Abstract

Florida’s Coral Reef is currently experiencing an unprecedented. multi-year outbreak of stony coral tissue loss disease (SCTLD). with the most highly susceptible species predominantly hosting algal symbionts in the genus Breviolum. We used a manipulative experiment to alter the algal symbiont communities in replicate cores of five species of Caribbean coral (Colpophyllia natans. Diploria labyrinthiformis. Meandrina meandrites. Pseudodiploria strigosa. and Orbicella faveolata) and tested whether these changes affected their susceptibility to SCTLD. We experimentally bleached and recovered a subset of cores to encourage algal associations in favor of Durusdinium. and then exposed them to SCTLD and compared their responses to control cores of the same genotype hosting native algal symbionts (Breviolum or Cladocopium). We found cores containing Breviolum were more susceptible to SCTLD and colonies exclusively hosting Breviolum were 2.5x more likely to present with SCTLD-lesions when compared with those with Durusdinium. These results suggest a hierarchy among Symbiodiniaceae in driving susceptibility to SCTLD ( Breviolum >> Cladocopium > Durusdinium >> Symbiodinium). This differential susceptibility suggests either that algal symbionts (or their specific microbial associates) are the active targets of a SCTLD pathogen. or that hosting certain symbionts increases the susceptibility of the coral’s immune system to SCTLD. We also assessed changes in holobiome composition (16S and 18S). host and symbiont gene expression. and histopathology. Our findings will help explain inter- and intra-specific variability in disease incidence and severity. with implications for interventions which could be applied to mitigate SCTLD impacts and/or restore affected reefs in the Caribbean

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