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.