Organic Controls over Biomineral Ca-Mg Carbonate Compositions and Morphologies

Abstract

Calcium carbonate minerals, such as aragonite and calcite, are widespread in biomineral skeletons, shells, exoskeletons, and more. With rapidly increasing pCO₂ levels linked to anthropogenic climate change, carbonate minerals face the threat of dissolution, especially in an acidifying ocean. Given the right conditions, Ca-Mg carbonates (especially disordered dolomite and dolomite) are alternative minerals for organisms to utilize, with the added benefit of being harder and more resistant to dissolution. Ca-Mg carbonate also holds greater potential for carbon sequestration due to both Ca and Mg cations being available to bond with the carbonate group (CO₃^2-). However, Mg-bearing carbonates are relatively rare biominerals because the high kinetic energy barrier for the dehydration of the Mg²⁺-water complex severely restricts Mg incorporation in carbonates at Earth surface conditions. This work presents the first overview of the effects of the physiochemical properties of amino acids and chitins on the mineralogy, composition, and morphology of Ca-Mg carbonates in solutions and on solid surfaces. We discovered that acidic, negatively charged, hydrophilic amino acids (aspartic and glutamic) and chitins could induce the precipitation of high-magnesium calcite (HMC) and disordered dolomite in solution and on solid surfaces with these adsorbed biosubstrates via in vitro experiments. Thus, we expect that acidic amino acids and chitins are among the controlling factors in biomineralization used in different combinations to control the mineral phases, compositions, and morphologies of Ca-Mg carbonate biomineral crystals.

Figure 1

Mineralogical analyses of precipitates from biosubstrate-rich solutions. (A) X-ray powder diffraction patterns (Mo Kα) of precipitates formed with different amino acids and chitin solutions. Patterns in bold lines show the presence of high-Mg calcite (HMC) phases with (104) peak positions located between the calcite and dolomite standards. (B) Transmission electron microscopy (TEM) image of HMC formed on a chitin-coated opal surface. (C) Selected-area electron diffraction (SAED) patterns of (B) along [100] zone axis showing distinct “a”-reflections indicating large coherent domains and no “b”-reflection suggesting no Ca–Mg cation ordering. (D) TEM image of HMC formed in an aspartic acid solution. (E) SAED pattern of (D) along the [182] zone axis showing distinct a-reflections indicating large coherent domains and no b-reflection suggesting no Ca–Mg cation ordering.

Figure 2

SEM images of precipitates on opal, glass slide (GS), and gold-coated glass slide (GCGS) surfaces in control, aspartic acid, and chitin experiments showing various morphologies of Ca–Mg carbonates, such as high-Mg calcite (HMC) with the {104} and {100} crystallographic forms and disordered dolomite (Dol) and aragonite (Arg). MgCO₃ values are obtained from both XRD and SEM–EDS.

Figure 3

EBSD pole figures showing a strong preferred crystallographic orientation of high-Mg calcite (HMC) on gold-coated glass slide (GCGS) surfaces and a lesser degree of preferred orientation on the glass slide (GS) surfaces from aspartic acid and chitin experiments.

Figure 4

Raman spectroscopy measurements of Mg-rich amorphous calcium carbonate (Mg ACC) to high-Mg calcite (HMC) transformations in precipitates from the aspartic acid on glass slide experiments, compared to carbonate standards. (A) Full spectra from 20 to 4000 cm^–1. Zoomed-in spectra featuring (B) low-wavenumber internal modes, such as translational and librational modes between 50 and 400 cm^–1, and (C) v₁ symmetric stretching mode between 1050 and 1130 cm^–1. Peak positions for the major carbonate modes in this sample fall between peak positions for calcite (light blue) and dolomite (dark blue) standards and have FWHMs that fall between the range for the low-Mg ACC example and the crystalline carbonate mineral standards. Photomicrographs in reflected light show where spectra were collected from (D) an Mg ACC example grown on a gold-coated glass slide aspartic acid experiment sample, and (E–H) ACC-to-HMC transformations in the glass slide aspartic acid sample.

Figure 5

(A) Mole percent of MgCO₃ in carbonates precipitated from the solution increases with increasing aspartic acid concentrations with [Ca²⁺] = 10 mM, [Mg²⁺] = 20 mM, and [CO₃^2–] = 40 mM. (B) Mole percent of MgCO₃ in carbonates precipitated from solutions and on solid surfaces (opal, GCGS, and GS). Results are compared for aspartic acid (14 mM, red symbols), chitin (0.0375 g/L, blue symbols), and control (no added biosubstrates, black symbols) experiments. Experiments with aspartic acid and chitin positively correlated with increasing [Mg²⁺] in the initial solution ([Ca²⁺] = 10 mM, [CO₃^2–] = 40 mM).

Figure 6

Schematic showing various factors controlling morphology and growth rate of carbonate minerals. For more detail on crystal morphologies, see Figures S2 and S3. Here we connect spherulitic growth associated with faster growth rates and lower Mg concentrations with solution-controlled systems that depend primarily on solution chemistry, whereas the euhedral crystal morphologies (that are also more Mg-rich) align best with surface controlled conditions where organic substrates and solid surfaces play a larger role.

Figure 7

Compiled data showing aspartic and glutamic acid proportions in regions with calcite and with aragonite in bimineralic organisms. Crassostrea irredescenes,Mytilus californiaus,Atrina rigida,^, and Stylophora pistillate.