Molecular Mechanisms of Coral Holobiont Heat Tolerance on Bleaching-resilient, Marginal Inshore Patch Reefs of the Florida Keys

Florida Keys coral reefs have experienced seven mass bleaching events since 1987. Many reefs have < 5% coral cover, but some inshore patch reefs have maintained higher cover (15-35%). This is counter-intuitive as inshore sites experience marginal conditions (greater thermal variability, turbidity, and sedimentation). It has been hypothesized that inshore corals are acclimatized and/or adapted to heat stress due to naturally hotter (+1^oC) and variable temperatures. The response of Florida Keys coral reefs with mass bleaching in 2014 and 2015 highlighted the resilience of the inshore patch reefs. Inshore sites demonstrated better recovery and higher bleaching resistance than offshore sites. 2015 and 2014 were the two hottest summers on record for the Florida Keys, yet total colony mortality at Cheeca Rocks, an inshore patch reef, was low with 94.7% of > 4000 colonies surviving. There was a reduction in bleaching severity and mortality with the second stronger thermal anomaly in 2015, which suggests that acclimatization may be possible with short recovery. Lab-based experiments have shown that inshore genotypes of the ESA-listed coral Orbicella faveolata exhibit a significantly greater heat tolerance relative to offshore genotypes. Dominance of the algal symbiont community by Durusdinium trenchii was associated with the greatest heat tolerance, however, there was evidence for host adaptation/acclimatization as inshore genotypes dominated by Breviolum were more heat tolerant than corals offshore with the same symbiont type. The most bleaching resistant inshore genotype had unique expression patterns of genes encoding proteins involved in the heat shock response, immunity, and protein degradation. This presentation will summarize these findings and present data from a reciprocal transplant experiment that aims to understand if the inshore-offshore dichotomy in heat resistance is due to acclimatization or adaptation. The existence of corals on the inshore patch reefs at temperatures that are +1.0^oC warmer than offshore sites in the Florida Keys may be an important source of heat-tolerant genotypes for restoration, as well as provide a roadmap for reef survival in the Anthropocene.