Field sampling
We selected Pseudodiploria strigosa (Dana, 1846) as the coral
host species because it is widely distributed across the Bermuda
platform. In Bermuda, a high-latitude subtropical coral reef system,
corals are exposed to a wide annual range in temperature fluctuations
and reef-zone specific thermal regimes ranging from more fluctuating
profiles in the inner lagoon patch reefs and milder conditions in the
outer reefs (de Putron and Smith 2011a; Courtney et al. 2017, 2020; Lima
et al. 2020; Wong et al. 2021). The reef zones sampled were
approximately 8 km apart (Lima et al. 2020) and P. strigosa is a
broadcast spawner; therefore, there is a high likelihood that gene flow
between the coral hosts colonizing inner and outer reefs is maintained
and that the host genetics is not structured into different populations.
Indeed, studies on other species have indicated high genetic exchange
among reef sites in Bermuda (Serrano et al. 2014, 2016). The sampling
period occurred between May 18th and May
22nd, 2017, late spring in the northern hemisphere,
when environmental conditions between the two reef zones, especially
temperature, are similar. The environmental gradient assessed here are
based on the knowledge that these two reef zones are exposed to
different regimes on a seasonal basis, with the most striking
fluctuations occurring in the winter and summer months (de Putron and
Smith 2011b; Smith et al. 2013; Courtney et al. 2017). Therefore, we
selected this period to capture a potential long-term acclimatization of
the coral holobiont to their reef zones, and not their immediate
response to acute temperature fluctuations. The mucus from P.
strigosa was collected from six colonies from the inner and outer reef
zones (n = 12 colonies total) to describe the natural spatial
variability in the coral microbiome (Lima et al. 2020, 2022). After the
mucus sampling, each coral colony was collected using a hammer and
chisel to be used in the experiments to assess heat resistance. The
colonies (n = 12 total, 6 from each reef zone) were carefully placed in
a cooler with seawater and transported to the Bermuda Institute of Ocean
Sciences. The collections were performed via SCUBA diving at 4-6m depth.