The role of background algal symbionts as drivers of shuffling to thermotolerant Symbiodiniaceae following bleaching in three Caribbean coral species

Reef-building corals host diverse dinoflagellate algal symbionts (Family Symbiodiniaceae) whose identity can influence host thermotolerance and whose relative abundance can be dynamic. Breakdown in this symbiosis during “bleaching” events can promote changes in symbiont communities in favour of thermotolerant types, particularly in the genus Durusdinium. We employed experimental bleaching to manipulate the symbiont communities of three common Caribbean reef-building species ( Montastraea cavernosa , Orbicella faveolata , and Siderastrea siderea ) and tested whether seasonal differences in the corals’ symbiont communities at the time of their collection affected their responses to manipulation. In O. faveolata and S. siderea , a minimum threshold of initial background proportion Durusdinium trenchii shaped recovery from bleaching with mainly Durusdinium . In contrast, in M. cavernosa , Durusdinium became highly dominant after recovery even when it was undetectable prior to bleaching. Seasonal changes were also detected in M. cavernosa and S. siderea dominated by Cladocopium , with significant increases and decreases, respectively, in symbionts per host cell in October (following annual temperature maxima) compared to the previous April (following temperature minima). These results demonstrate how background symbionts and seasonal differences in symbiont density can affect the disturbance and recovery dynamics of algal symbiont communities in different coral species, and prompt further research into how seasonal changes in algal symbiosis might inform projected future bleaching, which is increasingly relevant in light of predicted winter warming and prolonged warm summer temperatures under climate change.