Like Other Animals, Corals Can Get Diseases

Corals are animals that build structures called coral reefs. Corals live with other organisms, including microbes, such as tiny plant cells, bacteria, and viruses. These partners all depend on each other for food and energy [1]. Unfortunately, this relationship is sensitive and can break down when the coral is stressed. If any partner is stressed, the coral will become more sensitive to diseases. Yes, like all animals, corals can be affected by diseases. Coral diseases are a consequence of changing interactions between the corals, their microbes, and environmental conditions, such as high temperatures or the presence of too many nutrients in seawater. In our study, we investigated one of the most studied coral diseases, one that is caused by a specific group of microbes. This disease is called black band disease (BBD) and it is easy to recognize, because of the visible black bands on the surface of the corals (Figure 1).

What is Causing BBD in the Red Sea?

Because it is common on coral reefs around the world, BBD is the most studied coral disease, and it was also the first to be described [2]. This disease is a serious danger to coral reefs worldwide, because it may lead to the death of corals. Previous studies have shown that the presence of BBD increases during warm summer months [3].

BBD is known as a polymicrobial disease, which means that multiple kinds of bacteria cause this disease. The three types of bacteria that have been found to cause BBD are cyanobacteria (a blue-green type of bacteria), sulfate-reducing bacteria (SRB), and sulfide-oxidizing bacteria (SOB). Like plants, cyanobacteria can use the sunlight to generate sugars and oxygen. Conversely, sulfate-reducing bacteria live in environments that are free of oxygen. Sulfide-oxidizing bacteria by comparison can use chemical energy to make sugars. When they come together, these three types of bacteria create the clear black band on the corals by producing toxic chemicals. When a coral gets sick from BBD, the black band moves across the coral’s surface. This band contains toxic chemicals (sulfides) and has no oxygen, which eventually causes tissue death.

Do the Same Three Types of Bacteria Cause BBD in the Red Sea?

To study BBD in corals from the Red Sea, we collected parts of the black band from the surface of different corals and transferred the samples to small tubes. To figure out which bacteria were present in the samples and to get more information about those bacteria, we analyzed the bacterial DNA. Bacterial DNA, like ours, is made from four molecules called nucleotides (A, adenine; G, guanine; C, cytosine; and T, thymine) that are put together in a unique order for each type of bacteria. A specific region of the bacterial DNA can serve as a barcode or fingerprint that allows scientists to distinguish different types of bacteria. Telling the types of bacteria apart is done by a process called DNA sequencing, which “reads” the order of the four DNA nucleotides present in the fingerprint region of the bacterial DNA. From there, we can look at the different sequences of the bacteria in our samples and identify which types of bacteria are present.

We found that the three main BBD-causing types of bacteria, cyanobacteria, SRBs, and SOBs, all exist in the diseased corals from the Red Sea (Figure 2) [4]. We also calculated the percentage of each type of bacteria in our coral samples (Figure 2). Finally, we compared the bacteria from BBD corals in the Red Sea to the bacteria found in BBD corals around the world and found that they were similar.

Presence of BBD in the Red Sea

To understand how many corals in the Red Sea are sick with BBD, we collected samples and information from 22 reefs along the coast of the central Red Sea (Figure 3). The occurrence of BBD was different between reefs, and only a small percentage of corals were infected. However, toward the southern part of the Red Sea, we found a reef with many diseased corals, and we were interested in why there was so much BBD at this site. After looking into the surrounding environment, the higher incidence of BBD at this site might have been related to the high seawater temperature around this reef and the trash that humans throw into the sea. So, our results and the results of other research groups suggest that keeping the ocean clean helps corals be stronger to fight off diseases.

Conclusion

This article describes an investigation into black band disease of different coral reefs in the central Red Sea. Our results suggest that BBD in the Red Sea seems to be caused by interactions of the same three bacterial groups that cause BBD in other reefs across the world. Although the presence of this disease is overall rare, harmful environmental conditions, such as high seawater temperature and water pollution, makes coral more susceptible to disease. In conclusion, clean oceans make for healthier corals.

Glossary

Bacteria: ↑ Very small organisms that live everywhere and can be good or bad; some bacteria can cause diseases.

Polymicrobial: ↑ The involvement of multiple types of microbes.

Sulfate-reducing Bacteria (SRB): ↑ A group of bacteria that “breathe” sulfur, rather than oxygen.

Sulfide-oxidizing Bacteria (SOB): ↑ A group of bacteria that uses chemical energy to make sugars.

DNA: ↑ The genetic information inside an organism.

Sequencing: ↑ A method that determines the order of nucleotides in DNA.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.