It begins as a pale spot on an otherwise colorful mass of coral that the average snorkeler might not give a second glance. But to the trained scientific eye, that spot is a lesion that can be a disturbing harbinger of swift and complete destruction, even of healthy corals. Unfortunately biologists working in Florida and Caribbean waters have seen far too much of stony coral tissue loss disease (SCTLD), a persistent and widespread pathogen that is leaving an ever-growing trail of dead, white coral in its wake.
“It’s really devastating,” says Valerie Paul, head scientist at the Smithsonian Marine Station at Fort Pierce, Florida. “It can kill coral that’s hundreds of years old in a month.”
But Paul and colleagues have developed a new weapon against the coral killer: a probiotic cocktail that employs beneficial bacteria to save infected corals and prevent the disease’s spread. Their research, published Thursday in Communications Biology, isolated a strain of bacteria that demonstrated antimicrobial properties in corals that had successfully avoided SCTLD. The team used the strain to create a probiotic, an application of beneficial microbes that, in aquarium tests, stopped or slowed the disease nearly 70 percent of the time. Better yet, during a dozen different attempts to transmit the disease to non-infected coral dosed with the probiotic, the solution prevented SCTLD’s spread every time. The encouraging results create hope that probiotics could not only treat diseased corals but perhaps even lend protection to healthy ones against future SCTLD exposure.
SCTLD has quickly wreaked havoc on Florida’s coral reefs since it was first identified in 2014 near Miami. The mysterious coral killer has quickly spread along the entire 360-mile length of Florida’s Coral Reef, North America’s only barrier reef, and throughout the Caribbean into at least 22 different countries and territories. Last year, it appeared in the Gulf of Mexico, at Flower Garden Banks National Marine Sanctuary.
Colorful coral reefs that fall victim to SCTLD are transformed into dead relics, white calcium carbonate skeletons devoid of the biodiversity for which reefs are prized. Reef-building corals have a symbiotic relationship with algae called zooxanthellae. The corals provide a protected home to the zooxanthellae, which in turn, via photosynthesis, produce oxygen and essential nutrients for the coral polyps. When disease strikes, “the symbiotic algae that lives in the corals starts to kind of fall away,” Paul explains, “and all the tissue itself begins dying and disappearing—it sloughs right off the coral.”
In addition to being hot spots of marine diversity, coral reefs—the world’s largest living structures—also play a major role in protecting coastal settlements from storm damage. By dispersing waves’ energy before they hit shore, reefs dramatically reduce coastal flooding and erosion.
But the world’s reefs are facing extremely tough times. Ocean acidification and warming due to climate change, pollution and various diseases—including SCTLD—have placed many of the world’s reefs in obvious decline. According to some scientists’ estimates, the planet has lost half of its coral reefs since 1950, and the biodiversity associated with reefs has dropped by 60 percent.
Among those many problems, SCTLD is an immediate and fast-moving threat that scientists want to tackle, preferably before it appears on any Indo-Pacific reefs.
Despite extensive study by experts from many different groups, scientists don’t know exactly how the disease spreads. The affliction is waterborne, so direct contact with infected corals isn’t necessary. Because the disease has been observed island-hopping against prevailing currents, it may be that ships are spreading it with ballast water or biofilms that coat their hulls, or that tourists are transporting it via dive gear. Further frustrating efforts to pinpoint the disease’s exact cause is coral reefs’ complicated composition: a mix of coexisting algae and microbes amid ocean waters that feature their own complex cocktail of microbial life. “Imagine if we just went through the Covid pandemic and didn’t know what the causative agent was,” Paul says. “That’s still sort of where we’re at with this disease.”
One strong piece of evidence suggests bacteria are at least involved in the disease. The only previous treatment found effective against SCTLD is the application of antibiotics. Drugs like amoxicillin, when applied as a paste, are successful at slowing and stopping the spread of lesions on diseased coral. So bacteria play some role in the disease, though they may not be the primary culprit in its deadly effects; they could be a secondary infection that takes advantage of weakened corals.
