Close your eyes and picture a Caribbean coral reef.
You probably saw a beautiful landscape, with colorful patterns and exotic fish. But the sad truth, is that it is not all rainbows and butterflies. Lately, coral reefs have been attacked by infectious diseases and are currently facing deterioration. The Caribbean reefs have been referred to a “disease hot spot”1 and rightfully so. Over the past 30 years, they have lost 80% of their coral cover, leaving the coral vulnerable.2
Corals are living organisms that heavily rely on their environment and their surroundings to survive and thrive. Important factors to keep in mind are water temperature, chemistry and light. Inherently, under prime conditions an abundance of corals will flourish. However, this also allows for a prime environment where diseases are more likely to spread amongst the breadth of corals. It is easier for a disease to spread, within the same species (Figure 1). Therefore, if there is a primary coral in the coral reef that makes up the majority, they are more likely to be attacked.3
Figure 1. Right of coral being affected by Black Band Disease and the left has Christmas treeworms. Copyright: “Causes of Coral Disease.” Reef Resilience. The Nature Conservancy, 30 Aug. 2016. Web.
Now, imagine lighting a corner of a paper and observing the flame spreading and consuming the rest of the paper…this is what happens when a coral is affected by the Black Band Disease (BBD), the process just occurs across a span of months. This disease is due to bacteria overtaking the coral and exposing it to hydrogen sulfide and leaving it with no access to oxygen.4 Where the bacteria is attacking the coral at the moment a black band is seen and coral it has already affected is a pale white, this is depicted in Figure 2. The first case of BBD was reported in the 1970s and it was assumed that the bacteria, Phormidium corallyticum, was the root of evil.5 However, when researchers took a further look into the composition of the bacteria that makes up the black band, there were several cyanobacteria present. These attack the coral tissue and leave the coral’s skeleton exposed.5 The abundance of certain bacteria fluctuates between the different geographic locations around the Caribbean Sea. This group of bacteria forms a band and digests the tissue as it progresses along the coral colony.
Figure 2. Experimental site where surface area of coral affected by BBD is being measured. Copyright: Voss, Joshua D., and Laurie L. Richardson. “Nutrient Enrichment Enhances Black Band Disease Progression in Corals.”Coral Reefs. (2006) 25: 569.
BBD has been seen to affect other species of corals that grow close to the coral originally affected. Once affected by BBD, it is difficult for the coral to survive. If affected for several months, the coral colony dies.4 There are cases however, where if the cyanobacteria invader goes away, there is a possibility that the coral recover5; but it will never be the same.
In general, the decline in water quality, allows for the perfect environment for bacteria and microbes that attack corals, and it is linked to human pollution and increased water temperatures.The origin of this cyanobacteria or how it begins to attack the coral, still remains unclear. But there has been a correlation between the presence of coallivorous fish and the presence of BBD.3 Through studies, there has been proof that nutrient high environments help speed up the rate at which BBD affects the coral, partially because of increased nitrate production.2 Furthermore, there is a correlation between warmer seawater temperatures and the abundance of BBD amongst coral colonies as seen in Figure 3.6
Figure 3. a) Seawater Temperature b) Amount of colonies affected by BBD. Copyright: Edmunds, Peter J. “Extent and Effect of Black Band Disease on a Caribbean Reef.” Coral Reefs (1991): 161-65.
Corals are sensitive animals that are part of a delicate ecosystem and they must be taken care of. In 1991, BBD was still not a major concern and affected very few coral reefs; it was even seen in the perspective of diseases serving as a natural regulator for marine environments. 6 Now the position is very different and as noted above BBD had become a prevalent issue. So moving forward, scientists are researching the specifics of the origin of the disease and looking for ways to control the environmental factors that exacerbate the spread of the BBD.
1. Weil, E., G. Smith, and Dl Gil-Agudelo. “Status and Progress in Coral Reef Disease Research.”Diseases of Aquatic Organisms 69 (2006): 1-7. Web.
2. Voss, Joshua D., and Laurie L. Richardson. “Nutrient Enrichment Enhances Black Band Disease Progression in Corals.”Coral Reefs. (2006) 25: 569.
3. “Causes of Coral Disease.” Reef Resilience. The Nature Conservancy, 30 Aug. 2016. Web. <http://www.reefresilience.org/coral-reefs/stressors/coral-disease/causes-of-coral-disease/>.
4. “Causes of Coral Disease.”Reef Resilience. The Nature Conservancy, 30 Aug. 2016. Web. 21 Feb. 2017. <http://www.reefresilience.org/coral-reefs/stressors/coral-disease/causes-of-coral-disease/>.
5. Frias-Lopez, Jorge, George T. Bonheyo, and Qusheng Jin And. “Cyanobacteria Associated with Coral Black Band Disease in Caribbean and Indo-Pacific Reefs.” Applied and Environmental Microbiology. 69.4 (2003) 2409-2413. Web.
6. Edmunds, Peter J. “Extent and Effect of Black Band Disease on a Caribbean Reef.” Coral Reefs3 (1991): 161-65. Web.
