So whats still bleaching Oculina patagonica if Vibrio shiloi's not?

Note: this blog post is a tad longer than normal, but all information I felt was important to mention.

Even though the coral Oculina patagonica has developed resistance to infection by the coral pathogen Vibrio shiloi, it continues to demonstrate seasonal bleaching. These findings raise 2 important questions: (1) What is the current mechanism of bleaching of O. patagonica every summer in the eastern Mediterranean Sea? (2) How did the coral become resistant to V. shiloi infection and bleaching? Mills et al., 2013 provide evidence to explain the role of bacteria in the bleaching process of O. patagonica. It must be remembered that V. shiloi still adhere to the coral and penetrate the tissue, but it is then rapidly killed showing the corals defence mechanisms; which is surprising, because corals do not produce antibodies and are considered to lack an adaptive immune system.

A bleaching survey was first carried out in at Sdot Yam, Mediterranean off the coast of Israel which showed a clear correlation with bleaching profiles previously documented when V. shiloi was the infectant. Bleached colonies were often found next to or surrounded by apparently healthy colonies and bleached corals were on average larger than unbleached corals. This could lead to assumptions and further research that infections could be linked to coral age and it may not be passed on very easily.

Healthy fragments of O. patagonica in 11 aquaria were exposed to Nalidixic acid (Appendix I) for 24 hours, which was then removed and then the temperature was increased gradually (1°C) until it reached 31°C.  Infections with V. shiloi and strain EM1 were performed after the antibiotic had been removed (these strains were isolated from crushed fragments of O. patagonica) and temperatures had reached 29°C.  Also 8 aquaria were just exposed to the gradual increase of temperature. Both types of aquaria were left for 2-3 weeks. Zooxanthellae were also counted from tissue samples.

To test the hypotheses that corals now contained a bleaching bacterium other than V. shiloi and acquired one or more strains of bacteria that inhibit the growth of V. shiloi, several bacterial strains were isolated from crushed O. patagonica and checked their ability to bleach and/or their anti-V. shiloi activity. Out of 5 isolates tested, only 1 strain, EM1, bleached antibiotic-treated corals and reduced the zooxanthellae count by 74% (Table 1).  Strain EM1 were plated on TCBS Agar and based on its 16S rRNA gene sequence EM1 is a strain of Vibrio coralliilyticus (showing 99.8% identity to V. coralliilyticus). These data are suggestive but insufficient to demonstrate that V. coralliilyticus is the causative agent of the seasonal bleaching disease of O. patagonica. It was also clearly shown in (Table 1) that the beneficial bacteria that had caused the corals to become resistant to V. shiloi infection and bleaching were killed by nalidixic acid. Zooxanthellae counts were much lower in the EM1 strain treatment (Table 1) possibly indicating that the it is a more damaging bacteria than V. shiloi.

Table 1 presents the results. As you can see there is something else bleaching Oculina patagonica other than V. shiloi which could be the EM1 strain. 

One other strain, referred to as EM3, consistently inhibited the growth of V. shiloi after inhibition was observed on streak-plates of DNA this indicates the diffusion of an antibacterial compound. The inhibition by EM3 appeared to be specific to V. shiloi, but the chemical structure and mode of action of this material is not yet known. Further investigation into this strain would determine whether this was the antibiotic that saved the corals from V. shiloi bleaching.

This study perfectly links to the previously disputed coral probiotic hypothesis that stems from the concept of the coral holobiont. This is when the coral functions as the sum of the coral host and all of its symbiotic microorganisms, which means here that the coral acquired beneficial bacteria that inhibit infection and prevent bleaching.

It must be kept in mind that other mechanisms could be involved, such as temperature-induced virulence of certain viruses. Clearly, further multidisciplinary research, including a combination of coral microbiology together with coral host physiology is required to clarify the coral bleaching disease process. It will be interesting to determine whether antibiotics inhibit the bleaching of other corals too. But overall, the possibility that V. coralliilyticus could be involved in the bleaching is interesting as it is previously been know to be involved in necrosis, which shows the adaptive abilities of bacteria.

Reference: Mills, E., Shechtman, K., Loya, Y., & Rosenberg, E. (2013). Bacteria appear to play important roles both causing and preventing the bleaching of the coral Oculina patagonica. MEPS, 489, 155-162.

Can be found at: http://www.int-res.com/abstracts/meps/v489/p155-162/

Appendix I: Nalidixic acid was chosen as the antibiotic of choice after preliminary experiments indicated that it was efficient at killing coral-associated bacteria and had no apparent deleterious effect on the corals.