This study provides evidence that the coral holobiont and the coral host have different responses in terms of gene expression, during bleaching and through the recovery process. Here, we present evidence on previously unknown effects of bleaching; (i) Results of the metatranscriptome analysis indicate that each portion of the holobiont (i.e. ‘O. faveolata’, ‘Symbiodinium spp.’ and ‘other-eukaryotes’) has different responses to and recovery from bleaching; (ii) the coral host response appears to be masked by the responses of the associated organisms (i.e. ‘Symbiodinium spp.’ and ‘other-eukaryotes’); (iii) bleached colonies may not successfully recover from bleaching, while unaffected colonies do not experience as intense changes in gene expression; and (iv) the effects of bleaching on the host immune system extend beyond recovery of the Symbiodinium population and appear to result in immune suppression. These results support the hypothesis that coral bleaching affects the expression of innate immune genes of corals, and these effects can last up to a year after the event.
Analyses on thermal resistance of corals suggest that some individuals might be able to overcome rising temperatures associated with climate change [31–34]. Bleaching impacts have proven erratic, and corals that in the past survived such events have been locally exterminated in the same locations after new bleaching events [87]. Results in this paper suggest that bleaching has long-term effects, but at the same time provide evidence that unbleached corals remain better prepared to fight pathogenic infections. The relation between coral bleaching and immunity in corals is complex and variable [30]. Studies emphasizing the role of coral immunity as an important aspect in coral's resistance to stressors can help improve predictions of the future of corals and coral reefs [88,89].