http://www.reefkeeping.com/issues/2002-03/eb/index.php
The Amazing Case of Vibrio shiloi
Several years ago, an article appeared that described the bacteria,
Vibrio shiloi, as causing bleaching in the Mediterranean coral,
Oculina patagonica (Kushmaro 1996). ...
However, one could have heard a pin drop during the elegant and outstanding presentation by Dr. Eugene Rosenberg of Tel Aviv University (Rosenberg 2002). This man single handedly threw the proverbial monkey wrench into the coral world that morning. In the years since the original articles have been published, Rosenberg's team has not only fulfilled Koch's postulates for this pathogen in a textbook-like fashion, but has proceeded to describe the etiology in an extremely impressive manner. I would urge those interested to read the articles listed below that relate to this disease. ...
In short, Vibrio shiloi is a newly described species of bacteria, related to V. mediterranei, with an as-yet undetermined reservoir; that is, it is not known where or if the presence of this bacteria is normal to the environment, or if it is somehow just recently showing up to affect the area. It follows the temperature cycles of the area precisely, and causes bleaching in warm months followed by recovery as the water temperature declines....
A new species of bacteria, Vibrio corallyticus, was consistently found in the tissues of the bleached Pocillopora at a level that already fulfills the first of Koch's postulates. The virulence is even more amazing. At 23° C, there are no visible signs of disease. At 25° C, bleaching occurs. At 27° C, there is rapid tissue lysis. A virulence factor is being produced by this bacteria that correlates extremely well with the temperatures commonly cited as causing coral bleaching....
However, the importance of looking at bleaching in an entirely new light is now at hand. It has often been questioned why corals in the wild would bleach with only a 1-2° C temperature change when other areas (including tanks) routinely experience far greater vacillations without any bleaching incidence. The fact that virulence can be expressed with this small temperature increase makes such accounts explainable. Furthermore, with temperatures in the oceans having warmed over the past fifty years, and with bleaching events being more common in recent years, the existence of bacterial bleaching under such temperature increases may explain not only the increased incidence of bleaching, but also explain why mortality is so common in some bleaching events while recovery happens in others.
As an example, if corals have been growing in water averaging 26° C, more or less, for the past several thousand years, and over the past fifty years the temperatures in the water are now 27° C. Virulence of a microbe is expressed at 28° C to cause bleaching. Now, it only takes a 1° C change to cause virulence genes to be turned on and cause bleaching, and this occurs much more frequently than the 2° C change that it took previously. Furthermore, if the water temperature gets to 29° C, it may not be that the corals have exceeded their upper thermal limit, but that virulence genes that cause tissue lysis have been expressed. ..
As a final note to this incredible tale, and as if the reader has not had enough already, Rosenberg also found that Oculina in shallow water, even in high temperature and exposed to V. shiloi, rarely bleached. They found that UV radiation acted as an effective sterilizer for V. shiloi on the coral surface!