Florida corals had a rough 2014. During that year higher than normal ocean temperatures caused the most severe bleaching since record keeping had begun. Every single coral reef in the United States showed some signs of bleaching—the corals jettisoning the beneficial algae they normally rely on to survive.
This was the story that made headlines, but another equally detrimental ailment began plaguing the reef that year.
A new disease silently appeared in Southeast Florida reefs. Disease in corals is not uncommon, and there have been several outbreaks in recent years. This one was different. Now known as stony coral tissue loss disease, it is highly infectious and kills coral tissue, leaving a bony and bare skeleton behind.
“We didn’t understand the significance of it at that time because it was a stressful year. But the disease never stopped,” said Brian Walker, a coral researcher at the NOVA Southeastern University.
Today, over 20 coral species are known to be susceptible to the infection. The disease has spread south to the Florida Keys from Miami and is now even cropping up in reefs across the Caribbean. In July 2019, the disease made its first appearance all the way in Belize.
After initial infection, spots of dead coral called lesions appear. Then the disease kills tissue radially around the initial spots and can infect the entire coral colony in as little as a few days in some coral species and weeks in others. According to the National Oceanic and Atmospheric Administration, the disease has a mortality rate between 66 and 100 percent. While diseases generally target one or two related species, this one infects over 20 unrelated species.
“That was the intriguing thing, it wasn’t affecting one or two species it was affecting seemingly anything it came across. It’s both alarming and incredibly surprising to me,” says Blake Ushijima, a coral disease expert and a George Burch Fellow at the National Museum of Natural History, based at the Smithsonian Marine Station at Ft. Pierce.
Pillar corals, elliptical star corals, smooth flower corals, and maze corals are especially affected. Corals that are important for reef building, like boulder brain corals, are also affected, as are at least five endangered coral species.
Even some of the oldest, heartiest corals can’t seem to fend off the disease. In surveys conducted by Walker and his team, nine out of 115 of a selection of the oldest corals near Miami died between 2014 and 2018. These corals are the size of small cars and have been estimated from core samples to be over 300 years old. They have weathered hurricanes, coastal discharge from the Everglades, turbid waters after storms, and intense coastal development during the 1950s and 60s. Now the remaining are struggling to survive due to this new disease.
Yet, some corals are able to fend off the illness. Perhaps, scientists thought, those hearty corals hold a clue to helping others fend off the disease.
Stopping the Disease
The severity of the disease has spurred a collaboration of over 40 local, state, and federal organizations, including the Smithsonian Institution, to determine how to stop it. It has required the input of hundreds of scientists with expertise ranging from coral ecology, microbiology, disease, and restoration. Many teams are conducting several different projects related to the outbreak at the same time due to the fast-moving nature of the disease.
One of the first steps was to determine what was causing the disease. The prevailing perspective among scientists is that it is not just one disease agent, rather it is a number of pathogens or even a pathogen associated with an environmental condition that brings the onset of the disease. Ushijima is one of the many scientists tackling this angle of the research and he and his team have shown that not only is the disease transmissible, meaning it stems from an infectious agent, it also likely has a bacterial component. Preliminary experiments showed that adding an antibiotic to the surface of the affected area stops infection.
“Antibiotics can stop disease progression but it doesn’t completely rule out the role of viruses. Also, it still could be multiple different bacteria,” says Ushijima.
The use of antibiotics to determine whether bacteria are partially to blame is one thing—using them in the environment is tricky business. Due to concerns about how antibiotics will influence the natural ecosystem, their usage undergoes judicious review.
That does not mean there aren’t options for treating the infected corals. Walker and team treat infected corals with a chlorine epoxy mixture that has seen a 60 percent success in stopping the progress of infection. From the rim of the infection, the scientists leave a five-centimeter buffer of healthy tissue and then fill a one-centimeter trench around the area with the epoxy mixture. As the disease spreads it eventually hits the epoxy trench and is kept from spreading farther.
“It isn’t great but it’s still worth doing,” says Walker. “We’re going to treat as many corals as we can to save whatever we can.”
That’s why several teams are investigating other treatments. Ushijima’s team at the Smithsonian Marine Station at Ft. Pierce is taking a different approach using information gathered from the more resistant corals, and recently they’ve made heartening progress.
Ushijima is looking to probiotics as a means of slowing the disease and protecting healthy corals. Like humans, corals naturally host a microbiome of healthy microbes that act as a shield against harmful microbes. The concept is similar to the human consumption of probiotics to maintain a healthy digestive tract. In preliminary trials they were able to slow or stop disease progression in at least one coral species while also preventing disease transmission. And that got them thinking—perhaps a single or a suite of helpful bacteria could combat the disease.
Together with Smithsonian marine chemist Sarath Gunasekera, Ushijima is testing more than 600 bacteria known to live on coral to see if they hinder harmful bacteria found on infected coral. After weeding the candidate list down to just a few, one “probiotic” bacteria was particularly effective. When applied to diseased M. cavernosa in aquaria the bacteria were able to slow or completely stop the disease.
Soon scientists figured out why. Further studies allowed Gunasekera, Smithsonian Marine Station scientists Jennifer Sneed, Valerie Paul, and Sharon Thompson, and Julie Meyer of the University of Florida to discover that this bacteria naturally produce specific antibiotic compounds including korormicin, marinocine, and tetrabromopyrrole. While the antibiotics may or may not target the cause of the disease, which is still unknown, perhaps they are hindering secondary bacteria that take advantage of the damaged coral tissue.
“It’s one of the most exciting developments recently,” said Maurizio Martinelli, the coordinator for state and federal coral disease response efforts with Florida Sea Grant.
The hope is that these probiotics could be used to give healthy corals on the reef a leg up against the disease.
Despite all these efforts, there is still much to do to save the corals. Scientists from several labs are saving corals from the wild and housing them in aquaria around the country. The hope is that in the future when conditions are favorable again these corals can be used to repopulate damaged reefs.
This is where the public has a large role in helping future corals fend off disease. Research is now suggesting that nutrient runoff from agriculture and human waste may be weakening the corals’ system so that they are more susceptible to bleaching and disease. One of the simplest ways to help the corals may be cleaning up local waterways.
“I don’t think the reefs will be the same in my lifetime or my kids’ lifetime. I don’t know that they’ll ever return to being the same,” says Walker. “I do think there is hope, I don’t think all is lost, but if we continue down this path of prioritizing development and not prioritizing our water then I think the prognoses is not good.”