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The role of microorganisms in coral health, disease and evolution

Key Points

  • Scleractinian (hard) corals live in close association with abundant and diverse microorganisms. Different groups of microorganisms are present in the mucus surface layer, skeleton and tissues.

  • These symbiotic microorganisms benefit their host by various mechanisms, including photosynthesis, nitrogen fixation, digestion of complex nutrients and prevention of infections by pathogens. Conversely, under environmental stress conditions, certain microorganisms can cause coral bleaching and other diseases.

  • Climate change, water pollution and over-fishing are the three most frequently cited environmental stress factors participating in the rise of infectious diseases of coral. Recent studies have provided direct experimental evidence demonstrating how each of these factors contribute to microbial induced coral disease.

  • In the case of the best-studied coral disease, bleaching of Oculina patagonica by Vibrio shiloi, many of the virulence mechanisms have been shown to be induced by elevated temperature.

  • In the past five years, O. patagonica has become resistant to V. shiloi infection. To explain this finding, the coral probiotic hypothesis has been presented, which posits that a dynamic relationship exists between symbiotic microorganisms and corals at different environmental conditions, bringing about a selection for the most advantageous coral holobiont.

  • Generalization of the coral probiotic hypothesis has led us to propose the hologenome theory of evolution. This theory proposes that microorganisms have an important role in the evolution of animals and plants.

Abstract

Coral microbiology is an emerging field, driven largely by a desire to understand, and ultimately prevent, the worldwide destruction of coral reefs. The mucus layer, skeleton and tissues of healthy corals all contain large populations of eukaryotic algae, bacteria and archaea. These microorganisms confer benefits to their host by various mechanisms, including photosynthesis, nitrogen fixation, the provision of nutrients and infection prevention. Conversely, in conditions of environmental stress, certain microorganisms cause coral bleaching and other diseases. Recent research indicates that corals can develop resistance to specific pathogens and adapt to higher environmental temperatures. To explain these findings the coral probiotic hypothesis proposes the occurrence of a dynamic relationship between symbiotic microorganisms and corals that selects for the coral holobiont that is best suited for the prevailing environmental conditions. Generalization of the coral probiotic hypothesis has led us to propose the hologenome theory of evolution.

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Figure 1: The structure of coral tissue.
Figure 2: Infectious diseases of coral.
Figure 3: Infection of the coral Oculina patagonica by Vibrio shiloi.
Figure 4: Coral resistance to Vibrio shiloi infection.

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Acknowledgements

This research was supported by funds provided by the Israel Center for the Study of Emerging Diseases and the Coral Reef Targeted Research Program (CRTR). The CRTR is a partnership between the Global Environment Facility, the World Bank, the University of Queensland, NOAA and research institutions.

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Glossary

Scleractinian coral

Scleractinian, stony or hard corals as they are often referred to, are animals that are responsible for building coral reefs.

Mutualistic interaction

A close ecological relationship, between two (or more) species, from which both species benefit.

Symbiodinium

(Gr. Symbion living together and Gr. dinos whirling). A genus of dinoflagellate algae. It is the dominant genus of algal symbiont in reef-building corals.

Surface mucus layer

A chemically complex viscoelastic gel layer that surrounds coral. Much of the mucus originates from zooxanthellae. It is secreted from epidermal mucus cells and subsequently modified by resident microorganisms.

Holobiont

The host organism and all of its associated symbiotic microorganisms.

Oxygen radical

An atom or group of atoms that have one or more unpaired electrons. A prominent feature of radicals is that they have extremely high chemical reactivity.

SYBR gold staining

A technique for counting bacteria and viruses in environmental samples. Particles that contain either DNA or RNA emit a bright and stable yellow-green fluorescence that can be enumerated by epifluorescence microscopy.

Endolithic community

A group of organisms that live inside the pore space of rocks, in this case the space within the coral skeleton.

Coral bleaching

The whitening of corals due to the loss of their symbiotic zooxanthellae or the pigments associated with the algae.

Koch's postulates

The four criteria designed to establish a causal relationship between an infecting microorganism and a disease.

Heterotrophic bacteria

Microorganisms that use organic molecules as their main source of carbon and energy.

Alleopathy

The harmful effect of one organism to another caused by the release of chemical compounds.

Viable-but-not-culturable (VBNC) state

When in this state, bacteria can no longer grow and form colonies on conventional culture media, but they show metabolic activity, maintain pathogenicity and, in some cases, return to active growth under appropriate conditions.

Hologenome

The combined genomes of the holobiont.

Commensalism

A symbiosis in which one organism is benefited and the other is neither benefited nor harmed.

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Rosenberg, E., Koren, O., Reshef, L. et al. The role of microorganisms in coral health, disease and evolution. Nat Rev Microbiol 5, 355–362 (2007). https://doi.org/10.1038/nrmicro1635

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