What was done
The authors of this study examined in situ the effect of seawater temperature on the bleaching of the coral Oculina patagonica in the Mediterranean Sea from 1994-1996.  They also conducted aquarium experiments for a period of 45 days at controlled temperatures of 16, 20, 25, and 29°C.  Coral bleaching in the controlled experiments was induced by the bacterial agent Vibrio AK-1.

What was learned
Coral bleaching in the Mediterranean Sea was closely correlated with temperature.  The number of colonies showing bleaching rose in the summer, when seawater temperatures were about 28°C, to a high of around 80%; and it fell in the winter, when seawater temperatures were about 17°C, to values less than 20%.  Ninety-two percent of the corals that were infected with the bacterial agent Vibrio AK-1 and grown in the laboratory at 29°C experienced some degree of coral bleaching by 22 days into the experiment, whereas only 45% and 28% of the corals grown in water temperatures of 25°C and 20°C experienced any bleaching.  Furthermore, none of the corals in the controlled experiment with water temperature of 16°C experienced any coral bleaching over the 45 days of study.

What it means
This study demonstrates the temperature dependence of a specific bacteria-induced bleaching of the coral Oculina patagonica.  The authors note that similar correlations between seawater temperature and coral bleaching have led many people to "speculation that increased seawater temperature, resulting from global warming or El Niño events, is the direct cause of coral bleaching."  However, they continue, "coral bleaching is not necessarily the direct response to elevated seawater temperature," citing a number of studies of coral bleaching events not associated with any major sea surface temperature anomalies.  And they explicitly state that "it is not yet possible to determine conclusively that bleaching episodes and the consequent damage to reefs is due to global climate change."