What was done
Among the reef corals at Phuket, Thailand is a shallow-water Indo-Pacific coral, Goniastrea aspera, which traditionally suffers annual coral bleaching during the months of January through March on its west-facing slopes as a result of high solar irradiance.  However, during a period of anomalously high sea surface temperatures in May 1991 and again in May 1995, bleaching of G. aspera was limited to its east-facing surfaces only.  In an effort to explain this phenomenon, the authors did three things.  First, they looked for differences in symbiont algal genotypes on the east and west sides of the coral.  Second, they looked for differences in the amounts of solar radiation received by the east and west surfaces of the colonies during the bleaching event of May 1995.  Third, they subjected core samples from the east and west surfaces of G. aspera colonies to two different temperature regimes - ambient (27°C) and elevated (34°C) - under identical irradiance conditions for 3 days.

What was learned
Previous analyses had indicated a differential bleaching susceptibility among algal symbiont types of the coral Montastrea annularis, but as no genetic differences of this kind were found on the G. aspera corals, the authors were left to conclude that "the bleaching pattern observed in G. aspera is not due to genetic differences between their algae."

It was also determined that the east coral surface did not receive more solar radiation than the west at the time of the May 1995 bleaching event.  Neither were any differences in algal symbiont density or chlorophyll a content noted between the east and west coral surfaces in the ambient temperature treatment of their temperature study.  However, exposure to the elevated temperature resulted in the east-facing cores experiencing a significantly reduced algal density and an increase in the chlorophyll a content per algal cell, whereas the west cores remained unaffected.

What it means
The results of this study suggest that "the west surfaces of G. aspera colonies could have been protected against temperature bleaching in May 1995 because they were more tolerant than the east surfaces towards the combined stress of temperature and solar radiation," which tolerance "probably arose as a result of the increased solar radiation received by the west surfaces during January - March."  This ability of this coral and its symbionts to adapt to such temperature stress bodes well for the future.  If this adaptive mechanism is operational in other coral species, we may soon see a decline in the severity of the temperature-induced bleaching episodes that have plagued many reefs over the past two decades as corals begin to better tolerate such events.