Coral reefs are globally important ecosystems with high species diversity at both the macro and micro scales. In recent years, coral reefs have been heavily impacted by anthropogenic and natural stressors, including emerging diseases. Many of these diseases have been identified in reef-building corals, but other invertebrate taxa, such as soft corals, are increasingly at risk. This study focuses on a hybrid species complex within soft corals of the genus Sinularia, which dominate the shallow reefs of Guam, and the broader Indo-Pacific. These soft corals exhibit varying levels of disease susceptibility to Sinularia Tissue Loss Disease (STLD), a chronic wasting disease. In the current study, we used next-generation amplicon sequencing of prokaryotic and eukaryotic communities within these soft corals to characterize their microbiomes, and develop a better understanding of the etiology of STLD. There were slight differences in the microbiomes of healthy colonies of S. maxima, S. polydactyla and their hybrid species. There was also a decline in the relative abundance of putatively beneficial symbionts (Symbiodinaceae and Endozoicomonas) in STLD-affected soft corals, but no consistent shifts towards a specific microbial community associated with STLD. Our data suggest that the STLD phenotype may be due to a combination of factors including shifts in putatively beneficial symbionts, infection by a yet unknown etiologic agent, and anthropogenic impacts on this shallow, nearshore and popular reef.
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