Bacteria not the primary cause of coral bleaching

Stress factors that cause the breakdown of symbiosis between corals and their endosymbionts is what causes coral bleaching to occur. Rises in sea temperature have been correlated to mass coral bleaching events, however it has been suggested that bacterial infection of corals may also cause bleaching. Ainsworth et al. (2008) investigated the microbial ecology of the coral Oculina patagonica during an annual bleaching event to map bacterial populations within the tissue levels and determine whether they were related to the bleaching process.

The study was conducted at Sdot Yam on the Mediterranean coast of Israel between June and August 2005 during the annual bleaching event. Samples from 140 Oculina patagonica corals were taken every two weeks, with three replicate core samples from each tissue region (bleached and unbleached) from each coral colony. Control samples were collected from unbleached colonies in the surrounding area. Fluorescence in situ hybridization was conducted to analyse the identity and structure of the microbial communities in association with the coral tissue. Transmission electron microscopy was also used to assess the structure of the coral tissue and the location of the endosymbionts within the tissue.

The main finding was that Vibrio shiloi was not present when coral bleaching occurred or during coral death as a result of bleaching and therefore does not support the Bacterial Bleaching Hypothesis. They did find endolithic bacterial communities to associate with coral tissue, with a shift occurring from cyanobacterial dominated communities to green algal dominated communities during the process of bleaching. This demonstrates the complex interactions of microbial communities of the Oculina patagonica, and although are thought to be of importance, the specific nature of these bacterial colonies still remains unknown. Nonetheless, it is thought that environmental stressors are the main trigger of coral bleaching and the subsequent colonisation of opportunistic bacteria may also play an important role. Although this study showed Vibrio shiloi to not be involved in the primary cause of coral bleaching, further studies are required to determine if bacteria perhaps have a secondary role in the bleaching process and whether microbial treatments would be effective to decrease the severity of coral bleaching.

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

http://www.nature.com/ismej/journal/v2/n1/full/ismej200788a.html