Endangered coral can be protected with better wastewater treatment
By Bob Berwyn
SUMMIT COUNTY — By analyzing the genetics of bacteria found in partly treated sewage in the Florida Keys, researchers from Rollins College in Florida and the University of Georgia have identified human waste as the source of a pathogen that’s wiped out 88 percent of the elkhorn coral growing in Florida’s offshore reefs in the past 15 years.
The bacterium, Serratia marcescens, causes white pox disease in elkhorn coral. Because of the rapid die-off, the coral was listed under the endangered species act in 2006. Elkhorn coral across the Caribbean is affected by the same disease to varying degrees.
It’s the first time that a human pathogen has been found to cause disease in a marine invertebrate species, said University of Georgia ecology professor James Porter, one of the scientists who recently published their findings in the peer-reviewed open access journal PLoS ONE.
To give some perspective to the scale of the elkhorn die-off, Porter said it’s as if 88 percent of the redwood trees alone the Pacific Coast were to die in 15 years.
The good news is that elkhorn can recover from the disease when the pathogen is removed. Under good conditions, it can grow up to eight inches per year.
“We just have to give them clean water,” Porter said.
Most communities in the Keys are already in the process of upgrading their facilities to provide advanced wastewater treatment. That would help eliminate a primary source of the pathogen, although wastewater pumped from boats is another potential source, Porter said. The research gives solid grounds to the argument that developing countries around the Caribbean also need to do more to improve wastewater treatment, he added.
University of Georgia researchers have known since 2002 that the coral-killing bacterium is the same species as found in humans.
“When we identified Serratia marcescens as the cause of white pox, we could only speculate that human waste was the source of the pathogen because the bacterium is also found in the waste of other animals,” said Kathryn Sutherland, an associate professor of biology at Rollins College.
But they weren’t sure of the exact source, and that’s where the scientific sleuthing began, including genetic analysis and the collection and analysis of human waste samples from the wastewater treatment facility in Key West and samples from several other animals, such as Key deer and seagulls.
While Serratia marcescens was found in these other animals, genetic analyses showed that only the strain from human sewage matched the strain found in white pox diseased corals on the reef. The final piece of the investigative puzzle was to show that this unique strain was pathogenic to corals.
With funding from Florida’s Mote Marine Laboratory “Protect Our Reefs” grant program, Sutherland, Lipp and Porter conducted challenge experiments by inoculating fragments of coral with the strain found in both humans and corals to see if it would cause disease. The experiments were carried out in a laboratory in closed seawater tanks to eliminate any risk of infection to wild populations of corals.
“The strain caused disease in elkhorn coral in five days, so we now have definitive evidence that humans are a source of the pathogen that causes this devastating disease of corals,” Sutherland said.
“These bacteria do not come from the ocean, they come from us,” said Porter. Water-related activities in the Florida Keys generate more than $3 billion a year for Florida and the local economy. “We are killing the goose that lays the golden egg, and we’ve got the smoking gun to prove it,” Porter said.
Serratia marcescens is also a pathogen of humans, causing respiratory, wound and urinary tract infections, meningitis, and pneumonia. Human diseases caused by this bacterium are most often associated with hospital-acquired infections of newborn infants and immune-compromised adults.
This research reveals a new disease pathway, from humans to wildlife, which is the opposite of the traditional wildlife-to-human disease transmission model. The movement of pathogens from wildlife to humans is well documented—for example, bird flu or HIV—but the movement of disease-causing microbes from humans to marine invertebrates has never been shown before.
“Bacteria from humans kill corals—that’s the bad news,” said Porter. “But the good news is that we can solve this problem with advanced wastewater treatment facilities,” like one recently completed in Key West. “This problem is not like hurricanes, which we can’t control. We can do something about this one,” he said. The entire Florida Keys is in the process of upgrading local wastewater treatment plants, and these measures will eliminate this source of the bacterium.
The Rollins College and University of Georgia collaborative research group is currently funded by a $2.2 million grant from the National Science Foundation to investigate the ecology of white pox disease in the Florida Keys. The five-year study will focus on mechanisms of transmission of the coral pathogen and the factors that drive the emergence and maintenance of white pox outbreaks, including water quality, climate variability and patterns of human population density.
“We are concerned that disease incidence or severity may increase with rising temperatures,” said Erin Lipp, another member of the research team. The findings reinforce the importance of protecting near-shore water quality in a changing climate, she added.