Bacterial communities of juvenile corals infected with different Symbiodinium (dinoflagellate) clades

The coral holobiont consists of the host and its microbial partners, including the dinoflagellate endosymbiont Symbiodinium and bacteria living both on and within coral tissues. Although genetically different, Symbiodinium types have been shown to differentially affect the physiology of the coral host; their effects on the bacterial partners in the association are unknown. The present study compares profiles of the bacterial communities associated with juvenile corals of Acropora millepora and A. tenuis that had been experimentally infected with 2 different clades of Symbiodinium, Clade C1 and D, to investigate possible interactions between bacterial and Symbiodinium communities. Three culture-independent 16S rRNA gene profiling methods (clone library construction, terminal restriction length polymorphism and denaturing gradient gel electrophoresis) revealed no discernible pattern in bacterial communities on 9 mo old juvenile corals containing different clades of zooxanthellae, suggesting that coral-associated bacteria are not linked to Symbiodinium types in hospite in early ontogeny. In contrast to bacterial profiles of adult corals, bacterial communities associated with juvenile corals were highly variable, indicating that bacterial associates are not conserved in these early stages. When 12 mo old juveniles were sampled again in summer, bacterial communities associated with A. tenuis hosting Clade D Symbiodinium were dominated by sequences affiliating with Vibrio species, indicating that corals harbouring this symbiont may be more susceptible to temperature stress, allowing growth of opportunistic microbial community members possibly detrimental to coral health.