Corals and their friends...

A review of: The role of microorganisms in coral health, disease and evolution. Rosenberg et al (2007). Nature Reviews – Microbiology.

The coral holobiont contains microbial representatives from all three kingdoms including numerous viruses. Large amounts of molecular oxygen are produced during photosynthesis by the zooxanthellae Symbiodinium, allowing for efficient respiration by the coral and associated prokaryotic organisms. The high concentration of oxygen results in the formation of oxygen radicals which provide protection against infection.

Corals provide 3 habitats for bacteria: the surface mucus layer, the coral tissue and the calcium carbonate skeleton, each of which harbours a distinct bacterial population that differ between each habitat and occupy different niches. The endolithic community found in the skeleton is thought to be crucial to the survival of the coral when it loses its endosymbiotic algae because they provide the coral tissue with organic compounds. So far, little is known about the biological role of archaea in the coral holobiont and recent evidence suggests that they do not form specific associations with corals.

Climate change, over-fishing and pollution are the three most frequently cited stress factors responsible for the increase in the incidence of coral disease.

Expression of crucial bacterial virulence genes is temperature dependant. For example, at warm temperatures, V. shiloi expresses a cell surface adhesion that is required for bacterial adhesion to coral surfaces. At elevated temperatures, the microorganism also expresses Toxin P, which inhibits photosynthesis of the endosymbiotic algae, and superoxide dismutase, which is required for survival inside the coral. Elevated nutrients have been suggested as a cause of reef decline in coastal waters also, increasing the severity of aspergillosis and yellow blotch disease in some field experiments.

Overfishing reduces the number of fish that graze on algae, thereby increasing the amount of algae present on coral reefs, which have been found to influence the corals negatively by some studies – including alleopathy, shading, smothering, abrasion, overgrowth and harboring potential pathogens.

Corals possess innate or natural immunity. Their defenses include physical barriers such as the epidermis and mucus, cellular components (phagocytic cells) and soluble factors, including organic acids and anti-microbial products. It has also been found that a large proportion of the bacteria colonizing the mucus layer of some corals produce antibiotics, suggesting that coral-associated bacteria inhibit pathogen invasion and actively contribute to infectious disease resistance.

The ability of coral to adapt to environmental stresses, including elevated temperature conditions and infection to specific pathogens (such as V.shiloi) has lead to the development of the Coral Probiotic Hypothesis. It postulates that ‘a dynamic relationship exists between symbiotic microorganisms and corals at different environmental conditions that selects for the most advantageous coral holobiont in the context of the prevailing conditions’. By altering the structure of its resident microbial community, the holobiont can adapt to changing environmental conditions more rapidly and with greater versatility than on a process that is dependent on genetic mutation and selection of the coral host.

The authors conclude that when corals are exposed to environmental stresses, the microbial population undergoes a change, which can lead either to adaptation to the new condition or to coral disease.

I thought this paper explained the recent findings in coral disease and coral dynamics in a clear and simple manner. It brings in a lot of recent discoveries (although there have probably been quite a few more since 2007) and explains papers with short, clear summaries. I would suggest to anyone struggling with the basics of corals and their interactions with it symbionts and the hypotheses surrounding their survival mechanisms to give it a read.