Profiles of Symbiodinium spp. Unaltered by Heat Stress in a Coral Host

Barshis; Ladner; Oliver; Palumbi

Dinoflagellates of the genus Symbiodinium form an endosymbiosis with reef building corals, in which photosynthetically derived nutrients comprise the majority of the coral energy budget. An extraordinary amount of functional and genetic diversity is contained within the coral-associated Symbiodinium, with some phylotypes (i.e., genotypic groupings), conferring enhanced stress tolerance to host corals. Recent advances in DNA sequencing technologies have enabled transcriptome- wide profiling of the stress response of the cnidarian coral host; however, a comprehensive understanding of the molecular response to stress of coral-associated Symbiodinium, as well as differences among physiologically susceptible and tolerant types, remains largely unexplored.Here,weexamine the transcriptome-wide response to heat stress viaRNA-Seq of two types of Symbiodinium, the putatively thermotolerant type D2 and themore susceptible type C3K, resident within the same coralhost species,Acropora hyacinthus.Contrary topreviousfindings with coralhosts,wefindnodetectable change in gene expression across the dinoflagellate transcriptome after 3 days of elevated thermal exposure, despite physical evidence of symbiosisbreakdown.However,hundreds of genes identifiedas orthologs between theCandDtypes exhibited significant expression differences within treatments (i.e., attributable solely to type, not heat exposure). These include many genes related to known thermotolerance mechanisms including heat shock proteins and chloroplast membrane components. Additionally, both the between-treatment similarities and between-type differences remained pervasive after 12–18 months of common garden acclimation and in mixed Symbiodinium assemblages within the same coral host colony.