**4. Discussion**

This study compartmentalizes hypothetical abiotic conditions of the ECF, with the aim to gain a broader understanding of the chemical mechanisms relating to biomineralization among tropical marine calcifiers. We show that despite the same ion product of Ca and CO3, calcification rates vary with different Ca:CO3 ratios and Mg concentrations. In agreement with [31,36], this study emphasizes the importance of considering the ratio of Ca:CO3 when estimating the Ω*ara* within the ECF of marine calcifiers as exemplified in Figure 1. It is worth noting that in these experiments calcite and vaterite were also precipitated. The calculated Ωara in all experiments was 10, Ωcalcite was 15.15. Even though Ωvaterite has been reported to be lower than Ωara and Ωcalcite it is not possible to calculate Ωvaterite because this requires knowledge of the solubility product [49,50]. Our results show that high calcification rates, are not possible when the Mg concentration is equal to or half that of present-day oceanic concentrations, unless it is counterbalanced by a number of additional factors such as Ca:CO3 stoichiometry, temperature, Ω*ara*, proton pumping, or organic molecules. This study infers that the specific conditions required for CaCO3 precipitation among marine

calcifiers is positively amplified by the organism. It is difficult to fully understand the process that controls biomineralization without further in situ ionic measurements from the ECF or more in vitro experiments.
