Corals are living organisms and as such they are susceptible to diseases. High temperatures can affect the severity of diseases. High temperature is a stressor and as such it can decrease an organisms immune response and increase the virulence of a pathogen. This relationship between temperature and disease severity has been found in both terrestrial and aquatic systems1. There are a variety of different coral diseases2. A number of recent studies indicate that warmer water and bleaching events may help disease causing-organisms attack corals2. A link between temperature and severity of coral disease has been investigated through many different studies that have shown that diseases are spread more quickly throughout a colony during the summer1. These pathogens can have devastating effects on the ocean’s coral reefs, affecting not only the corals but many other organisms that depend on coral reefs2.
Acropora palmata, or Elkhorn coral found in Florida have been particularly affected by white pox disease (figures 1 and 2)3. Kathryn Patterson and colleagues studied the growth of lesions on Elkhorn Coral colonies in the Florida Keys National Marine Sanctuary3. They used photographs and videos taken between 1996 and 2000 to test the dispersion of the disease and tissue loss on infected colonies. The team found that some lesions increase in size very quickly3. These lesions are less common than the more slowly progressive lesions, but are still the most important to coral tissue death3. The more common lesions, which increase at a much slower rate but are more numerous, have less of an impact on coral tissue death3. Patterson and colleagues also determined that the disease was caused by the bacteria S. marcescens, a bacteria commonly found in human fecal matter; therefore, sewage may be the source of the microbe responsible for the S. marcescens disease known as white pox disease3. Their most important result, however, was that loss of living coral due to white pox disease was greater during the summer months than during the winter months3. This result indicates that tissue loss due to white pox disease is correlated with temperature3. As ocean temperatures rise due to climate change, the effect of diseases such as white pox will be magnified and occur more rapidly. In short, diseases will be more deadly.
Figure 1: Healthy Elkhorn coral colony (Acropora palmate). Photo by US Fish and Wildlife Service. Licensed by CC BY 2.0.
Figure 2: An Elkhorn coral colony with white pox disease. White pox disease is characterized by white patches or lesions and tissue loss everywhere on the coral colony. Photo by Jim Stuby. Licensed by Public Domain.
Research by John Bruno and his colleagues suggests that both temperature and coral cover, the percentage of an area that is covered by live coral colonies, affect the frequency of coral disease1. Other studies have shown that diseases spread more quickly throughout coral colonies in the summer and this could be explained by higher temperatures, but it could also be explained by other seasonal factors1. Bruno and his colleagues wanted to specifically see if temperature had any effect on the frequency at which diseases developed1. To investigate this aspect of disease, they developed studies to look at the relationship between how frequently white syndrome developed in certain kinds of corals and when warmer than usual water temperatures occurred across the Great Barrier Reef1. White syndrome is a disease of corals that is characterized by a white band of exposed coral skeleton that progresses across a coral colony1. It has been found in 17 species of Pacific reef-building corals1. The scientists looked at forty-eight reefs for six years, in particular, studying the times when the sea surface temperature was abnormally high1. They found that the frequency of disease increased 20 times over the usual progression of the disease in years following a particularly warm summer1. Moreover, the frequency of disease varied between reefs with different coral cover1. Those reefs that had the highest coral cover also had the greatest frequency of white syndrome1.
These researchers believe that the higher temperature may have caused physiological stress which compromised the corals’ immune systems1. Corals may need the cooler waters during wintertime to attract greater amounts of helpful symbiotic algae present in their tissues as well as promote greater growth of the coral tissue, both of which are needed to help the corals fight disease1. Warmer winters may inhibit algae accumulations and growth, and the corals may therefore be more susceptible to disease and growth of pathogens1.
High coral cover may help white syndrome to spread from colony to colony. One way that coral cover may help diseases spread is through the so-called “line of death”1. Although corals appear to be lifeless rocks, they are vicious defenders of their space when they come into contact with another colony1. They use stinging tentacles to harm competitors, creating lesions on their competitors1. Pathogens may be able to take advantage of these lesions to infect the colony1. The disease was the most widespread during the warmest summer throughout the study1. Bruno and his colleagues suggest that coral cover was such an important part of predicting disease outbreaks that warmer temperatures may actually inhibit white syndrome by decreasing coral cover due to mortality from bleaching1. This hypothesis has not been tested, but at the end of the day, increased coral mortality is increased coral mortality, no matter the source. In addition, while white syndrome may be heavily influenced by coral cover, other diseases may not be affected at all and may be able to take advantage of the weakened coral caused by higher temperatures.
So what does all of this mean in terms of global warming? It means that not only do corals have to deal with bleaching and the stress of higher temperatures, but also this stress may help them get sick and may play a part in the severity of the disease. The Great Barrier Reef is a marvelous natural environmental resource for many organisms that depend on it for food sources as well as protection. Its loss, or even loss of part of it, would have devastating effects not only on oceanic organisms and animals, but would be a severe loss to humanity as well. Coral reefs provide a livelihood for fishermen and a great source of tourism dollars. We must address global warming and the subsequent warming of the oceanic waters to prevent such an overwhelming loss.
Footnotes:
- Bruno, J.F., Selig, E.R., Casey, K.S., Page, C.A., Willis, B.L., Harvell, C.D., Sweatman, H., and Melendy, A.M. (2007). Thermal stress and coral cover as drivers of coral disease outbreaks. PLoS Biol 5, e124.
- Global Warming and Coral Reefs – National Wildlife Federation. https://www.nwf.org/Wildlife/Threats-to-Wildlife/Global-Warming/Effects-on-Wildlife-and-Habitat/Coral-Reefs.aspx.
- Patterson, K.L., Porter, J.W., Ritchie, K.B., Polson, S.W., Mueller, E., Peters, E.C., Santavy, D.L., and Smith, G.W. (2002). The etiology of white pox, a lethal disease of the Caribbean elkhorn coral, Acropora palmata. Proceedings of the National Academy of Sciences 99, 8725–8730.