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 °C had net photosynthetic rates significantly less than zero (Pnet max < 0), suggesting that daytime O2 respiration rates were significantly greater than rates of O2 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 °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.

► We examine the effect of a thermal priming event on subsequent coral bleaching.
► The onset of thermal bleaching was not altered by the priming event.
► Photosynthetic productivity and pigments were altered by the priming event.
► Pre-stressed corals were able to dump greater amounts of light energy as heat.