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Below the clear waters of the Caribbean Sea lurks a deadly virus known as the white plague. Unlike the similarly named Black Plague, the
White plague was first recognized in the Florida Keys in 1977. In those days white plague wasn't much of a concern. In 1995 it reemerged again in the same area but this time the disease spread rapidly amongst the corals. White plague was reported in the Caribbean in 2001.
White plague is debilitating to coral reefs. It causes rapid tissue loss in corals, which leaves their white skeletons exposed to the elements. Multiple species of coral are affected by white plague. Ultimately, the disease can cause partial or complete colony mortality in corals. Soffer says that white plague is not difficult to spot; "You have living, healthy tissue, and then immediately below that you have a straight band of white on the bottom of the coral." The white band can spread quickly to the rest of the colony.
Scientists have identified three types of white plague that differ only in the rate they spread. The first kind identified in 1977 is known as Type I and destroys roughly one tenth of an inch (3 millimeters) of coral tissue per day. Type II is more lethal, with a mortality rate of .8 of an inch (2 centimeters) per day. At this rate Type II can annihilate a small colony in one to two days. The most recently discovered kind is Type III, which was first found in 2000. Type III kills coral tissue at a rate greater than .8 of an inch a day and mainly targets large reef-building corals.
Debate has persisted whether white plague is caused by bacteria or, as new evidence suggests, viruses. Several studies throughout the years have been performed to identify the cause of white plague. However, a solid consensus has yet to appear. In 2003 scientists thought that the bacterium Aurantimonas coralicida was responsible for Type II. Some researchers disagree by noting that in studies this bacteria was not found in corals with Type II. Ironically, Aurantimonas coralicida was only found in healthy coral. Three years later in 2006 scientists found that the bacterium Thalassomonas loyana may cause a similar disease to white plague in Red Sea Corals. This finding further supports the idea that white plague is bacterial in origin. On a side note, we now know that the disease affecting Red Sea Corals can be treated with a bacteriophage, a virus that infects bacteria. Studies have also shown that corals suffering from white plague have "increased bacterial diversity" and carry more microbes from some specific bacterial families.
Viruses too may be the cause although limited studies have tested their role in white plague. Collin Closek, a University of Pennsylvania biologist who studies yellow blotch disease among corals in the Caribbean says "viruses are challenging to work with and thus are often overlooked." White plague's tendency to spread and kill rapidly is akin to a virus's behavior. Therefore it's logical to believe that a virus is behind the disease.
To test the viral theory Soffer and her team researched a 2010 outbreak of white plague - mostly likely Type I - infecting boulder star corals in the Virgin Islands. The researchers compared the viral density in tissue samples from healthy corals, diseased corals, and
Up to 90% of the corals Soffer studied were bleached while 7% exhibited both bleaching and white plague (no colonies only showed signs of white plague). Three different types of tissue samples were taken. Samples were taken from colonies that were only bleached, from colonies that were both bleached and diseased, and from the few entirely healthy corals.
DNA Sequencing and an "imaging technique" known as transmission electron microscopy were used to determine the viruses present in the samples. Suffice it to say the viral makeup of each kind of sample differed. Non-lethal Herpes-like viruses were found in healthy tissue specimens. Samples from bleached only corals for the most part contained large DNA viruses (which includes the poxvirus) but also contained herpes-like viruses and small-strand DNA viruses (SCSDVs). Diseased tissue predominantly contained SCSDVs.
Of course, further research will be needed to cement the role viruses play in white plague infections. "[The research] is a great addition to the study of coral diseases, as it highlights the potential role of viruses in coral disease," notes Christian Voolstra, a marine scientist at the King Abdullah University of Science and Technology in Saudi Arabia who did not partake in the study. Soffer and her colleagues are currently trying to elicit white plague in the lab by infecting corals with SCSDVs. If Soffer is successful in her attempts then the virus theory will gain further supporting evidence. Soffer also says that future studies on coral diseases will need to place greater emphasis on the role of viruses. "Viruses can be tracked down to a source," adds Soffer. "If the viruses behind white plague are tracked down to, say, human sewage, then we may have a way to mitigate disease infections." Your move SCSDVs.
References:
Castro, J. "Coral ‘white plague' epidemic could be caused by virus." October 15, 2013.
National Oceanic and Atmospheric Administration. "Coral Reef News." NOAA's Coral Reef Conservation Program. March, 2005.
Oregon State University. "Viruses associated with coral epidemic of ‘white plague'." Oregon State University. September 12, 2013.
Images:
1. Oregon State University (via Flickr).
2. Oregon State University (via Flickr).
