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Scientists Race to Decode Disease Devastating Florida Coral Reefs

As a mysterious but persistent pathogen threatens the last remaining healthy coral ecosystem in the continental United States, researchers are struggling to understand how the disease spreads and to devise ways to stop it.

Written by Amelia Urry Published on Read time Approx. 7 minutes
Coral disease in the Florida Keys.Rick Loomis/Los Angeles Times via Getty Images

In 2014, coral reefs in Florida started to turn bright white. But this was not the heat-stress bleaching that has become a familiar and deadly phenomenon in recent years – this was a disease. Coral tissue sickened and died, leaving a bare white skeleton. Like a contagious flu strain, it spread quickly through the corals of southeast Florida and the upper Florida Keys. Then, unlike almost every other coral disease scientists know of, it refused to go away.

Four years later, the mysterious disease – one of a group of tissue-loss diseases that affect corals – continues to spread, though its precise cause is still unidentified. On some reefs it has reached, between 60 and 100 percent of corals are infected; many eventually die.

A group of scientists, government agencies and nonprofits, backed by an emergency $1 million grant from the state, is now trying to understand the disease as it moves toward the lower Florida Keys, where it threatens the healthiest remaining stretch of coral reef in the continental United States – and the estimated $6 billion that reef-related activities bring to the state.

“It’s surprised us as scientists, because most disease outbreaks don’t last this long,” said Erinn Muller, science director of the nonprofit Mote Marine Laboratory’s Center for Coral Reef Research and Restoration. “We all kept expecting it to go away with the winter months but it never did.”

The surprise underscores how little is understood about coral diseases at a time when they’re becoming more frequent and extensive, particularly in the Caribbean, where 8 percent of the world’s reefs experience 70 percent of reported disease, according to the United Nations’ Global Coral Disease Database. Though individual outbreaks may receive less attention than dramatic global bleaching events, their cumulative effect can dramatically transform reefs by eliminating key species, thinning out coral cover and reducing the diversity and resilience of the ecosystem.

As climate change continues to put pressure on corals around the world, diseases are becoming more common, exacerbated by other sources of stress such as pollution from sewage and agriculture runoff. Muller describes this as the “death by a million cuts” corals face: If climate change doesn’t kill them, there are other threats that might.

Coral disease spread as of 2017. (Florida Department of Environmental Protection)

Most coral diseases tend to flare up during the summer, when warm water temperatures create favorable conditions for bacteria and viruses to grow while simultaneously stressing out corals. Occasionally, a seasonal infection will make the leap to full-blown epidemic.

That’s what happened in the late 1970s and 1980s, when an infection called white-band disease showed up in the United States Virgin Islands and eventually swept through the Florida Keys and the Caribbean. White band targeted two common branching corals, elkhorn and staghorn, which are large and fast-growing species that made up much of the structure of Caribbean reefs. Once infected, these corals rapidly began to shed their living tissue, with ribbons of flesh peeling away from skeletons that quickly grew over with algae and eventually broke apart.

Like the disease spreading today, white band was able to jump vast geographic gaps between reefs that usually keep diseases contained. It eventually killed more than 90 percent of elkhorn and staghorn corals in Florida and the Caribbean, Muller said. Today, the once ubiquitous corals are listed under the Endangered Species Act, and reefs that were previously dominated by dense thickets of coral have never fully recovered.

The recent infection around Florida is poised to do to the massive “mounding” corals that make up much of the remainder of the state’s reefs what white band did to the elkhorn and staghorn. It is highly contagious and deadly. For species such as the endangered pillar coral, already reduced to just 70 individual colonies in Florida, this disease could lead to local extinction.

“Corals only die a certain number of ways, visually,” Muller said. “They either bleach or they lose tissue.” That can make even basic steps such as identifying a disease difficult: Outwardly, they all look more or less the same. Hence some scientists referring to the new disease as “white syndrome,” as it bears a similarity to a disease first seen in Florida in the 1990s, at the time called white plague. But even if the diseases resemble each other, it’s very difficult to say for certain that it’s the same pathogen progressing in the same way.

