Ghosts of Elkorn Coral Past

To round out my posts about coral disease, I thought I would choose a disease that hits close to home for the United States, and is one we do know a *bit* about. The paper I am summarizing today also happens to be co-authored by one of my favorite graduate professors, so shout-out to Dr. Jeffery Turner! If you ever read this, hopefully you approve and don’t revoke my ‘A’ in Ocean’s Health 😉

Once ubiquitous throughout Caribbean waters, the species of coral called Acropora palmata (pictured above), or as it is more commonly known in English, elkhorn coral, used to be one of the main reef dwellers in that part of the world. It’s large, branching calcium cased skeletons is a major component of reef structures. However, its living population numbers have severely declined over the past few decades. So much so that it is now one of the most endangered coral species. It appears that a wide host of reasons have contributed to its decline. These include increasing water temperature, changing water chemistry, and hurricane damage. However, one of the biggest factors contributing to its decline is disease, primarily white pox disease (pictured below). This disease is characterized by irregularly shaped white blotches across the coral’s body. These blotches are basically bare skeleton that are exposed due to tissue necrosis (death) caused by the disease. The disease is extremely contagious and can rip through an entire reef quickly. Starting in the 1970’s, white pox and white band disease infected and caused the mortality of large numbers of elkhorn coral in the Caribbean. Since then, the species has continued its decline.

Until the late 1990’s, the elkhorn coral in the Florida Key’s did not have the same disease problems as the rest of the Caribbean. However, between 1996 and 2002, an outbreak of white pox disease ravaged the Florida Key’s population, reducing its cover by nearly 88%. Recent years has seen a decline in the prevalence of white pox disease and white band disease however. The reason for this decline is not well understood, but the two main hypotheses are 1) it may only be due to a severe decline in its host species, and therefore not a true reduction in the disease itself, or 2) the new generations of corals are more resistant to the disease.

As I said in a previous post, there is not as much known about coral diseases as we would like. In 2003 though, a startlingly discovery was made about white pox disease. Researchers found that this disease was very likely partially caused by the bacteria species called Serratia marcescens, a pathogen that comes from untreated human fecal matter. Ugh, let that sink in for a minute. Researchers were able to show this possible connection through a series of corroborating evidence. In 2003, a series of white pox out breaks in nearby elkhorn coral coincided with the identification of a strain of this bacterium in untreated human sewage, coral eating snails, the water around the coral reefs, and two other species of nearby corals. The short time frame of the outbreaks, combined with the wide variety of organisms the bacteria was found in supports the idea that this particular disease is a zoonotic. This means it’s a pathogen that infects multiple species, including humans. In this particular case, the bacteria starts in the human gut, and passes into the marine environment via improper sewage dumping or leaking sewage tanks. GROSS ☹ Once it’s in the water, it can then infect other organisms, like elkorn coral.

I would like to stress that this study found the bacteria in raw, untreated sewage, not in any of their post-treatment samples. This is some slight good news! It indicates that current treatment methods are effective in eliminating at least this particular pathogen. Phew! Now, to make sure no more untreated wastes gets into the environment! Something I think EVERYONE can be in favor of!

This is one of the first times a disease like this has been studied and reported on. Most known zoonotic diseases infect in the opposite direction, starting in a wildlife population and crossing over to human populations. Discovering that we can unknowing pass diseases onto the world around us is a bit of a game changer. We knew that sewage not properly taken care of could make other people sick, but now it appears that more organisms are in the firing range. As always, more study is needed to learn if this is a new trend, or simply the discovery of an old phenomena.

Alright, there you have it. Hopefully you have learned a bit more about the current plight of the world’s corals (and hopefully I never have to talk so much about human waste again 😊 ).

Sutherland, K., Porter, J., Turner, J., Thomas, B., Looney, E., Luna, T., . . . Futch, J. L. (2010, May). Human sewage identified as likely source of white pox disease of the threatened Caribbean elkhorn coral, Acropora palmata. Envrionmental Microbiology, pp. 12(5), 1122-1131.