Bacteria are not the primary cause of bleaching in the Mediterranean coral Oculina patagonica

After recently reviewing the paper by Rosenberg, E., et al. published in 2009, titled ‘The role of microorganisms in coral bleaching’ which held the view that high temperatures act upon the coral microorganisms as well as the host, causing a change in microbial community that can either directly or indirectly lead to coral bleaching; I have read a paper from 2008 that opposes the idea that bacteria are the primary cause of coral bleaching.

It is well documented that during times when the sea surface temperature is warm, symbiont photosynthesis is reduced due to an amplified susceptibility to photo-inhibition, which leads directly to active oxygen production and results in the breakdown of the symbiosis. Alternative recent studies have proposed that bacterial pathogens are the primary source of bleaching in reef-building corals. The study explained within this paper aimed to investigate the in situ bacterial involvement, and in situ coral microbial ecology, in ecological patterns of bleaching O. patagonica across the Israeli coastline.

Extensive monitoring of O. patagonica during the bleaching event of 2005 (14/06/2005-22/08/2005) took place along the Israeli Mediterranean Sea. Both bleached and non bleached corals were monitored with samples taken every 2 weeks at Sdot Yam as well as Ashkelon, Bat Yam and Acziv; giving a total of over 140 samples. From each colony 3 replicate core samples were taken from each tissue region. Regions of sampling were designated either bleached or unbleached tissue- and bleached tissue cores were taken around the bleached lesion to ensure both bleached tissue and the active region of bleaching were sampled. Flourescent In Situ Hybridization (FISH) was used with oligonucleotide probes and coupled with spectral imaging to explore identity and structural complexity of the microbial communities. The authors also used transmission electron microscopy to examine intracellular bacterial proliferation. They decalcified the coral cores in 20% EDTA and stained 1μm thin tissue sections with 1% Toluidine blue. Samples were photographed using standard light microscopy, following which ultra-thin sections were viewed in transmission electron microscope at acceleration voltage 90kV and images taken.

The results showed that V. sholoi is not associated with O. patagonica bleaching. No evidence of bacterial populations was found associated with any of the 48 bleached coral samples, 48 samples of unbleached tissue or 48 samples of healthy corals during their study. Each sample was probed with a general probe mix and a Vibrio sp probe, with a resulting 144 FISH experiments conducted on each region type; FISH was even repeated on arbitrarily selected samples. Bacterial populations penetrating or multiplying were not found within either the epithelium or gastrodermis of the bleached regions. The only microbial communities found interacting with and in close association with the tissue layers of field bleached corals were members of the endolithic community. Endolithic communities were found not only on the bleached corals but also healthy corals. They therefore suggest that there is no evidence to support a primary role of bacteria in causing coral bleaching as in the basis of the ‘Bacterial Bleaching Hypothesis’.

If bacteria do not play a primary but rather secondary role during coral bleaching or some diseases (being corals are susceptible to microbial attack during stress) it needs to be determined if the use of microbial remedies on a local or regional scale could reduce the impact of disease events.

A review of:

Ainsworth, T. D., Fine, M., Roff, G., and Hoegh-Guldberg, O. (2008). Bacteria are not the primary cause of bleaching in the Mediterranean coral Oculina patagonica. The ISME Journal. Vol. 2. pp. 67-73.

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