The role of algal symbionts (genus Breviolum) in the susceptibility of corals to Stony Coral Tissue Loss Disease in South Florida

The role of algal symbionts in the susceptibility of corals to SCTLD remains a priority for ongoing disease research. This project tests the hypothesis that hosting algal symbionts in the genus Breviolum, either exclusively or predominantly, plays a role in determining why some coral species (and individuals) are more susceptible to stony coral tissue loss disease (SCTLD) than others. In this report we describe the physiological and molecular outcomes of a multifaceted experiment investigating the role of algal symbionts in five different Caribbean coral species whose susceptibility to SCTLD varies (Colpophyllia natans, Diploria labyrinthiformis, Meandrina meandrites, Oribicella faveolata, and Pseudodiploria strigosa). Through experimental bleaching and recovery, we manipulated a subset of cores from each colony away from their native symbionts towards Durusdinium and subsequently exposed cores containing Breviolum (and in some cases Cladosporium) or Durusdinium to SCTLD. Tissue biopsies were taken throughout the algal manipulations and disease exposure to monitor changes in coral-algal symbiosis and a subset of samples taken throughout the disease exposures were selected for downstream transcriptomics and microbial dynamics analyses (16S and 18S).

Generally speaking, we found that 87% of cores containing Breviolum presented with SCTLD-like lesions between 10 and 50 days after disease exposure, compared to 36% of unmanipulated Cladosporium cores (only found in O. faveolata) and only 19% of cores manipulated to contain Durusdinium. These results, suggest that in addition to a species-oriented disease susceptibility hierarchy, there is a hierarchy among different Symbiodiniaceae in their susceptibility to SCTLD, which we tentatively rank as Breviolum >> Cladosporium > Durusdinium >> Symbiodinium. Further analyses of host gene expression, symbiont gene expression, 16S microbial dynamics, 18S microbial dynamics, and histology associated with this project is still ongoing.