The significance of this carbon dioxide sink becomes evident when considering the extensive coral reefs and vast fields of blooming algae, visible even from space. With ocean acidification likely impeding this cycle, there is a threat of a negative feedback loop, where reduced carbon capture leads to increased ocean acidification, further disrupting CO 2 sequestration. Understanding biomineralization mechanisms, and more specifically, the impact of external factors on organisms' ability to form biogenic calcite, is critical for assessing their impact on these complex mineral formation pathways. Biogenic calcite often comprises intricate, complex crystals that serve as skeletons and protection against predation. Additionally, these crystals can function as materials for locomotion, buoyancy, and even light detection. To form such multifunctionality from just a few available building blocks, organisms must exert meticulous control over the nucleation and growth of the biomineral by regulating the physicochemical boundary conditions under which the mineral forms. Although synthetic efforts, especially under standard conditions, have yet to succeed in creating similarly sophisticated materials, understanding biomineralization pathways may pave the way for new approaches in forming synthetic, bioinspired materials. A commonly recognized pathway that plays a significant role in shaping biominerals involves using a transient, amorphous intermediate called amorphous calcium carbonate (ACC). This intermediate can be molded into various forms, shapes, and textures. While classical physicochemical parameters, such as supersaturation-dependent on concentration and pH in the case of ACC-determine nucleation pathways, it is also recognized that specific additives, both organic and inorganic, play a crucial role in the mineral's nucleation and growth. Organic additives, particularly acidic macromolecules, have been shown to decrease ACC nanoparticle size by locally increasing supersaturation. They can also stabilize ACC and influence polymorph selection during crystallization, such as forming calcite or vaterite. Additionally, these additives may become incorporated into the host crystal during crystallization, inducing anisotropic lattice distortions. Inorganic additives, especially magnesium and strontium ions, are found in biogenic calcite with concentrations of up to 45 mol% often incorporated into the crystal structure. Magnesium incorporation into ACC, producing magnesium-stabilized ACC (Mg-ACC), can significantly enhance the stability of the amorphous intermediate against crystallization for several months, ultimately influencing the final polymorph. While this has significant implications for Mg-ACC biomineralization, the exact mechanism remains debated. It has been proposed that the relatively higher dehydration energy of Mg 2+ than Ca 2+ limits water diffusion, which is responsible for the amorphous to crystalline transformation; however, studies have shown that water diffusion in pure ACC is slower than in Mg-ACC, challenging this hypothesis. Additionally, the increased incorporation of hydroxyl groups along with Mg 2+ ions may contribute to the enhanced stability of Mg-ACC. While the stabilizing effect of magnesium has been extensively studied, its influence on the initial formation of Mg-ACC, which often precipitates as nanoparticles, remains unclear. Specifically, magnesium's effect on particle size and size distribution-crucial for determining ACC stability and polymorph control-has not been thoroughly investigated. Numerous biomineralization pathways can be rationalized within the concept of non-classical nucleation theory. Unlike classical ion-by-ion nucleation, it describes the formation of crystals from metastable inter-mediates that can assemble via different pathways to produce the final solid. One of these pathways is crystallization via particle attachment (CPA) of transient nanoparticles forming a mesocrystalline intermediate that macroscopically mimics classical ion-by-ion growth, first observed in sea urchin larvae. CPA is widespread and is observed in calcite-forming species such as corals and nacre-forming mollusks, but even across phyla in mammalian teeth and plants. The formation of intermediate mesocrystals requires a very narrow nanoparticle size distribution. Considering the process as the assembly of a colloidal crystal, successful mesocrystal formation necessitates relative size distributions below 5-10%. Since CPA is widespread and depends on a controlled and narrow nanoparticle size distribution, we aim to understand how various biomineralization-relevant parameters influence ACC nanoparticles' size and size distribution.