Coral reefs are fragile things, and there has been concern for years about the various ways human activities manage to inflict harm on them. Some are physical—over-collecting coral for aquarium use or damaging reefs while diving—and some more insidious, like acidification and warming of the oceans.
Two recent studies show renewed threats—some from warmer waters, and some from something a bit closer to home for those who deal with water-quality issues.
Last week, the ARC Centre of Excellence for Coral Reef Studies and James Cook University reported die-off rates of up to 80% for some species of coral in the Indian Ocean. They’re attributing it to warmer temperatures—on average, about 4 degrees Centigrade warmer than usual in the affected regions. As the water heats up, the corals lose the algae that normal live on the reefs and provide nourishment. This process eventually starves the coral and leads to a “bleached” appearance.
The other recent report is in some ways even more disturbing, because it suggests a phenomenon many scientists hadn’t thought possible: human disease spreading to and killing the coral. We worry a great deal about viruses jumping from animals to humans—think bird flu—but less about pathogens moving in the other direction. Researchers at the University of Georgia have identified a bacterium, Serratia marcescens, that causes intestinal illness in humans and that has been transmitted to elkhorn coral in the Caribbean. It causes a disease called white pox in the coral, and researchers now believe the disease has been destroying reefs for a decade and a half.
As one scientist points out, it required an unusual “triple jump” for the bacterium to reach the coral—from vertebrate to invertebrate, from a terrestrial to a marine environment, and finally from anaerobic to aerobic conditions. Studies are ongoing to see just how the bacteria may have spread and why some corals are more susceptible than others, but it seems clear that the bacterium entered the water from land-based sources, and how we treat wastewater—and possibly urban and agricultural stormwater runoff as well—determines which types of pathogens enter the ocean.