Prolonged thermal stress determines the degree of symbiont shuffling and Platygyra verweyi survival after bleaching

Rising seawater temperature caused by climate change is threatening the survival of reef-building corals. One of the mechanisms for corals to overcome thermal stress is to shuffle in hospite, the relative dominance of Symbiodinium from thermally susceptible types to tolerant ones. Although the shuffling of Symbiodinium has been observed in a several of coral species, the influence of thermal history on the process of symbiont shuffling and subsequent survival of coral hosts remains unclear. In this study, we conducted in situ reciprocal transplantation experiments (RTE) on nubbins of brain coral, Platygyra verweyi, collecting from a nuclear power plant outlet (NPP-OL), inlet (NPP-IL) and Wanlitung (WLT) of Kenting National Park in 2014 and 2015. While the symbiont community of P. verweyi in NPP-OL was dominated by stress-tolerant types D1/D1a, the ones in NPP-IL and WLT were dominated by the thermal- susceptible types C3/C3cc. In 2014, all the nubbins transplanted from NPP-IL and WLT to NPP-OL bleached in July after the prolonged high seawater temperature (Degree Heating Week, DHW=10.43) in NPP-OL. In contrast, those ones transplanted from NPP-OL to the other two sites remained unbleached. Forty percent of bleached nubbins showed sign of shuffling from C3/C3cc to D1/D1a, although 75% of them died in the subsequent fall. In 2015, 73% of the RTE nubbins from WLT to NPP-OL bleached under less prolonged high seawater temperature (DHW=5.7) with 73% shuffling their symbiont community from C3/C3cc to D1/D1a dominant and surviving after a reduction of the thermal stress. Our results show that in the facing of climate change impact, P. verweyi is capable of shuffling symbiont community towards the stress-tolerant type in order to respond to the rising seawater temperature. However, the degree of prolonged thermal stress may play a crucial key role in determining the success of shuffling and final survival of the coral host. In addition, our study also indicates that the Symbiodinium D1/D1a dominant P. verweyi can have a comparable or higher growth rate than Symbiodinium C3/C3cc dominant one under field condition. This suggests a potential acclimatization and/or adaptation might occur in the P. verweyi population at NPP-OL after long-term association with Symbiodinium D1/D1a.