Dataset: Palau Coral Reef Experiment 2018: Isotopes

Eight colonies of the coral Psammocora digitata and Pocillopora verrucosa were sampled from the offshore western barrier reef, Rebotel reef (7.248833° N, 134.235817° E) at 5–10 m depth, and from Nikko Bay (also known by Ngermid Bay, 7.3245° N, 134.4939° E) at 5 m depth. Samples were transported to the Palau International Coral Research Center (PICRC), and each colony sample was cut into nine replicate ramets that were placed in flow-through sea water tables and allowed to heal for 48 hours before mounting on labeled PVC tiles with marine epoxy (Splash zone compound A-788).

Temperature experiments were conducted in indoor aquarium systems. Each system used a semi-enclosed design that consisted of a series of 44 L plastic bins connected to a central 220 L sump that was supplied by a continuous slow-feed supply of fresh seawater. The control system (4 bins) was maintained at an average temperature of 28.27 ± 0.33°C by an in-line chiller and titanium heater. The heated system (6 bins) was ramped from 28°C to 31°C (1°C day^-1) and held at 31.86 ±0.14°C by an in-line titanium heater. All bins were lighted to an irradiance of 600 μmol photons m^-2s^-1 by LED lights set to daily ramping. At the end of each experiment (day 13), control and treatment ramets were placed in glass beakers containing 400 mL of freshly filtered seawater (0.45 µm) that was enriched with 0.633 mM of NaH¹³CO₃ (99 atom % ¹³C, Cambridge Isotope Lab Inc., Andover, MA, USA), and 1.5 µM of Na¹⁵NO₃ (98 atom % ¹⁵N, Cambridge Isotope Lab Inc., Andover, MA, USA). Beakers were fitted with false bottoms and continually stirred with magnetic stir bars. All beakers were held for 5 hours at the experimental temperatures under the continuous maximum light level (600 μmol photons m^-2s^-1) used in the experiment. Fragments were then removed, rinsed in filtered seawater, and immediately frozen at -60° C until further processing.

Coral tissue was removed with an airbrush and filtered seawater (0.45µm), followed by the addition of 0.02% (w/v) sodium dodecyl sulfate (SDS) and homogenization for 10 s with a Tissue-Tearor (Biospec Products, Inc). Algae and coral tissue were separated by 2–3 centrifugation washes (550 g for 5 min) with 10 s homogenization steps between each wash. Algal cells were pelleted via centrifugation and frozen at -20°C. Accumulated supernatants (animal portion) were microscopically verified to not contain symbiotic algae or skeletal material and were filtered onto pre-combusted (450°C for 5 h) glass 0.7 μm glass fiber filters (Whatman GF/F) until clogged and then frozen at −20°C. Due to the relatively high concentration of ¹³C assimilation by the symbiotic algae during the labeling incubations, coral skeletons were placed in 100% bleach for 24h to remove remnant organic material, rinsed in fresh water for 24h, and dried under low heat. Approximately 20 mg of the outermost CaCO₃ was sampled from both the corallite and coenosarc regions of the coral skeleton using a Dremel tool with a diamond bit. Skeletal samples were stored at -20°C. Elemental isotope analyses were performed on a Carlo Erba CHN Elemental Analyzer (Model NA1500) coupled to Thermo Finnigan Delta V Isotope Ratio Mass Spectrometer via a Thermo Finnigan Conflo III Interface at the University of Georgia, Center for Applied Isotope Studies.

Instruments:

• Seawater temperature was controlled by an in-line chiller and titanium heater DeltaStar DS-3, and Cygnet Mini (Aqualogic Inc.), and light was supplied to each experimental bin by a custom LED array (XP-G3 Cool White LEDs, Cree) controlled with a digital Storm Controller(Coralux).
• Water was continuously mixed in each bin by a small submersible pump (Sicce Micra, 90 GPH).
• Coral tissue was removed with an airbrush (Paasche VL-3AS) at 100 psi and homogenized with a hand-held homogenizer (Tissue tearor, Biospec Products, Inc).
• Coral homogenates were centrifuged in a clinical centrifuge (IEC)
• Isotope analyses were performed on a Carlo Erba CHN Elemental Analyzer (Model NA1500) coupled to Thermo Finnigan Delta V Isotope Ratio Mass Spectrometer via a Thermo Finnigan Conflo III Interface at the University of Georgia, Center for Applied Isotope Studies.