.
First identified in the waters off Miami in 2014, stony coral tissue loss disease has since spread across the Caribbean, likely hitching rides from port to port in ships’ ballast water. An indiscriminate killer, it attacks roughly half of the region’s 40-odd reef-building corals. Mortality in some species has reached as high as 95%. One of the worst affected species, the pillar coral — Dendrogyra cylindricus — is now listed as critically endangered by the International Union for the Conservation of Nature.
Scientists still don’t know what pathogen causes stony coral tissue loss, hampering efforts to treat the disease or slow its spread. By the time its characteristic white lesions appear, the original culprit has been lost in a swarm of opportunistic microbes that dine on the coral’s weakened and dying tissue. Faced with hundreds, even thousands of types of bacteria, viruses and fungi, researchers are left searching for a needle in a microbial haystack.
Bacteria are a likely component of the disease, for the simple reason that antibiotics effectively heal most lesions. By now, divers in Florida have successfully treated over 20,000 coral colonies with a paste of amoxicillin and marine epoxy. The antibiotic can, however, only stop the spread of existing lesions, not prevent new ones from forming elsewhere on the coral. The work is costly and time-consuming, and poses the risk of fostering antibiotic-resistant bacteria — a particular concern given that amoxicillin is used in human medicine as well.
Thanks to a stroke of scientific luck, reef advocates may soon have a better weapon in the fight against stony coral tissue loss. In his lab at the University of North Carolina, microbiologist Blake Ushijima had been studying how the disease passes from sick to healthy great star corals, Montastraea cavernosa, when he noticed something interesting: certain colonies failed to fall ill.
Curious if his subjects’ microbiomes might have something to do with their robust good health, Ushijima and his team took samples of coral mucus and smeared them on nutrient-filled petri dishes. Out of what Ushijima guesses were thousands of different kinds of bacteria tested, he and his colleagues identified 222 strains with some ability to inhibit the growth of their neighbors.
The team further winnowed down this selection by challenging the microbes with three different types of pathogenic bacteria, all found in the lesions of stony coral tissue loss. The winner was a smooth, round, yellow colony of bacteria, with an empty ‘zone of inhibition’ around it — a clear sign it was excreting antibacterial compounds to fend off competitors. Identifying these bacteria, a strain of Pseudoalteromonas called McH1–7, “did require some luck,” Ushijima says. “You end up with stacks and stacks of petri dishes all over the place.”
The team isolated the yellow microbes, cultured them, and applied them to great star corals infected with stony coral tissue loss. Remarkably, in 70% of the colonies, the microbes slowed or stopped the spread of the disease. In a follow-up experiment, the team dosed healthy corals with the beneficial microbes, and then exposed them to sick colonies. “None of the fragments got infected if we treated it with probiotics,” Ushijima says, calling it an “amazing result.”
Valerie Paul, the study’s other lead author and the director of the Smithsonian Marine Station in Fort Pierce, Florida, cautions that McH1–7 is far from a cure-all. In follow-up experiments, the bacteria helped a different species of star coral avoid getting sick, but not brain corals. Which species of coral McH1–7 will benefit is just one of many big questions still to be answered, Paul says. “If you treat the corals, how long do the effects of the probiotics last? [Do] you have to keep treating them repeatedly?” Unlike humans, who can get our probiotics easily by downing pills or yogurt, corals live in the open ocean, where currents can wash away treatments. An aquatic environment also limits scientists to one-hour dives. “Doing anything with corals is a thousand times harder,” Ushijima says.
Raquel Peixoto, a coral microbiologist at King Abdullah University of Science and Technology in Thuwal, Saudi Arabia, praised the new research for finding an alternative to antibiotics, and for showing that probiotics can be effective for “different species in different areas of the globe, and against different types of threats.” Probiotics are safe, she says, because they use bacteria that are naturally present on the coral, rather than introducing something new. Her own research has shown that beneficial bacteria can help corals survive oil spills, and can shield them from the deadly effects of rising water temperatures.
In the coral-filled waters outside of the KAUST research center, Peixoto and her colleagues are conducting pilot tests to determine how probiotics work “in the real world,” she says. They’re experimenting with different ways of delivering the treatment — an underwater irrigation system is one promising option — and are also working to determine the minimum effective amount of probiotics, and whether the bacteria spread into the surrounding environment. “I don’t think probiotics will save the world, or even the reef,” Peixoto says. They are “one in the bricks in the wall” of coral reef protection, which also requires mitigating local stressors and cutting carbon emissions.
Erinn Muller, who manages the coral health and disease program at Mote Marine Laboratory in Sarasota, Florida, agrees that probiotics such as McH1–7 are just one of many tools scientists need to restore reef habitats. The utility of probiotics is limited due to the fact that beneficial bacteria seem to be species-specific, she says, and because scientists can currently only culture “a very small percentage” of bacteria found in the ocean. While the research “is still kind of in its infancy,” Muller anticipates that it will advance “quite quickly within the next few years.” Muller was not involved with this study, but collaborates with Ushijima and Paul on other coral microbiome research projects.
Coral probiotics could bring unexpected benefits for human health as well. Although the yellow bacteria in Paul and Ushijima’s study excrete a known antibiotic, korormicin, future research might identify novel therapeutics, potentially leading to new treatments for bacterial infections in humans as well. Ushijima is working with a chemist at the University of North Carolina to search for these compounds, while Mote Marine Laboratory recently launched its own program to hunt for promising compounds in marine microbes.
With stony coral tissue loss showing no signs of abating, help for the Caribbean’s reefs can’t come soon enough. Ushijima and Blake are now working to identify additional bacteria that can combat the disease, with the hopes of someday concocting a probiotic cocktail that can treat corals in the wild.
In the meantime, stony coral tissue loss has spread far and wide, knocking out reefs and the many benefits they provide, from habitat for fish to coastal protection against increasingly fierce storms. In April, the disease was sighted for potentially the first time in Curaçao, an island only 40 miles north of the South American coast. “I’ve been working on corals for, gosh, 40 years almost,” Paul says. “This is one of the worst things I’ve ever seen.”
