Since 2014, a water-borne disease called the stony coral tissue loss disease (SCTLD) has been rampantly killing Florida’s coral reef which is North America’s only living coral reef barrier. So far SCTLD has infected coral colonies comprising two dozen coral species found across the coasts of 22 countries in the Caribbean. What’s more shocking is that nobody has a clue about the exact source of this mysterious coral infection. 

However, a team of researchers from the Smithsonian’s National Museum of Natural History recently published a study in which they formulated a bacterial probiotic that can treat SCTLD infection and prevent it from spreading further.

Coral reef ecosystems are biodiversity hotspots of very high ecological value. They serve as homes to an endless number of fish and other marine species. Plus, at the same time, they also protect coastlines against erosion, destructive waves, and storms. Many businesses, and communities that depend on fishery, recreation, and nautical tourism cannot thrive in the absence of coral reef colonies. 

Over the last 50 to 70 years our planet has already lost more than 50 percent of its coral ecosystem due to water pollution, coral bleaching, and ocean acidification. Unfortunately, half of the remaining coral reefs are also finding it hard to survive due to climate change, and now there is this new threat in the form of SCTLD which wipes out even hundred-year-old corals within a month. According to the Smithsonian researchers, SCTLD is probably the most destructive coral outbreak that has ever been recorded in human history. 

Valerie J. Paul, one of the study authors and the Director of the Smithsonian Marine Station at Fort Pierce in Florida, said “It’s really devastating, it just eats the coral tissue away. The living tissue sloughs off and what is left behind is just a white calcium carbonate skeleton.” She further added, “It can kill coral that’s hundreds of years old in a month. We need to do everything we can to help them so they don’t disappear.”

How a bacterial probiotic can rescue the corals

Some past studies reveal that treatments involving antibiotics like amoxicillin were also able to deliver positive results against SCTLD infection. At present, these antibiotics are the only available SCTLD treatment option. According to the study authors, the effectiveness of the antibiotics against the disease hints that pathogenic bacteria also play a role in its spread. They further point out that apart from supporting marine animals coral reefs also host numerous microorganisms, and it is possible that some of the bacteria that they host offer them protection against pathogens. 

To test these assumptions, the researchers first conducted aquarium experiments in which they placed many healthy coral species nearby or in contact with SCTLD-infected corals. They were surprised to see that some of the corals didn’t get the infection. Valerie and her team then identified three SCTLD-causing bacterial strains and about 222 strains that were likely to show some resistance against different pathogens. 

Later, 83 of the 222 strains were found to be really effective against harmful microbes, and among these also, only one particular bacterial strain, McH1-7, showed the ability to control SCTLD. The study authors created a probiotic cocktail containing the McH1-7 strain and tested it on different coral species. The strain was able to reduce the progression of SCTLD in 22 live coral fragments by over 60 percent and successfully prevented the infection from spreading, during all the transmission experiments.       

“McH1-7 has broad-spectrum antibacterial activity against SCTLD-associated bacterial isolates. McH1-7 arrested or slowed disease progression on 68.2% of diseased Montastraea cavernosa fragments treated, and it prevented disease transmission by 100%. McH1-7 is the most chemically characterized coral probiotic that is an effective prophylactic and direct treatment for the destructive SCTLD as well as a potential alternative to antibiotic use,” the researchers note.

This is just one probiotic, we need more

The probiotic prepared using McH1-7 has been found to be effective in preventing SCTLD infection in the great star coral (Montastraea cavernosa) and a few other hard coral species. The study authors aren’t sure if the same treatment will be beneficial for all SCTLD-affected species. They suggest that they’d probably have to develop new probiotics for treating different species. However, the approach is still better than antibiotic treatments because those can also lead to the rise of antibiotic-resistant pathogens in corals. 

The researchers admit that they also need to improve the current McH 1-7 probiotic treatment method because it is very labor-intensive and complex. They are required to cover the corals in plastic bags and sheets and then have to inject the probiotic dose into individual corals. The SCTLD infection has already spread across the 360-mile-long Florida reef region, and therefore a fast and scalable treatment approach is needed to solve the problem. Hopefully, scientists will be able to overcome all these challenges with further research and new probiotic treatments.

The study is published in the journal Communications Biology.