Dataset: Coral microsensor profiles collected during exposure to low pH in outdoor mesocosms in Hawaii from July 2023 to August 2023

Coral Collection and Acclimation

Six large colonies each of Montipora capitata and Pocillopora acuta were collected approximately 300 m offshore of the Hawai‘i Institute of Marine Biology (HIMB) in Kāne‘ohe Bay, Hawai‘i (21.4337 N, 157.7893 W). Colonies were transported to HIMB in ambient seawater and placed into a flow-through mesocosm (area 1.37 m2; volume ~50 m3). Mesocosms received natural seawater from Kāne‘ohe Bay (<1 km from collection site) with a residence time of about one hour. Temperature, dissolved oxygen, salinity, and carbonate chemistry were not manipulated and followed natural diel patterns. Light was natural but shaded to approximately 50 percent (midday PAR ~800 µmol photons m-2 s-1). Colonies acclimated for 10 days, were then halved and tagged by genotype, and acclimated for an additional 3 days before experiments began. Initial buoyant weights were recorded prior to treatment placement.

Experimental Treatments

Two carbonate chemistry conditions were established: an ambient control and an elevated pCO2 treatment (target pH offset of approximately -0.3 units relative to the control). Treatment seawater carbonate chemistry was manipulated by bubbling pure CO2 or CO2-scrubbed air into mesocosm mixing pumps using a pH-stat approach. All other seawater variables (temperature, salinity, nutrients) were unmodified. Colonies were placed in similar positions within each mesocosm to minimize spatial variation during the 19-day exposure.

Seawater Chemistry Measurements

Temperature, salinity, dissolved oxygen, and pHNBS were measured with a YSI ProDSS multiparameter instrument. pHNBS values were corrected to total scale (pHT) using Tris buffer calibrations following Dickson et al. (2007).
Total alkalinity (AT) was measured via open-cell titration using a Metrohm Titrino 877 Plus with 0.1 M HCl. All AT values were corrected using certified reference materials from A. Dickson (batch 205). Carbonate system parameters (DIC, HCO3-, CO32-, pCO2, and Omega aragonite) were calculated using the R package seacarb.

Seawater Sample Collection, Filtration, and Storage

For AT and DIC measurements, seawater samples were collected in 250 mL borosilicate glass bottles with no headspace. Samples were poisoned immediately with 100 µL saturated HgCl2 to halt biological activity. Bottles were sealed with ground-glass stoppers greased with Apiezon L and stored in the dark at room temperature for no more than 7 days prior to analysis.  No nutrients or chlorophyll samples were collected for this study.

Flume System

A custom acrylic flume was used for microsensor profiling. The system consisted of a concentrator section (2.09 m2) supplied by a 24 V Hygger pump (6511 L h-1), a flow-reducing baffle, and a hexagonal flow straightener leading into a test section (88.9 cm by 22.3 cm; 1.8 m2). Flow velocity was approximately 1250 µm s-1 (residence time 44 minutes). Lighting was provided by a 180 W full-spectrum LED aquarium light delivering approximately 800 µmol photons m-2 s-1.
The flume received water from the control mesocosm each morning. For elevated pCO2 flume conditions, CO2 was bubbled into 1 L of seawater until pH 4.00, which was then mixed into the flume reservoir and equilibrated with circulation for one hour, achieving a pH offset similar to the mesocosm treatment.

Microsensor Measurements

Oxygen was measured using a PreSens PSt7 micro-optode (230 µm tip diameter), and pH was measured using a Unisense glass microelectrode (tip diameter 100 µm). Measurements were recorded every 6 seconds.
Sensors were mounted on a dual-probe automated micromanipulator (Zaber T-LSR075A). Movement followed scripted profiles with 100 µm vertical steps. At each height, sensors paused for 60 seconds before recording. Profiles extended from the coral surface (0 µm) upward until bulk seawater values were reached, followed by an additional 2000 µm at 500 µm intervals.
Sensor placement targeted branch tips oriented into flow and positioned between polyps. Sensors were lowered in fine increments (1–10 µm) until maximum steady values of oxygen and pH were observed at the tissue surface. Bulk seawater values were measured with the YSI instrument and used to standardize microsensor readings.

Calculation of Diffusive Boundary Layer Thickness

Diffusive boundary layer (DBL) thickness was calculated from log-transformed concentration profiles. A linear model of log10(concentration) versus distance was used to estimate the distance at which the concentration equaled the bulk seawater concentration.

Proton and Oxygen Flux Calculations

Fluxes were calculated using Fick’s first law of diffusion. The linear portion of each concentration gradient was used to determine the slope (m). Proton flux was calculated using a diffusion coefficient of 9.31 × 10-5 cm2 s-1, and oxygen flux using a diffusion coefficient of 2.20 × 10-5 cm2 s-1 at 26.5 C and salinity 35. These methods follow approaches described by Martins et al. (2020, 2021) and Pacherres et al. (2023).