Thermal priming affects symbiont photosynthesis but does not alter bleaching susceptibility in Acropora millepora

This study experimentally investigated the effect of an early summer short-term summer thermal anomaly in November on Heron Reef (Great Barrier Reef), on the performance of the coral-algal symbiosis during a subsequent, thermally induced bleaching event (defined by loss of symbionts) in February 2009. Fragments of the reef flat coral, Acropora millepora, exposed to the two heating events lost 78% of their symbionts and showed a 64% decline in dark adapted quantum yield (Fv/Fm) to values less than 0.25. Whereas corals that were only heated in the second event lost 57% symbionts and displayed a 75% decline in Fv/Fm to values less than 0.2. Coral in both pre-stressed and non pre-stressed treatments at 34 degrees C had net photosynthetic rates significantly less than zero (P-net max<0), suggesting that daytime O-2 respiration rates were significantly greater than rates of O-2 evolution through photosynthesis. Increases in the daily light field and/or increases in temperature led to the observation of dark-adapted xanthophyll de-epoxidation suggesting that the potential for non-photochemical quenching is maintained into the dark (after 1 h dark adaption). Pre-stressed corals were also found to have de-epoxidation ratios three times greater than non pre-stressed corals at 34 degrees C and xanthophyll pool to Chl a, much greater than non pre-stressed corals. Combined, these results indicate that symbionts in pre-stressed corals have a greater ability to dump incoming light energy as heat We conclude that whilst differences were observed between pre-stressed and non pre-stressed populations in measurements of photosynthetic productivity, photosynthetic pigments and areal protein, thermal bleaching as defined by symbiont loss did not vary between treatments. (C) 2012 Elsevier By. All rights reserved.