Coral reef ecologists have laid a persistent and troubling puzzle to rest. The elkhorn coral, named for its resemblance to elk antlers and known for providing valuable marine habitat, was once the Caribbean's most abundant reef builder. But the "redwood of the coral forest" has declined 90% over the past decade, in part due to highly contagious white pox disease, which causes large lesions that bare the coral's white skeleton and kill its tissue. Now, after nearly a decade of data collection and analysis, researchers have fingered the cause of the affliction: human excrement. The finding represents the first example of human-to-invertebrate disease transmission and suggests a practical approach for halting the disease's spread.
"This is a really important bit of work," says coral researcher Thomas Goreau of the Global Coral Reef Alliance in Cambridge, Massachusetts. "I would say they've now proven their case beyond any doubt."
Nine years ago, a research team led by coral reef ecologists Kathryn Sutherland, now of Rollins College in Winter Park, Florida, and James Porter of the University of Georgia, Athens, linked white pox to a bacterium called Serratia marcescens, which is found in the intestines of humans and a handful of other animals. In humans, Serratia can cause respiratory and urinary tract infections. But although Sutherland and her team strongly suspected human waste—stemming from septic tanks that leak sewage into the Florida Keys's porous bedrock—was the culprit, they had no proof that the disease didn't start with Key deer, cats, seagulls, or any of the Caribbean's other Serratia-harboring wildlife. "There was considerable skepticism—it was too easy to blame other things," Porter says.
The duo and colleagues spent years collecting Serratia samples from healthy and diseased corals, from humans via a wastewater treatment facility in Key West, and from other animals. To obtain each sample's genetic fingerprint, they added an enzyme that breaks up the bacterium's genome wherever a specific gene sequence is found.
Because every strain's genome differs slightly, each one yields a unique pattern of breaks. Comparing the patterns among all their samples, the team found only two that matched each other exactly: the Serratia strain found in white pox-afflicted coral and the one drawn from human waste.
To dispel any remaining doubt, the researchers cultivated small fragments of healthy, Serratia-free coral in the lab, and then exposed these to the human-specific strain. Within as little as 4 days, the healthy coral showed signs of white pox infection, they report today in PLoS ONE.
In the Florida Keys and the Caribbean, where sea-based tourism and recreation pump billions into the economy each year, the discovery has significant implications, Porter says.
Sutherland and Porter hope their new evidence will encourage communities throughout the Caribbean to upgrade their waste management facilities, replacing septic tanks ill-suited for the region's geography and geology with wastewater treatment plants. Key West has not seen a single new case of white pox since its transition to an advanced wastewater treatment facility in 2001, the researchers say.