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.