The persistence of coral reefs in the Florida Keys reef tract is of concern as coral bleaching, due to increased ocean temperatures, and human-linked disease outbreaks have led to a reduction in coral cover of 40% since the 1980’s. Evidence suggests a variation in stress susceptibility of conspecific coral from inshore and offshore reefs in the Florida Keys. However, the mechanism behind the disparity in stress susceptibility is unknown. Variation in genetic composition of a coral’s symbiotic algae (Symbiodinium spp.; referred to as zooxanthellae) has been proposed as a mechanism to withstand stress. As such, I investigated zooxanthellae composition of Porites astreoides and Montastraea cavernosa from an inshore and offshore reef in the Lower Florida Keys to determine links to stress susceptibility. Additionally, I investigated variation in the expression of metabolically related genes in zooxanthellae of P. astreoides reciprocally transplanted between reefs, as well as exposed to elevated temperatures and disease. Chapter one examines changes in the dominant zooxanthellae subclade type in P. astreoides and M. cavernosa throughout a two-year reciprocal transplant study. The goal of this study was to determine if zooxanthellae subclade type could explain higher rates of bleaching in offshore reef coral, as well as assessing the possibility of acclimatization to different environments. Increased complexity and diversity was seen in the composition of zooxanthellae subclade types from coral collected at offshore reefs, compared to inshore reefs. As a result, offshore reef zooxanthellae displayed less stability, possibly explaining higher bleaching susceptibility. Additionally, zooxanthellae composition patterns were retained throughout the reciprocal transplant, demonstrating a lack of acclimatization. Chapter two examined site-specific variation in the expression of metabolically related genes in zooxanthellae from P. astreoides following the reciprocal transplant. Symbionts from offshore corals experienced significantly increased expression in PCNA, SCP2, G3PDH, PCP and psaE (p<0.05) compared to inshore symbionts, a pattern consistent with increased bleaching susceptibility. Significant differences in gene expression between zooxanthellae from inshore and offshore reef indicate functional variability and are likely a result of localized adaptation. Similar to results in chapter one, gene expression patterns from site of origin were retained throughout the reciprocal transplant, suggesting no acclimatization. Chapter three investigated variation in the response of the same zooxanthellae genes when P. astreoides was exposed to extreme temperatures and disease. Here disease was mimicked by the application of lipopolysaccharide from Serratia marcescens, the causative agent of acroporid serratosis. Gene expression did not differ in zooxanthellae from inshore and offshore reefs, nor as a consequence of extreme temperature or disease. Several factors may explain the lack of variation including zooxanthellae response to acute versus moderate stress, host protection and targeting of symbionts by S. marcescens. These results suggest that symbionts of P. astreoides may be locally adapted to chronic moderate stress, but respond similarly to acute extreme conditions.
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