Sea urchins have a spherical outer shell and spines that extend from their shell composed of calcium carbonate (CaCO3). The shell and spine are connected by muscles that allow the spine to move. Shells and spines are biominerals, defined as minerals created inside and outside living organisms, and consist of CaCO3 in the form of calcite, amorphous calcium carbonate (ACC), and intracrystalline organic matter (Albéric et al., 2018, Lauer et al., 2020, Seto et al., 2012). CaCO3 in the spines is composed of an inner porous spongy stereom and outer radial septa. These biominerals contain large amounts of Mg. For example, the spines of Paracentrotus lividus contain 3.8–4.9 mol% Mg (Zolotoyabko et al., 2010), that of some other sea urchins contain 2–12 mol% Mg (Magdans and Gies, 2004). Furthermore, some skeletons of some other echinoderms (bittle stars and sea stars) are composed of calcite with 16 % Mg (Bianco-Stein et al., 2022). The amount of Mg in spines increases from the tip to the base; however, the mechanism and function of heterogeneous Mg substitution are unknown (Magdans and Gies, 2004).

Mg is an element in the second group of the periodic table and shows similar chemical properties, including its electron configuration and reactivity, to those of Ca. The radius of the Mg ion is smaller than that of the Ca ion. Therefore, Mg ions are theoretically more reactive with anions than Ca ions. However, it is more difficult for Mg ions than Ca ions to combine with CO32–; therefore, the solubility of MgCO3 is 1.2 mM and that of CaCO3 is 0.15 mM in aqueous solution because of the interactions between Mg ions and water molecules (Qafoku et al., 2017, Saldi et al., 2009). MgCO3 is among the most important carbonate minerals, along with CaCO3; however, this compound is difficult to synthesize in an environment with ambient temperature and pressure. Nevertheless, amorphous MgCO3 (AMC) is readily synthesized at ambient temperature and pressure. With increasing temperatures, AMC evaporates water molecules and changes its crystal structure from nesquhonite to hydromagnesite, and then to magnesite via a dypingite (Hopkinson et al., 2012). On the other hand, crystalline MgCO3 hydrated with various crystal structures can be synthesized at ambient temperature and pressure. Dolomite, represented by CaMg(CO3)2, is a carbonate mineral that has formed carbonate rocks together with limestone throughout the history of the Earth under high pressure and temperature. Dolomite is also produced by microbial action (Petrash et al., 2017, Vasconcelos and McKenzie, 1997). Carbonate rocks containing dolomite account for more than one-fifth of the world's widely distributed sedimentary rocks (Al-Shargabi et al., 2023). Furthermore, Some methods for synthesizing CaMg(CO3)2 in vitro have been reported. Mg is incorporated into calcite by crystal formation in a mixture of water and alcohol (Falini et al., 1996). Presence of some organics, such as crosslinked gelatin films with entrapped poly-L-aspartate (Falini et al., 1997), porous silica gel (Fernández Díaz et al., 1996) and acidic amino acids and chitins(Fang et al., 2023), or presence of CO2 fine bubbles (Kimura et al., 2020) also leads Mg-containing calcite.

Using calcein-labeled Ca, Ca2+ was found to penetrate larval sea urchin embryos and were deposited intracellularly (Vidavsky et al., 2014). There are some researches about the Mg translocation pathways within the bodies of larval sea urchins. For example, larval primary mesenchyme cells contain a variety of vesicles with different elemental compositions, among which Mg-rich vesicles contribute to Mg transport. (Kahil et al., 2023), However, many of the pathways remain unclear.

The mechanism of synthesis of high-Mg-containing calcite constituting adult sea urchin spines is unclear. In this study, we investigated the factors that increase the Mg content in calcite using sea urchin Heliocidaris crassispina spines, which are composed of high Mg-containing calcite.