Bad news for coral: fecal bacteria is the killer

In 1996, researchers in the Florida Keys noticed that a strange new disease, dubbed white pox, was attacking the coral reefs. It has since killed 85 percent of the elkhorn coral there. The cause of the disease, common fecal bacteria, spells bad news for coral offshore from any human development. James Porter, professor of ecology and marine science at the University of Georgia, discussed the disease with Ira Flatow on NPR’s Talk of the Nation on June 21. Here is a synopsis of what he said.

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The one coral species being attacked is the magnificently branching elkhorn coral, the giant redwoods of the reefs. A variety of plants and animal species are wholly dependent upon that structure. Coral reefs are the rain forests of the ocean. There are more varieties of plants and animals there than in a rain forest.

Elkhorn coral is the shallowest of the Caribbean corals. It used to be the most common Caribbean coral, but in May it was proposed for inclusion on the endangered species list. Now, on some reefs off Key West, the mortality is as high as 98 percent. We have found white pox in the Bahamas, the US Virgin Islands, the Caribbean coast of Mexico and it might also be in Puerto Rico.

A fecal coliform bacteria — serratia marcescens, the standard human fecal coliform bacteria — is causing white pox and killing the coral. A DNA analysis makes this one hundred percent certain. It is the same species of fecal coliform bacteria found in humans and animals and human and animal sewage.

Proving the exact origin of the bacterium is going to be difficult. We have not found it in open ocean, but we have identified serratia marcescens in the very near shore coastal area. It could be delivered by tourists and divers but it’s more likely from concentrated sewage sources, like leaking septic tanks, runoff from the coasts, or pumpout from boats.

To make the jump from a fecal coliform bacteria to a marine invertebrate pathogen is a big evolutionary leap. It’s a very contagious disease. Whether coral gets it depends on whether its nearest neighbor has it. It follows a standard contagion pattern of spread.

Global warming may influence it. Elevated temperatures promote microbe growth and depress the immune system of the coral. You’ve got a double whammy going on.

We hope that it may have run its course. Not all the specimens have been lost in the Keys, but the mortalities are very high. Maybe some specimens are resistant.

I suspect, however, that what we’ve discovered is a small subset of a general problem that’s occurring on the coastlines of both oceans and lakes. We need to find out if there is a correlation between human population density and the spread of this disease.