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Acclimation to Thermal Stress in Reef-Building Corals
Reference
Middlebrook, R., Hoegh-Guldberg, O. and Leggat, W. 2008. The effect of thermal history on the susceptibility of reef-building corals to thermal stress. The Journal of Experimental Biology 211: 1050-1056.

What was done
The authors collected branches of the reef-building coral Acropora aspera -- which contains the dinoflagellate symbiont Symbiodinium (clade C3) -- from three large colonies on the reef flat adjacent to the Heron Island Research Station at the southern end of Australia's Great Barrier Reef. Multiple upward-growing branch tips were placed on racks immersed in running seawater within four 750-liter tanks that were maintained at the mean local ambient temperature (27C) and exposed to natural reef-flat summer daily light levels. Then, two weeks prior to a simulated bleaching event -- where water temperature was raised to a value of 34C for a period of six days -- they boosted the water temperature in one of the tanks to 31C for 48 hours, while in another tank they boosted it to 31C for 48 hours one week before the simulated bleaching event. In the third tank they had no pre-heating treatment, while in the fourth tank they had no pre-heating nor any simulated bleaching event. At different points throughout the study, they measured photosystem II efficiency, xanthophyll and chlorophyll a concentrations, and Symbiodinium densities.

What was learned
Middlebrook et al. report that the symbionts of the corals that were exposed to the 48-hour pre-bleaching thermal stress "were found to have more effective photoprotective mechanisms," including "changes in non-photochemical quenching and xanthophyll cycling," and they further determined that "these differences in photoprotection were correlated with decreased loss of symbionts, with those corals that were not pre-stressed performing significantly worse, losing over 40% of their symbionts and having a greater reduction in photosynthetic efficiency," whereas "pre-stressed coral symbiont densities were unchanged at the end of the bleaching."

What it means
In the words of the three researchers, "this study conclusively demonstrates that thermal stress events two weeks and one week prior to a bleaching event provide significantly increased thermal tolerance to the coral holobiont, suggesting that short time-scale thermal adaptation can have profound effects on coral bleaching." In addition, they say that "both corals and Symbiodinium have been shown to possess a wide variety of genes that encode for stress response proteins, which can impart protection, indicating that a more comprehensive study is required to elucidate all of the underlying mechanisms of thermal bleaching." All things considered, therefore, it may well be that earth's reef-building corals are not nearly as helpless before the specter of possible future global warming as the world's climate alarmists have made them out to be.

Reviewed 11 June 2008