The role of microorganisms in coral bleaching

Coral bleaching is the disturbance of the symbiotic relationship between the coral and its endosymbiotic zooxanthellae (of the genus Symbiodinium). The severity of the disease is often correlated with high seawater temperature; and in recent years the main hypothesis to explain coral bleaching is that the high temperature of the water causes irreversible damage to the symbiotic algae, resulting in loss of pigment/ algae from the holobiont- the results of this is that the coral tissue becomes transparent, showing the calcium carbonate skeleton underneath; other signs of bleaching include reduction in mucus and often inhibition of sexual reproduction. If bleaching is not reversed then corals will die. This paper discusses evidence for an alternative hypothesis- the microbial hypothesis of coral bleaching.

Although the running hypothesis is mainly based on a raise in seawater temperature, other findings have proven that salinity, cyanide exposure, sediments and seawater temperature decrease have all been causes of bleaching as well as the often disregarded microbial pathogens.

Coral bleaching was discovered in the Eastern Mediterranean Sea, as it occurs every summer amongst the species Oculina patagonica. It was found that an infection by Vibrio shiloi was the cause using ‘Koch’s postulates’ (a criteria for establishing a causal relationship between a causative microbe and a disease), but the effects could only occur if both the causative agent was present and an elevated temperature of above 25^oC. The pathogenic bacteria are chemotactic to the coral mucus and adhere to the β-galactoside-containing receptor, on the coral surface; they penetrate through into the epidermal layer and then multiply intracellularly (reaching 10⁸- 10⁹ cells per cm³). The V. shiloi produce an extracellular peptide toxin (PYPVYPPPVVP) which inhibits photosynthesis in the alagae. All of the reactions are temperature dependent, relying on the conditions being 25-30^oC, explaining why it only occurs in the summer and not all year round.

The authors consider the ideas of coral bleaching being due to ultraviolet radiation, however, they discuss a paper that argues against this, by using samples of corals from high ultraviolet radiation points (water 0-80cm in depth) with results showing that UV radiation actually inhibited coral bleaching by killing the pathogen.

O. patagonica is not actually a reef coral, so to test the theory of microbial bleaching, it was important to check the hypothesis on a reef coral; for this they chose Pocillopora damicornis, from the Zanzibar coral reef. It was shown to have also been bleached by a pathogenic species of Vibrionacea known as Vibrio coralliilyticus.

There are two main and differing viewpoints on the matter of coral bleaching.

Most coral biologists take the view that high temperatures and light act directly on the symbiotic algae to inhibit photosynthesis and produce reactive oxygen species. According to this idea, microbes play no role in the bleaching of the corals, and the change in microbial community is a result and not a cause.

The second hypothesis which is possible (and is taken to be the view of the authors of this paper) is that high temperatures act on the coral microorganisms as well as the host, causing a change in microbial community that can either directly or indirectly lead to bleaching.

Clearly there is not enought evidence to rule out one hypothesis and further research needs to be conducted, combining coral microbiology together with coral host physiology is required to clarify the bleaching process.

A review of:

Rosenberg, E., Kushmaro, A., Kramarsky-Winter, E., Banin, E., and Yossi, L. (2009). The role of microorganisms in coral bleaching. The ISME Journal. Vol. 3. pp. 139-146