Corals in the Florida and Caribbean waters have been largely affected with stony coral tissue loss disease or SCTLD. It is a persistent and widespread pathogen which has destroyed the corals, turning them into white calcium carbonate skeletons, devoid of the biodiversity, and basically lifeless.
Valerie Paul, head scientist at the Smithsonian Marine Station at Fort Pierce, said, “It’s really devastating. It can kill coral that’s hundreds of years old in a month.”
After many trials, Paul and colleagues have developed a new weapon that can be a potential cure in the future: a probiotic cocktail to give infected corals beneficial bacteria which can save them and prevent the spread of the disease.
The scientists’ research, which was published in Communications Biology, isolated a strain of bacteria that demonstrated antimicrobial properties in corals that had previously avoided SCTLD.
Previously, studies have identified three strains of harmful bacteria in infected corals. Then, Paul’s team examined 222 bacterial strains found in disease-resistant corals to find out if any of them could be turned as a weapon against the bad strains.
The team found 83 strains that showed some level of antimicrobial activity, but one stood out from the rest. A strain, named McH1-7, was able to resist bacteria associated with SCTLD.
With this discovery, scientists put the potentially successful strain to chemical and genetic analysis to find out which compounds made it effective against harmful bacteria. They also wanted to know which genes produced the compounds.
So, they tested a probiotic application of McH1-7 on 22 different live samples of great star coral (Montastraea cavernosa). In aquarium tests, the probiotic was successful in stopping or slowing the disease nearly 70% of the time.
Moreover, when the researchers tried to transmit the disease to healthy coral slathered with the probiotic beforehand, they found that the cocktail prevented SCTLD’s spread.
Better than antibiotics
The result from Paul’s team was better than employing antibiotics to fight the disease.
Before, there was an attempt to cure the corals with drugs amoxicillin applied as a paste. This treatment is successful at slowing and stopping the spread of lesions on diseased coral, but just like in humans, antibiotics have the potential to backfire: producing antibiotic-resistant bacteria.
Since that risk doesn’t happen with probiotics, they’re the more advantageous choice for the corals.
Of course, this creates hope for the corals. With these probiotics, it won’t just be the infected corals that get the treatment; healthy corals may also get protected against future SCTLD exposure.
However, this early discovery has its challenge: application in the field.
To dose corals with probiotics, scientists have to make some kind of underwater aquarium. They need to surround a colony with a large sheet of plastic and weighing down the edges. Essentially, they’re creating a bubble which then they infuse with probiotics.
This process, while quite doable in the lab, will be labor-intensive in real application.
According to Jennifer Moore, the protected coral recovery coordinator at the National Oceanic and Atmospheric Administration, the probiotics are great at the individual coral level, showing very promising success.
“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,” Moore said.
What caused SCTLD?
Although the corals have been studied extensively by biologists and scientists from different groups, they still don’t know the exact cause of this disease.
What they know so far is that it’s not necessary for the disease to spread by direct contact with infected corals, and that SCTLD spreads through the water.
And, since this disease is found from island to island against prevailing currents, scientists suspect that it latches onto ballast water or biofilms that coat ships’ hulls. It can also spread when tourists unknowingly transport it with their diving gear.
There are only speculations so far as to what is causing the disease, and this is made worse by coral reefs’ complex composition. They’re a mix of coexisting algae and microbes within ocean waters with their own intricate cocktail of microbial life.
“Imagine if we just went through the Covid pandemic and didn’t know what the causative agent was. That’s still sort of where we’re at with this disease,” Paul said.
Corals developing their own resistance
Now, we know that bacteria are at least involved in the disease because probiotics and antibiotics are effective against SCTLD, but scientists didn’t hit the bull’s eye. In this case, bacteria play some role in the disease. Although they may not be the main culprit of its fatality, scientists thought that they could be a secondary infection that takes advantage of weakened corals.
Coral reefs aren’t just the homes of fish, shellfish, and sponges; they’re also the host of community of organisms, and some of them can be effective in fighting off pathogens.
As mentioned, Paul and colleagues found some unaffected, healthy coral colonies that stayed disease-free when they were put in contact with infected corals. The scientists wanted to find out about the corals’ natural defenses so they could further develop a treatment that will help diseased corals.
Further developments are needed because thus far, the probiotic treatments seem to work most effectively on great star coral and a few other species. Unfortunately, on at least one other coral species, the solution doesn’t have much impact.
Moreover, since SCTLD is waterborne and fast-moving, scientists want to nip this threat in the bud before it appears on any Indo-Pacific reefs.
Making this harder for corals to survive
We all know that coral reefs all over the world are experiencing a lot of problems in recent times. Ocean acidification, warming caused by climate change, pollution, and other diseases have put the reefs in great peril. It may get worse if SCTLD is let loose.
So far, since 2014 when it was first identified, the disease has devastated Florida’s coral reefs. The coral killer has 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. In 2022, it appeared in the Gulf of Mexico, at Flower Garden Banks National Marine Sanctuary.
Not backing up so easily
The urge to stop this disease from devastating other coral reefs due to its contagiousness has driven scientists and conservationists to keep trying to save the corals.
And despite the challenges mentioned above, the scientists won’t stop developing the probiotics. Therefore, Paul’s team plan to expand their research to find new probiotics that could help all two dozen or so types of hard corals impacted by SCTLD.
In addition, since the probiotics can also protect healthy corals, the current ones can be used for ongoing coral restoration efforts. Nowadays, any efforts to save coral reefs is better than nothing at all.
According to Lisa Gregg, co-leader of the Florida Coral Rescue and Propagation Team, the probiotics are useful. “How will we protect the offspring when we’re putting them back out in the water? Inoculation with a probiotic bath could be one of those options to provide some added layer of protection,” Gregg said.
Paul added that the research and conservation projects are a part of collaborative efforts to keep coral reefs thriving, and for now, free of SCTLD. “It really has been all hands on deck, and people have really worked closely together. Everybody would just love to see this thing gone,” Paul said.