*3.1. Precipitation Rates*

A highly significant interactive effect of Mg ion concentration and Ca:CO3 concentration was observed (Figure 3; Table 2; F[2,24] = 150.924, *p* < 0.001). When Mg was included into the aquatic solution, neither a significant weight change of the CaCO3 seed nor a difference between the two Ca:CO3 scenarios were observed (Table 2; Table A1). Conversely, both Mg-free Ca:CO3 scenarios had significant weight increases. The Mg-free 47:1 Ca:CO3 scenario had a calcification rate four times that of the Mg-free 2.8:1 Ca:CO3 scenario (Figure 3, points labelled A and B). The average weight increase (± SD) in the Mg−free 47:1 Ca:CO3 treatment was 1.017 (± 0.130] mg d−<sup>1</sup> and 0.229 (± 0.061) mg d−<sup>1</sup> in the Mg-free 2.8:1 Ca:CO3 treatment. The high amount of newly formed CaCO3 measured in the Mg-free 47:1 Ca:CO3 <sup>2</sup><sup>−</sup> is partly explained by spontaneous nucleation, which was only observed in this scenario. Slight dissolution was observed under the intermediate (26.5 mM] and high (53 mM) Mg scenario with a 47:1 Ca:CO3 concentration (−0.001 mg d−<sup>1</sup> ± 0.001 and −0.002 mg d−<sup>1</sup> ± 0.003, respectively). The intermediate and high Mg scenario with a 2.8:1 Ca:CO3 concentration had no significant weight changes (0.000 mg d−<sup>1</sup> ± 0.001 and 0.002 mg d−<sup>1</sup> ± 0.007, respectively), the Ca:CO3 concentration had no effect on the growth rate when Mg ion concentration was equal to or half that found in the ambient ocean, indicating that in this situation Mg has a stronger inhibiting effect towards calcification than Ca:CO3 concentrations.

**Figure 3.** Scatter plot showing the standardized total weight change per day [mg d−1] of each experimental scenario. The data shows a clear inhibiting effect of Mg and 2.8:1 Ca:CO3 stoichiometry towards inorganic mineralization. Treatments not connected by the same alphabetical symbol (A, B, or C) are significantly different as shown in Table 2; F[2,24] = 150.924, *p* < 0.001.


