Coral reefs create underwater ecosystems that are not only beautiful but also play a fundamental role in sustaining our planet’s health. They provide sustenance for millions of people who rely on them for food, livelihoods, and culture. But coral reefs face an unprecedented confluence of perils: rising sea temperatures, ocean acidification, land and marine pollution, and overfishing.
A healthy brain coral. Coral reefs across the world are facing multiple stressors, including ocean acidification, pollution, overfishing, and a tissue loss disease. Photo by Nick Hobgood.
As the planet continues to warm, the future of these underwater sanctuaries becomes more uncertain. But researchers now say they may have found a promising way to take some of the strain off, by treating a disease that is devastating to many types of coral
Stony coral tissue loss disease, or SCTLD, was first discovered in Florida in 2014 and has been devastating reefs in Florida and the Caribbean since then.
“Since SCTLD arrived, it’s been killing coral, and it hasn’t gone away,” says Valerie Paul, head scientist at the Smithsonian Marine Station at Fort Pierce, Florida, who has worked with coral for four decades. “It’s continued to spread, with people getting more and more concerned about the extent to which this disease is killing coral. It’s pretty scary.”
SCTLD is an underwater pandemic amid coral reefs. It has a high mortality rate, with the capability of wiping
SCTLD attacks hard corals, infecting tissue and creating small lesions that then spread across the coral. The disease drains the vibrant hues from the coral and leaves behind a canvas of desolate white. The problem with SCTLD is that its origin remains unknown, so finding an effective treatment is complex.
A brain coral afflicted by stony coral tissue loss disease (SCTLD) in Looe Key, Florida. Photo courtesy of FWC Fish and Wildlife Research Institute.
Close up of the sick coral. You can see the disease moving across the coral, leaving a white band of recently dead skeleton in contrast to the healthy, yellow/brown tissue. Photo courtesy of FWC Fish and Wildlife Research Institute.
“We know it can spread from coral to coral through direct contact and in the water column,” Paul says. “So something is being transmitted, but we just don’t know what it is. After all this time, we don’t know if it’s bacterial or viral or a combination of both.”
Viruses have been hypothesized to play a role in the disease, and scientists are starting to learn more about the role of viruses in coral health. For example, a recent study found evidence that ocean warming may create more intense viral outbreaks within corals. “If viruses are involved in coral disease or bleaching, we need to understand this so that we can develop measures to counter these infections,” says Lauren Howe-Kerr, a postdoctoral fellow at the Minderoo Foundation in Perth, Australia.
Scientists are also aware that pathogenic bacteria play a role in the disease, which is typically treated with the antibiotic amoxicillin. But this has proved only somewhat effective. The treatment works by mixing the antibiotic into a paste and then smearing it on the coral at the lesion, which allows for the slow release of medicine. This stops the disease in around 75 percent of cases. The problem, however, is that it’s not a permanent solution, with corals becoming reinfected and the disease showing up in other parts of the colony.
“And then there’s the additional concern with antibiotic resistance development, especially if you have to go and repeatedly treat the coral,” Paul says.
Paul and her team of researchers have been working to find a treatment for SCTLD since 2017. After years of research, they have discovered the first effective bacterial probiotic for treating and preventing SCTLD.
They tested 222 bacterial strains from healthy star coral fragments, investigating which strains might have anti-bacterial, disease-resistant properties. Out of the 222 healthy strains tested, 83 had promising results, with one in particular standing out: Pseudoalteromonas sp. strain McH1-7.
“When we were looking at some of the isolates from this coral, trying to culture some bacteria, we saw that it had a lot of bacteria associated with it making antimicrobial compounds,” Paul says. “So it seemed like perhaps these bacteria, the natural microbiome of this coral, were helping protect it from disease.”
The researchers also found that if they pretreated the corals with the McH1-7 probiotic for two days and then exposed them to the disease, none became infected. This study showcases the possibilities of manipulating coral microbiomes for coral health.
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“Efforts such as these to boost the coral’s disease-fighting toolkit are crucial,” Howe-Kerr says. “SCTLD may be a complex disease, with multiple factors and/or pathogens involved, and the more we learn about this deadly disease, the more ways we can hopefully add to this toolkit,”
The majority of their testing took place in tanks, so its effectiveness in oceans remains to be seen. “It gets a little trickier when applying it to the ocean,” Paul says. “The application just isn’t as efficient.”
To test treatments in ocean coral, Paul and her team put a weighted plastic bag over the coral to create a mini aquarium. They then injected the healthy bacteria, leaving it for around two hours to allow for the microorganisms to be absorbed, before removing the bag.
“It’s hard to treat a large number of corals in this way, but of the corals we have been targeting and following, we can see beneficial effects,” Paul says. “We’d really like to take this to the next step to see if we could find a better way of deploying the probiotics in the field.”
With little funding to advance this research, understanding of the treatment remains limited to this study. To scale up would require better deployment methods and collaboration with teams across disciplines, Paul says, including bioengineering. Further use of slow-release gels or polymers could be created, for example, that could be deployed to gradually release the probiotics around the coral. Such methods have yet to be proven, however.
“We’ve definitely demonstrated that you can manipulate the microbiomes and make the corals more resistant,” Paul says. “But whether we can extend that to be an effective field treatment is still the big question.”
Still, with the future of coral reefs uncertain, any potential treatment to help preserve corals or bolster their defenses against climate change is useful.
“Whilst working with reefs I’ve seen a lot of changes, unfortunately, mostly in a negative direction,” Paul says. “But there are places in the world and there are certain reefs that can maintain a better condition than others, and a lot of that depends on what people do.”
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