Â鶹ÒùÔº

Scientists with the Smithsonian's National Museum of Natural History have discovered that a bacterial probiotic helps slow the spread of stony coral tissue loss disease (SCTLD) in already infected wild corals in Florida.

The findings, published in the journal , reveal that applying the probiotic treatment across entire coral colonies helped prevent tissue loss.

The new treatment provides a viable alternative to antibiotics, which only offer temporary protection and also run the risk of creating resistant strains of SCTLD.

"The goal of using the probiotics is to get the corals to take up this beneficial bacterium and incorporate it into their natural microbiome," said Valerie Paul, the head scientist at the Smithsonian Marine Station at Fort Pierce, Fla., who co-led the new study. "The probiotics then will provide a more lasting protection."

SCTLD emerged in Florida in 2014 and has rapidly spread south throughout the Caribbean. Unlike other pathogens, which usually target specific species, SCTLD infects more than 30 different species of stony corals, including boulder-shaped brain corals and limb-like pillar corals.

As it spreads, the disease causes the corals' soft tissue to slough off, leaving behind white patches of exposed skeleton. In a matter of weeks to months, the disease can devastate an entire coral colony.

Researchers have yet to identify the exact cause of SCTLD. But the pathogen appears to be linked to harmful bacteria. To date, the most common treatment for SCTLD is treating diseased corals with a paste that contains the antibiotic amoxicillin.

But antibiotics are far from a cure-all. While the amoxicillin balm can temporarily stem the spread of SCTLD, it needs to be frequently reapplied to the corals' lesions. This not only takes time and resources but also increases the likelihood that the microbes causing SCTLD could develop resistance to amoxicillin and related antibiotics.

"Antibiotics do not stop future outbreaks," Paul said. "The disease can quickly come back, even on the same coral colonies that have been treated."

To find a longer-lasting alternative, Paul and her colleagues have spent more than six years investigating whether beneficial microorganisms, or probiotics, could combat the pathogen.

Like humans, corals host communities known as microbiomes that are bustling with bacteria. Some of these minuscule organisms, which can be found in both coral tissue and the sticky, protective mucus that corals secrete, produce antioxidants and vitamins to keep their coral hosts healthy.

The team first looked at the microbiomes of corals that are impervious to SCTLD. The goal was to harvest probiotics from these disease-resistant species and use them to strengthen the microbiomes of susceptible corals.

The scientists tested more than 200 strains of bacteria from disease-resistant corals. In 2023, they published a study about the probiotic Pseudoalteromonas sp. McH1-7 from the great star coral (Montastraea cavernosa), which produces several different antibacterial compounds.

This comprehensive antibacterial toolbox made McH1-7 an ideal candidate to combat a versatile pathogen like SCTLD.

Paul and her colleagues initially tested McH1-7 in the lab on live pieces of M. cavernosa. They discovered that the probiotic reliably prevented the spread of SCTLD. However, the team was eager to test McH1-7 on corals in the wild.

In 2020, the team conducted field tests on a shallow reef near Fort Lauderdale. They focused on 40 M. cavernosa colonies that displayed signs of SCTLD. Some of the corals received a paste containing McH1-7 that was applied directly onto disease lesions. Other corals were treated with a solution of seawater containing McH1-7.

The team covered the colonies treated with the solution using weighted plastic bags and administered the probiotics inside the bag to cover the entire colony.

"This created a little mini-aquarium that kept the probiotics around each coral colony," Paul said.

The team monitored the coral colonies for two and a half years and took multiple rounds of tissue and mucus samples to gauge how the corals' microbiomes changed over time. They discovered that the McH1-7 probiotic successfully slowed the spread of SCTLD when delivered to the entire colony through the bag and solution method.

The samples revealed that the probiotic was effective without dominating the corals' natural microbes. In contrast, corals treated with the probiotic paste lost more tissue than the untreated control corals. This revealed that applying the probiotic directly to the lesions was the least effective way to control SCTLD.

While the probiotic appears to be an effective treatment for SCTLD among Florida's northern reefs, more work is needed to calibrate the treatment for other regions. For example, Paul and her colleagues have conducted similar tests on reefs in the Florida Keys. Preliminary results are mixed, likely due to regional differences in the disease itself.

But Paul contends that probiotics could become a crucial tool for combating SCTLD across the Caribbean, especially as researchers continue to fine-tune how they are administered at scale to corals in the wild. These beneficial bacteria support what corals already do naturally.

"Corals are naturally rich with bacteria and it's not surprising that the bacterial composition is important for their health," she said. "We're trying to figure out which bacteria can make these vibrant microbiomes even stronger."

This interdisciplinary research is part of the museum's Ocean Science Center, which aims to consolidate the museum's marine research expertise and vast collections into a collaborative center to expand understanding of the world's oceans and enhance their conservation.