Will probiotics save Florida's Coral Reef?

Imagine watching a forest slowly wither away, not from logging or fire, but from a creeping disease that kills tree after tree. Now picture that forest underwater, made not of wood but of living coral, the backbone of an entire ocean ecosystem. This is the reality facing the Coral Reef in Florida, which has been ravaged over the past decade by a mysterious and devastating illness called stony coral tissue loss disease (SCTLD).

Recently, scientists have begun testing an unexpected ally in the fight: probiotics, the same concept used in yogurt or dietary supplements to keep our gut microbes healthy. And it turns out, these "friendly bacteria" may give corals a fighting chance.

Florida's Coral Reef is the only barrier reef in the continental United States, stretching for hundreds of miles along the state's southeast coast. It's a natural wonder, a tourist magnet, and home to countless marine species. Yet since 2014, it has been under siege from SCTLD, a fast-spreading disease that eats away at corals, leaving behind bone-white skeletons where thriving colonies once lived.

Unlike most coral diseases, which typically affect just a handful of species, SCTLD is terrifyingly broad in scope. It has infected over 30 species of reef-building corals. Some species have seen catastrophic declines, to the point of "functional extinction" in certain areas. And the disease hasn't stayed put: it's spread from Florida to more than 30 countries and territories across the Caribbean.

Scientists still don't know exactly what causes SCTLD, though bacteria are prime suspects. Antibiotics, specifically an amoxicillin paste, can halt its progress, but they don't prevent reinfection. Worse, widespread antibiotic use risks creating drug-resistant "superbugs," which could make the problem even harder to treat. Clearly, new strategies are needed.

This is where probiotics come in. Just like humans, corals host a rich community of microbes, tiny bacteria, archaea, and other microscopic partners that help them fight disease, process nutrients, and withstand stress. Some scientists believe that boosting corals with carefully chosen beneficial bacteria could strengthen their natural defenses against SCTLD.

A team of researchers from the Smithsonian Marine Station in Fort Pierce (Florida), the University of Maryland Center for Environmental Science, the University of North Carolina Wilmington, the University of Florida, and Nova Southeastern University, decided to test this idea in the wild. Their study, published in Frontiers in Marine Science, is one of the first large-scale field trials of coral probiotics.

The team focused on a particular bacterium called Pseudoalteromonas strain McH1–7. This microbe was originally isolated from a coral fragment that seemed naturally resistant to SCTLD. In lab experiments, it had already shown promise: it could slow or stop the disease in aquarium tanks, and even prevent healthy corals from catching it.

But would it work in the open ocean, where conditions are harsher, more complex, and impossible to fully control? That was the big question.

How do you give medicine to a coral?

Treating corals with probiotics is no small task. The researchers devised two creative methods. The first one is the whole-colony "bagging" method. Divers covered infected corals with large, weighted plastic bags, creating a kind of underwater tent. Into this enclosed space, they injected a suspension of McH1–7 bacteria mixed with seawater. For two hours, the coral "bathed" in the probiotic solution, giving the bacteria a chance to colonize its tissues. The second method is the Probiotic paste method. Using syringes, divers applied a thick, alginate-based paste laced with McH1–7 directly onto the visible disease lesions. Think of it like spreading an ointment on a wound.

Both techniques were tested on colonies of the boulder star coral (Montastraea cavernosa) near Fort Lauderdale, Florida. The corals were monitored for 2.5 years, using 3D photogrammetry (a kind of digital modeling from photographs) to measure changes in live tissue. The researchers also analyzed the corals' microbiomes to see how their microbial communities responded.

The whole-colony bagging method worked. Corals treated with the probiotic bath lost significantly less tissue over the 2.5-year study than untreated corals or those given control treatments. On average, they retained nearly 80% of their living tissue, compared to just 53% for control corals. The treatment didn't overwhelm or destabilize the corals' microbiomes, which is good news, it suggests McH1–7 can integrate smoothly without throwing the system out of balance.

The probiotic paste failed (and even backfired. Surprisingly, the paste method performed worse than the controls. Corals treated with probiotic paste actually lost more tissue than those given paste without bacteria. The reasons aren't entirely clear, but the researchers suspect the concentrated bacteria at the lesion sites may have stressed the already damaged tissue.

So, a full-colony "bath" appears to give corals the microbial boost they need, while spot treatments may be ineffective, or even harmful.

This research is a hopeful step in an otherwise bleak story. For years, Florida's corals have been on life support, with antibiotics as the only available treatment. Probiotics offer a fresh, more sustainable alternative that doesn't carry the same risks of antibiotic resistance.

The success of the whole-colony method also shows that beneficial microbes can help corals without taking over their microbial communities. That's important, because it means probiotics can work in harmony with the natural microbiome rather than disrupting it.

There are still hurdles. The bagging technique is labor-intensive, requiring divers to individually treat each coral colony. Scaling it up to protect entire reefs would be a massive undertaking. Still, the proof-of-concept is here: probiotics can work in the wild.

Will probiotics save Florida's reefs? Alone, probably not. The threats to coral reefs are vast: warming seas, ocean acidification, pollution, and overfishing all add to the burden. But in combination with other conservation efforts, such as coral nurseries, selective breeding of disease-resistant corals, and habitat protection, probiotics could become a powerful tool.

Think of it as giving corals a dose of resilience. Just as probiotics help us fend off illness and maintain balance in our own bodies, these friendly bacteria might help corals survive long enough for broader solutions, like reducing carbon emissions and restoring reef habitats, to take hold.

If you want to learn more, read the original article titled "Evaluating the effectiveness of field-based probiotic treatments for stony coral tissue loss disease in southeast Florida, USA" on Frontiers in Marine Science at http://dx.doi.org/10.3389/fmars.2025.1480966.