While coral reefs house some of nature’s most visually appealing ecosystems, teeming with fish, shellfish and sponges, they also host a less obvious community of organisms: the microbiome. Microscopic bacteria, fungi and other organisms live on corals and, as in humans, some of them can be helpful in fighting off pathogens.
During aquarium experiments on the disease’s transmission, Paul, microbiologist Blake Ushijima at the University of North Carolina Wilmington and colleagues found that some healthy coral colonies didn’t get the disease when they were placed in the same aquarium or even put in contact with diseased corals. They sought to find out what natural defenses made these corals resistant, in the hope that they could fuel an effective treatment that might be applied to others.
Previous studies of infected corals identified three strains of harmful bacteria associated with SCTLD. With those in hand, the team then studied the bacterial strains, 222 in all, found with corals that are resistant to disease to see if any of them showed promise at acting against the pathogenic strains. In all, 83 showed some level of antimicrobial activity, but one was a standout at resisting bacteria associated with SCTLD—a strain named McH1-7. Scientists then put that promising strain to chemical and genetic analysis to find out which compounds made it effective against harmful bacteria and which genes produced those compounds.
Finally, they tested a probiotic application of McH1-7 on 22 different live samples of great star coral (Montastraea cavernosa) and found that it stopped or slowed the disease nearly three quarters of the time. And during every attempt to transmit the disease to non-infected coral treated with McH1-7, the probiotic prevented the spread of disease—a result unmatched by even the effective antibiotics. Amoxicillin treatments also run the risk of producing antibiotic-resistant bacteria, but that isn’t a concern with probiotics.
Still, researchers have to overcome some hurdles for probiotics to become a widespread weapon in efforts to protect coral reefs.
So far, treatments seem to work best on great star coral and a few other species—but on at least one other coral species, the solution doesn’t seem to have much impact. That may be because the strain’s effectiveness is species specific. If so, the work may be expanded to find new probiotics that could help all two dozen or so types of hard corals that are impacted by SCTLD.
Dosing corals with probiotics in the field is also a challenge. Scientists have worked to create a kind of underwater aquarium by surrounding a colony with a large sheet of plastic and weighing down the edges, creating a bubble into which probiotics are infused. But the process is labor-intensive.
“I think these probiotics, along with most of our tools, are great at the individual coral level,” says Jennifer Moore, the protected coral recovery coordinator at the National Oceanic and Atmospheric Administration. “The success that they are having is really exciting, and it’s very promising. But scaling that to the scope that we’re actually dealing with—300 nautical miles of reef tract with millions and millions of corals on it that are being impacted by this disease in Florida alone—that is the real challenge. Our tools aren’t quite ready to meet that scale.”
But scientists won’t let these challenges stop them. Because these probiotics have also been found to protect healthy corals, the Florida Fish and Wildlife Conservation Commission’s Lisa Gregg sees them as a potential valuable tool for ongoing coral restoration efforts.
Since 2018, the Florida Coral Rescue and Propagation Team, which Gregg co-leads, has rescued healthy corals from the reef and created a kind of coral gene bank for future restoration. The project oversees 2,500 corals, from 20 different species, being held in 25 holding facilities across 13 different states. It safeguards the corals and their genetic diversity so they can be used to breed future generations that will restore Florida’s Coral Reef. “How will we protect the offspring when we’re putting them back out in the water?” says Gregg. “Inoculation with a probiotic bath could be one of those options to provide some added layer of protection.”
Paul emphasizes that her group’s probiotic work has been one part of a huge collaborative effort, alongside projects like Gregg’s, to explore antibiotics, raise corals in captivity and test other treatments in hope of finding as many tools as possible to fight the deadly disease. “It really has been all hands on deck, and people have really worked closely together,” she says. “Everybody would just love to see this thing gone.”