Bacterial Species and Their Implications in Coral Death

Various bacterial species have implications incoral-reef2.jpg

both coral health and death and methods to understand the microbial communities and disease processes of corals worldwide are becoming increasingly important. Coral reefs are an integral part of the marine ecosystem, providing habitats for a diverse spectrum of life, controlling the level of carbon dioxide in the ocean, and protecting coasts from strong currents. Researchers studying the normal microbial associations and disease-causing interactions are uncovering new and fascinating

information that changes our way of viewing coral health and death.



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Corals and Their Microbial Communities- Background

Corals provide three general habitats for bacteria: the surface mucus layer, coral tissue, and the calcium carbonate skeleton, each of which has a distinct bacterial population (Bourne and Munn, 2005; Koren and Rosenberg, 2006). According to Rosenberg, et al., some generalizations about coral and their normal bacterial associations are beginning to emerge:

(1) the variety of bacterial species associated with a particular coral species is high, including a many new species
(2) bacteria in a given coral community are different from those in the seawater surrounding the coral; thus, the microbe-coral association is specific
(3) the cultured bacterial population is completely different from the species composition of the uncultured population
(4) similar bacterial populations are found on the same coral species, regardless of geography, and different bacterial populations are found on different coral species
(5) different bacteria are found in the mucus layer as well as tissue from the same coral fragment

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Viruses, Bacteria, and Archaea. Photo courtesy of Rosenberg et al., 2007





Viruses are a common component of the coral reef environment, but are usually associated with the bacterial communities inhabiting the coral (Rosenberg, Kellogg, and Rohwer, 2007). Corals are also home to protists, algae, and non-specific archaea.












I. Coral White Plague Disease - White plague is similar in appearance to White Band Disease, but it affects different species. The disease is characterized by an sharp white line or band, exposed coral skeleton that separates living tissue from algal-colonized skeleton, and possibly a narrow band of bleached tissue is visible adjacent to exposed skeleton. Typically initiating at the base of a coral colony, the condition spreads quickly upward and outward. There are three types of Coral White Plague: I, II, and III, with type II and III causing higher rates of morbidity (Richardson and Aronson, in press). The disease was first discovered in the Florida Keys and has been seen in more than 40 species of coral around the world. The particular type and species discussed here is type II, is caused by the c-proteobacterium Thalassamonas loyana (Efrony et al., 2007), and occurs on the Eilat coral reef (Gulf of Aqaba, Red Sea) in the species Favia favius.

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Coral White Plague Disease of //Favia favius//. Photo courtesy of Rosenberg et al., 2007
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Coral White Plague Disease. Photo courtesy of A. Bruckner, CORIS, NOAA
















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Coral White Plague. Photo courtesy of NOAA.




A. Bacterial species identification - Thalassomonas loyana

The white plague-like disease of the coral Favia favius by the gram negative pathogen T. loyana was studied using phage therapy. This was a new characterization of the pathogen; T. loyana is a new genus and species as of 2005 (Barash et al., 2005).
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White Plague Pathogen. Photo courtesy of NOAA

1. Materials and methods

The researchers first sequenced the BA3 genome in order to determine the processes necessary for bacterial adsorption of the phage and to monitor mutation rates. Next, 12 corals showing the disease and healthy F. favius were installed in aquaria and monitored for 48 hours. After 48 hours, three aquaria were inoculated with phage BA3 immediately, three aquaria were inoculated with phages 1 day after the infection, three were treated with phages 2 days after the infection, and three aquaria served as no-phage controls. The phage inoculation concentration in all treatments was 5 x 10E3 phages ml-1, the minimum number required to reproducibly achieve protection based on preliminary studies.

After 37 days all the corals treated 1 day after infection were crushed and the average phage titer in the crushate was determined. In all infection experiments, the percentage of healthy corals was determined visually, both directly and after photography.

Phage Protection Against Transmission of Disease from Coral to Coral - After the first changing of the aquarium water (day 3) of the above-described phage therapy experiment, new healthy corals were added to the aquaria (three corals to the phage-treated coral aquarium and two corals to the control aquarium). Transmission of the disease was observed by examining the condition of the freshly added corals (Efrony et al., 2008).

2. Results

The coral white plague disease progresses rapidly. In the control group, with no phage addition, the coral tissue was observed to have been lysed after only 4-6 days. Phage BA3 was not found in either healthy corals or in diseased corals that were not inoculated with BA3. When corals were treated 1 day after infection, they were protected by the phage for at least 37 days. However, the corals inoculated 2 days after infection were not protected (see photo). It was determined that the time of addition of the phage is crucial to prevent coral disease.

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Photographs of corals during the phage protection experiment. Photo courtesy of Efrony et al., 2008


The water was changed in the aquaria several times during the 37 days; however, the phage remained present in the water for the duration of the experiment. Healthy corals placed in an aquarium with diseased corals became infected and died within 2 weeks, showing a rapid transmission of the disease. However, both sea-infected and laboratory-infected corals that were treated with the phage, when placed in an aquarium with healthy corals, did not transmit the disease.

Because the phage addition on day two did not prevent coral tissue loss and death, it is suggested that the phage therapy would be useful in preventing the spread of the disease, rather than curing an already infected coral. Since positive results were observed when diseased corals treated with the phage failed to infect healthy corals in the aquarium, it is possible that the phage therapy is a means to reduce the disease to manageable levels in localized areas (Efrony et al., 2008).


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Eilat Coral Reef from above. Photo courtesy of bibleplaces.com
3. Implications for field trials -- Due to the mutation rates of BA3, it is uncertain how phage therapy will progress in the field, or at what concentrations the phage titer will be most efficient. Clinical field trials will need to be initiated to determine the efficacy of phage therapy in treatment of Coral White Plague disease.
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Eilat Coral Reef from above. Photo courtesy of Auscillate.com


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Along the Eilat Coral Reef. Photo courtesy of businessweek.com.

Sources:
Bourne, D.G., and C.B. Munn. 2005. Diversity of bacteria associated with the coral Pocillopora damicornis from the Great Barrier Reef. Environmental Microbiol 7:1,162–1,174.

Rosenberg, E., A. Kellogg, and F. Rohwer. 2007. Coral Microbiology. Oceanography 20, 2:146--154.


Richardson, L.L. and R.B. Aronson. In press. Infectious diseases of reef corals. Proc. 9th Intl. Coral Reef Symp., Indonesia.

Efrony, R., I. Atad, and E. Rosenberg. 2008. Phage Therapy of Coral White Plague Disease: Properties of Phage BA3. Curr Microbiol 58:139–145.

Barash Y., R. Sulam, Y. Loya, E. Rosenberg. 2005. Bacterial Strain BA-3 and a filterable factor cause a white plague-like disease in corals from the Eilat coral reef. Aquat Microbiol Ecol 40:183–189.


For More Information: Bacterial Species and Their Importance in Coral Death 2

How Stuff Works: Coral Reefs

Coral Disease Information

Coral White Plague Disease Information