Responses of coral-associated bacterial communities to heat stress differ with Symbiodinium type on the same coral host

This study compared the effect of heat stress on coral-associated bacterial communities among juveniles of the coral, Acropora tenuis, hosting different Symbiodinium types. In comparison to a control temperature treatment (28 degrees C), we documented dramatic changes in bacterial associates on juvenile corals harbouring ITS 1 type D Symbiodinium when placed in a high (32 degrees C) temperature treatment. In particular, there was a marked increase in the number of retrieved Vibrio affiliated sequences, which coincided with a 44% decline in the photochemical efficiency of the D-juveniles. Interestingly, these Vibrio sequences affiliated most closely with the coral pathogen, Vibrio coralliilyticus, which has been implicated in some coral disease outbreaks. In contrast, A. tenuis hosting ITS 1 type C1 Symbiodinium did not exhibit major bacterial shifts in the elevated temperature treatment, indicating a more stable bacterial community during thermal stress; concomitantly a decline (10%) in photochemical efficiency was minimal for this group. D juveniles that had been exposed to moderately elevated sea temperatures (30 degrees C) in the field before being placed in the control temperature treatment displayed a decrease in the number of Vibrio affiliated sequences and bacterial profiles shifted to become more similar to profiles of corals harbouring type C1 Symbiodinium. In combination, these results demonstrate that thermal stress can result in shifts in coral-associated bacterial communities, which may lead to deteriorating coral health. The lower resilience of A. tenuis to thermal stress when harbouring Symbiodinium D highlights the importance of inter-kingdom interactions among the coral host, dinoflagellate endosymbiont and bacterial associates for coral health and resilience.