By: Jenny Marder

The bacterium Serratia marcescens infected this coral from Looe Key in the Florida Keys, revealing the dead, white limestone skeleton underneath. Photo by James W. Porter.

A Florida biologist has linked a vicious coral-killing pathogen in the Caribbean and Florida Keys to human sewage that leaks into the ocean from improperly treated wastewater.

The Caribbean Elkhorn coral was at one time the most common coral in the Caribbean, but has declined by 90 percent over the last 15 years and is now an endangered species. Among the many factors contributing to its decline is a disease known as white pox, caused by Serratio marcescens, a common fecal intestinal bacteria found in the guts of many humans and other animals, including seagulls, Key deer and cats. But whether it came from humans or another source has been a mystery.

"The medical mystery was a whodunit," said James Porter, a professor of ecology at the University of Georgia and an author of the study, published Wednesday in the journal PLoS One.

When the pathogen attacks, it sears bright white blotches onto the coral, dead tissue which lifts off, leaving the skeleton exposed and killing the organism.

To determine the source of the pathogen, the team tested human sewage, along with that of several other organisms, including seabirds, fireworms, snails and deer. A genetic analysis showed that only the strain found in human sewage and snails matched the strain in the coral.

Using a PVC pipe cutter, the researchers broke off small coral fragments, brought them back to the lab, and swabbed the coral with various strains, including the human strain, which had been isolated from raw sewage. The result: only coral inoculated with the human strain of the bacteria got sick.

"You could smell the coral's response," said Kathryn Sutherland of Rollins College, the study's lead author. "It was releasing mucous like crazy, it was changing the clarity of the water and it was giving off a distinct smell." As it died, tissue lifted off from the coral in stringy fragments. It was usually dead within 24 hours from when it started showing disease signs, she said.

The team's hypothesis is that the white pox originated in human sewage and was transmitted by the snail, which serves as a vector, carrying the disease from coral to coral.

White pox in humans causes respiratory infections and has been associated with meningitis and pneumonia.

While the Florida Keys is going through an expensive process of upgrading to advanced wastewater treatment plants, waste disposal there has traditionally involved septic tanks, which have a tendency to leak. Septic tanks do best in areas with low population density and soil, which filters out many contaminants before they can pollute the waterways. The Florida Keys instead are situated on a porous limestone bedrock.

"In the Keys, we don't have that filter. Just Swiss cheese bedrock," Sutherland said. "So a faulty septic tank that leaks goes right through the bedrock and leaks into the ocean." And the Caribbean has a widespread lack of wastewater treatment, she added.

Carribean elkhorn coral is a reef-building coral with antler-like branches, and it serves as a habitat for reef species like lobster, snapper and other fish, which use the branches as shelter and protection against predators.

"In many ways, this particular species are like the redwoods of a forest," Porter said. "They're the large branching iconic species. An analogy would be as if 97 percent of redwoods died in Sequoia National Park."

The hope, Sutherland said, is that these findings will help spread the message on the importance of upgrading sewage treatment throughout the Caribbean.

"Hopefully eliminating this source will lead to declines in the disease and recovery in the coral," she said.