Tracking Bacterial, Viral, and Gene Expression Shifts throughout SCTLD Development in the Dry Tortugas

The rapid spread of SCTLD throughout Florida’s Coral Reef has had devastating impacts on these essential coastal ecosystems. This rapid spread has hampered efforts to study many aspects of disease biology, including investigation of causative agents and factors which contribute to host resilience. Characterization of these traits is essential to creation of improve management strategies for Florida’s Coral Reefs, but depends on availability of samples from disease-susceptible corals with no prior disease exposure. This project leveraged unique samples from Dry Tortugas National Park (DRTO) samples before and during SCTLD arrival to investigate numerous aspects of SCTLD biology. By combining integrative ‘omic and histological analyses of corals sampled through time we provide novel insight regarding the patterns underlying SCTLD outbreaks across multiple species of corals. Histological analyses of samples confirmed signatures of disease consistent with previous analysis of SCTLD corals, including vacuolization of symbionts. Similarly, while microbial communities of DRTO corals pre-disease were unique compared to other Florida

Keys samples, signatures of disease progression in the microbial communities were highly similar to those previously sequenced. Notably, while corals from DRTO had high levels of beneficial microbes, including Endozoicomonas, presence of these bacteria failed to protect against disease onset, and populations of these beneficial microbes declined through disease onset. This suggests intervention to maintain beneficial microbial communities, through methods such as probiotic treatment, may be necessary to maintain these communities and prevent disease. Viral community analyses similarly identified patterns consistent with previous studies of SCTLD. Viruses associated with Symbiodiniaceae were present in both healthy and diseased samples, pointing to a role of opportunistic pathogens in SCTLD. Viruses with potential impacts on coral immunity were also documented, further implicating roles of opportunistic associations in SCTLD. Finally, transcriptomic analyses revealed notable patterns of changes in host coral immunity across disease progression in three out of the four studied species. Notably, all three of these species were characterized by decreases in expression of key immune genes over time healthy and apparently healthy coral tissue. These patterns of immune decline corresponded with peak disease onset in each species, suggesting a biological rather than environmental driver. Combined, our analyses to date suggest that even remote, previously healthy corals can succumb to SCTLD, and emphasize the need for proactive and comprehensive management approaches. Ongoing analyses of these data, supported by a 2024-2025 FDEP contract, will incorporate integrative approaches to generate important biological insight which will guide the development of improved management approaches necessary for the protection of theses reefs.