“We may never know,” said Cindy Lewis, deputy director of the Keys Marine Lab, one of almost two dozen organizations working with the state on the problem. “That’s just the way it is in the marine environment.”

A researcher working on Erinn Muller’s team, taking samples of healthy corals on Looe Key to compare to sick corals. (Mote Marine Laboratory & Aquarium)

Unlike a human being with flu-like symptoms, when a coral exhibits signs of disease it is much harder to isolate the bacterium or virus behind the illness. Only a few of the thousands of species of bacteria that grow on a coral can be cultured in the lab; others can be sniffed out using DNA analysis, but often only if scientists know what they are looking for. That makes it hard to assess exactly how coral diseases arise and spread.

Reducing environmental stresses by cleaning up water quality and lowering greenhouse gas emissions is the only real way to avoid disease, but researchers do have some other tools to try to lessen their harm in the short term. At Keys Marine Lab, Lewis has been running trials of experimental treatments for white syndrome, the most promising of which involve a broad-spectrum antibiotic applied in small doses to the diseased tissue.

By first cutting as much dead and diseased tissue from a coral colony and then digging a “firebreak” channel into the coral skeleton to separate healthy tissue, Lewis and her colleagues are attempting to isolate the disease. Then they apply the antibiotic in a sticky layer – right now, they’re testing several substances to use as a medium, including a specially compounded paste adapted from human dentistry and shea butter.

In the controlled trials, even small doses of antibiotics applied in this targeted way have been able to stop the spread of disease in individual corals. One colony of pillar coral that Lewis rescued from a diseased reef over the winter bounced back in weeks, after losing nearly 30 percent of its tissue to the disease.

“It’s growing all over the place, over where we cut the disease,” she said, “just growing over everything like icing on a cake.”

But applying the treatment on a reef is a different matter. The time it takes to prune diseased tissue from individual corals, then establish firebreaks and apply antibiotics, would be prohibitive in every location where white syndrome has hit. There are also risks, such as pathogens evolving antibiotic resistance or hurting the beneficial bacteria on which corals rely.

A healthy female pillar coral releases eggs in the Florida Keys National Marine Sanctuary in 2012. (Karen Neely/FWC via the Florida Keys News Bureau)

Florida Keys National Marine Sanctuary would also have to grant a permit to conduct any treatments on the reef. Superintendent Sarah Fangman said in an emailed statement that, “the current disease outbreak is unprecedented in terms of geographic range, duration, mortality and affected species” and cited management strategies such as “addressing known coral stressors, such as pollution and sedimentation, and identifying disease-resistant types of corals,” but did not comment on antibiotic usage. Florida Department of Environmental Protection spokesperson Elyssa Finkelstein pointed out only that the discussion of treatment options “is ongoing and constantly evolving.” Any field treatment, she said, would be made in consultation with an advisory committee, composed of leading coral disease scientists, reef managers and regulatory agencies.


Diseased corals being studied for experimental antibiotic treatments at Keys Marine Lab. (Cindy Lewis, Keys Marine Lab)

Muller at Mote Marine Lab pointed out that the corals Mote grows for its reef restoration efforts have so far shown a curious resilience to the disease. In experimental tanks, diseased corals collected from the reef sit side-by-side with healthy nursery-raised corals.

This gives her hope, she said, that these corals could still be planted on reefs and resist infection. But it also shows how tough the natural environment has become – corals have spent year after year in warmer, more polluted waters, getting battered by hurricanes, their natural defenses weakening.

Lewis believes it’s likely the Sanctuary could eventually grant her permit application for limited use of antibiotics on diseased corals. One place to start could be with the 70 individual pillar corals left in the wild. (Update: Lewis’s permit application to apply an antibiotic treatment to pillar corals in the sanctuary was granted several days prior to publication of this piece, the sanctuary’s superintendent said in an emailed update.)

“If we don’t do anything, we’re looking at probably losing this species on the reef in the next five to 10 years,” said Lewis. “I hate to jump immediately to antibiotics, but if it’s the only thing we have between us and the extinction of some of these corals, I think it’s worth the risks.”

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