Disease from human sewage is killing Caribbean corals

Forty years ago, the elkhorn coral was one of the most common species in the Caribbean. Five years ago, it was listed as critically endangered. The coral’s woes are many but, aside from the warming temperatures, predators and storms that affect all corals, the elkhorn is also plagued by a highly contagious malady called white pox disease. White lesions erupt all over the coral’s branches, representing areas where its animal tissue has wasted away to leave the white skeleton.

Now, Kathryn Patterson Sutherland from Rollins College in Florida has discovered the cause of white pox disease, and it’s an unexpected one – us. We have literally landed the elkhorn in s**t.

The white pox is caused by a bacterium called Serratia marcescens which we donated to corals via a gift of sewage. S.marcescens is a gut bacterium found in human faeces. It’s also an opportunist. It grows where the atmosphere is moist, such as sinks and showers, and where immune systems are weak, such as catheters, wounds and hospital equipment. But how dd it end up infecting coral reefs?

In 2003, Sutherland found a clue. During an outbreak of white pox disease, she identified a unique strain of S.marcescens, known as PDR60, in both local elkhorns and untreated human sewage. Now, she has confirmed that this strain causes white pox disease. In aquarium tanks, she let the strain loose upon colonies of elkhorns and saw that it caused white wounds in as little as four days. Other strains from human wastewater did the same.

Sutherland also found PDR60 in two species of star corals and one snail that eats them. These samples could also infect elkhorns with white pox, but less often and after a longer time than those taken directly from human sewage. These other species could act as carriers that spread the bacteria to the elkhorn, or staging posts on its path from human intestines to coral skeletons.

It isn’t clear why PDR60 is so problematic for corals but Sutherland thinks that it probably has special adaptations that allow it to survive in the sea. It can survive for up to 20 days in seawater, while other strains of the same bacterium die in just a few hours. Indeed, Sutherland has isolated the strain from several different sites, suggesting that it’s now an entrenched part of the Caribbean.

It seems astonishing to think that a human bacterium could be infecting corals, but animal diseases jump into humans all the time – think swine and bird flu, ebola, Nipah virus, and many more. These are known as zoonoses and reverse zoonoses, where human diseases infect animals, are also fairly common. But this is the first instance of a human disease affecting a marine invertebrate. Humans and elkhorn corals are separated by almost a billion years of evolution, and it will be fascinating to understand how one bacterium can target both species.

Meanwhile, the elkhorn coral is still in danger. “The good news is that we have a solution and that solution is wastewater treatment,” says Sutherland. She says that the Florida Keys have traditionally used septic tanks to dispose of waste, but these rely on soil to filter out contaminants that spring from leaks. Instead of soil, the ground in the Keys is mostly made of porous limestone bedrock, which allows waste from a leaking septic tank to quickly reach the shore.

“A decade ago the people of Key West upgraded their sewage treatment from septic systems to advanced wastewater treatment and today the entire Florida Keys is in the process of upgrading local wastewater treatment plants,” says Sutherland. “Wastewater treatment upgrades are expensive but they are definitely tax-payers money well spent.”

Reference: Sutherland, K., Shaban, S., Joyner, J., Porter, J., & Lipp, E. (2011). Human Pathogen Shown to Cause Disease in the Threatened Eklhorn Coral Acropora palmata PLoS ONE, 6 (8) DOI: 10.1371/journal.pone.0023468