*2.3. Preparation of Stock Solutions*

All experiment stock solutions were prepared with Millipore® water, which was initially boiled to drive out dissolved CO2 and then kept in a constant N2 atmosphere to prevent CO2 in-gassing. For all stock solutions pH was measured with a WTW-Multi 3430 Set K pH senor and calibrated with the pH 4 and 10 buffers at 25 ◦C. Among all the experiments temperature, salinity, and pH remained constant at 25 ◦C, 36, and 8.7, respectively. The aragonite saturation state, Ωara in all incubations was 10, with a saturation index, SIara = log(Ωara), of 2.8, which should induce aragonite precipitation. These parameters represent conservative estimates of realistic scenarios for the coral ECF [11,37]. The aquatic properties chosen for the stock solutions in these experiments reflect the ECF parameters known for *Galaxea fascicularis*, but may not be representative for other coral species, e.g., [38,39]. The full details of the quantity of chemical compounds used for each experiment can be found in Table 2. Each chemical compound was weighed on a Mettler Toledo® scale with a 1μg precision (room humidity 30% and temperature 22 ◦C). Concentrations of CaCl2 and magnesium chloride [MgCl2] in the calcium stock solution and NaHCO3 in the carbonate stock solution were double the target concentrations for calcium and carbonate ions because the fluids enter the incubation chambers at a 1:1 ratio and dilute each other's concentration by half (quantiles are given in Table 2A). After adding all the necessary chemical compounds to the stock solutions, they were transferred into 1-L Tedlar® gas sampling bags [36] and put into the climate cabinet at constant 25 ◦C (±0.5 ◦C). This study would improve greatly if microsensors were installed in the incubation chambers to monitor the real time chemistry. Unfortunately, our approach relies on the calculated parameters inside the chambers similar to the work of [31,36].

The calcium stock solutions were prepared by dissolving CaCl2 in 5-L of carbon-free Millipore® water. For the experiments containing magnesium, MgCl2 was added to the stock solution of CaCl2. The Mg concentrations were chosen to represent a strong ion removal mechanism (0 mM, control Mg treatment), a medium ion removal mechanism [26.5 mM, equivalent to half the concentration in present day seawater], and a weak ion removal mechanism (53 mM, equivalent to the concentration in present day seawater). The amount of sodium chloride [NaCl] was then adjusted to maintain a final salinity of 36. Neither carbon nor alkalinity was present in the CaCl2 stock solution (pH = 7), therefore maintaining a zero DIC and TA concentration.

The carbonate stock solutions were prepared by dissolving NaHCO3 in 5-L of carbonfree Millipore® water. Sodium hydroxide [NaOH] was added via titration to adjust TA and to reach a pH of 8.716. The pH of the carbonate stock solution was 8.716 because when it mixes with the CaCl2 solution (pH = 7) in the incubation chamber the pH will adjust to 8.700 because DIC and TA are known to mix conservatively [40]. DIC and TA were calculated for equilibrium carbonate chemistry in NaCl using the dissociation constants of [41].


**Table 1.** Solutions setup for incubation experiments. Calcium, magnesium, and DIC concentrations are arranged by the amounts of CaCl2, MgCl2, and NaHCO3 added to the solutions. Temperature and salinity remained constant at 25 ◦C and 36 g kg<sup>−</sup>1, respectively. Carbonate concentrations and TA are calculated for equilibrium carbonate chemistry in NaCl using the dissociation constants of [41].

Although the concentration of calcium in the ECF is known to vary, previous studies have recorded values between 9–15 mM from cold-water corals 9–12.3 mM with a mean of 9.9 mM; [42] and the tropical corals *Pocillopora damicornis* and *Acropora youngei* range between ca. 9–15 mM; [43]. The target value of 10.6 mM calcium, was chosen for the incubations with a high Ca:CO3 (47:1) stoichiometry because it represents conditions measured with microelectrodes in the ECF of *Galaxea fascicularis* 9–11 mM; [11]. Although the target value of 2.6 mM calcium is perhaps unrealistically low, it was chosen for the incubations with a low Ca:CO3 (2.8:1) stoichiometry to emulate a strong proton removal from the ECF (resulting in elevated TA four times greater than ambient seawater), as well as a DIC concentrating mechanism (three times greater DIC than ambient seawater) as proposed by a number of authors [28,44–46]. These Ca:CO3 stoichiometries were also chosen to maintain constant pH and Ωara between the treatments.
