Introduction
Since its first observation near Miami, Florida in 2014, stony coral tissue loss disease (SCTLD) has spread throughout Florida’s Coral Reef and to at least 25 jurisdictions across the wider Tropical Western Atlantic (Kramer et al., 2019; NOAA, 2018; Precht et al., 2016). The disease affects at least 24 scleractinian species and is characterized by subacute to acute tissue loss leading to colony mortality, often with formation of rapidly-progressing focal or multifocal lesions (G. Aeby et al., 2021; G. S. Aeby et al., 2019; Landsberg et al., 2020; NOAA, 2018). The pathogen(s) have not yet been identified, though examination of microbiomes of diseased samples suggest a bacterial (Becker et al., 2021; Meiling et al., 2021; Meyer et al., 2019; Rosales et al., 2020, 2022; Studivan et al., 2022; Ushijima et al., 2020) and/or viral (Veglia et al., 2022; Work et al., 2021) consortium. Antibiotic treatments have shown high rates of success in halting the progression of disease lesions, and in some cases, the quiescence of visible disease signs on treated colonies (Forrester et al., 2022; Neely et al., 2020; Shilling et al., 2021; Walker et al., 2021). Attempts to treat SCTLD-affected corals with chlorinated epoxy, which was hypothesized to affect more potential pathogenic taxa relative to targeted antibiotics, have been largely unsuccessful (Shilling et al., 2021; Walker et al., 2021). The mechanisms by which antibiotic application affects disease progression are unknown, particularly the potential impacts on the coral host and its microbiome in processes such as recovery and potential antibiotic resistance.
The molecular mechanisms underlying coral immune responses to SCTLD are poorly understood relative to other diseases (Traylor-Knowles et al., 2022). A recent study utilizing an untargeted metabolomic approach identified several lipid and tocopherol classes of Symbiodiniaceae origin that distinguished healthy and diseased corals, providing further evidence of algal symbiont involvement in SCTLD (Deutsch et al., 2021). Transcriptomic approaches have also been used to examine gene expression of disease lesion tissue in the coral speciesMontastraea cavernosa and Orbicella faveolata , finding differential expression of numerous genes implicated in stress, extracellular matrix rearrangement, immunity, and apoptosis pathways (Traylor-Knowles et al., 2021). Relatively few similarities, however, were observed between coral species, indicating a need for additional cross-species comparisons to identify consistent disease response mechanisms. To date no transcriptomic studies of SCTLD, or any coral disease, have focused on the effects of intervention methods, identifying a critical need to understand the potential consequences of antibiotic treatment of host and symbiont responses.
To address this knowledge gap, we conducted paired ex-situ transmission and in-situ intervention experiments, and examined whole-transcriptome gene expression patterns of corals in response to disease exposure and antibiotic treatment, respectively. These experiments focused on the coral species M. cavernosa and O. faveolata due to their ongoing use in field-based disease intervention and monitoring efforts (Shilling et al., 2021; Walker et al., 2021), ecological importance as primary reef-builders (González-Barrios & Álvarez-Filip, 2018; Walton et al., 2018), and growing use in reef restoration (Koval et al., 2020; Rivas et al., 2021). Through these experiments, we 1) identified transcriptomic responses to SCTLD exposure in a controlled lab setting, 2) compared responses to disease exposure between species, 3) examined transcriptomic modulation following antibiotic treatment in a field-based time series, and 4) compared trends between diseased and treated corals in lab and field settings. In doing so we seek to better understand coral immune responses to SCTLD, and to provide transcriptomic resources to the development of disease exposure diagnostics. These experiments also evaluated disease intervention effectiveness at the molecular level and identified potential patterns of recovery following treatment of SCTLD-affected wild colonies with antibiotics.