Seasonal stability of coral-Symbiodinium associations in the subtropical coral habitat of St. Lucie Reef, Florida

The coral community at St. Lucie Reef (Stuart, Florida; 27 degrees 8' N, 80 degrees 8' W) is found near the northern latitudinal range limit for Florida reefs and persists under environmental variability from freshwater discharges, summer upwelling, and thermal instability. Since aspects of coral physiology can be attributed to the composition of endosymbiotic zooxanthellae (genus Symbiodinium), we examined the dynamics of Symbiodinium strains in St. Lucie corals to gain insight into the organization of coral-algal symbioses under local stressors. Two scleractinian coral species that dominate the reef, Montastraea cavernosa and Pseudodiploria clivosa, were repeatedly sampled at 4 reef sites over 17 mo, during both wet and dry seasons. Symbiodinium cellular density and photosynthetic pigments differed between the 2 coral hosts, where Pseudodiploria clivosa had higher cell densities and chlorophyll concentrations than Montastraea cavernosa. Over time, these parameters varied, but were not significantly altered following freshwater discharge events. Symbiodinium diversity and abundances were identified using ITS2 region amplification and next-generation sequencing, which revealed remarkable stability of the relative proportions of different Symbiodinium genotypes throughout the sampling period. Novel associations with unique Symbiodinium strains observed for each coral species as well as the stability of these symbioses could indicate local adaptation of St. Lucie Reef corals to their marginal environmental conditions.