**5. Conclusions**

To understand the nuances of how coral reef calcifiers can adapt to global change, such as ocean acidification, we need to better understand the ionic composition at the site of calcification. Unfortunately, in the short-term tropical calcifying organisms show little acclimatization potential to ocean acidification [101] particularly when coupled with thermal

stress [102], but there are few examples of resistance by altering the ionic concentrations in the ECF, for example Ca [43]. Additionally, this study considered the influence of various Ca:CO3 stoichiometry and Mg concentrations on the precipitation rates and morphology of CaCO3 in a homeostatic experiment. Although there is still a need to conduct more experiments covering a range of other possible scenarios, we believe that our findings are highly relevant within the field of coral reef research for the following reasons:


**Author Contributions:** C.E.R. and S.H., contributed equally to all aspects of this study from the conceptualization, methodology, validation, analysis, drafting, writing, and editing, visualization, project administration, and funding acquisition of this study. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project was funded by the Leibniz Center for Tropical Marine Research [ZMT, Bremen] who is supported by the Ministerium für Kultur und Wissenschaft des Landes Nordrhein-Westfalen, the Regierende Bürgermeister von Berlin-inkl. Wissenschaft und Forschung, and the Bundesministerium für Bildung und Forschung.

**Acknowledgments:** Special thanks to Matthias Birkicht for analytical support and Gernot Nehrke for use of the CRM to identify the crystal polymorphs. We greatly appreciate the constructive comments from the three anonymous reviewers.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
