Mg Content of Calcite Enhanced by Peptides: Insights to Interpreting Paleoenvironment from Biominerals

The magnesium content of biogenic and abiotic calcite has been related to both seawater chemistry and temperature, making it an important tool for reconstruction of paleoenvironment. The Mg content of biomineralized calcite precipitated at a given temperature often deviates from that of calcite formed inorganically at the same temperature. These deviations, known as the ‘vital effect,' are believed to have biological origins but the mechanistic basis for measured offsets remains unclear. Mineralization occurs within an organic-rich matrix comprised of largely acidic amino acids. Though these acidic amino acids have been implicated in biomineralization, their specific role is not well understood. Natural humic and protein substances in marine sedimentary environments have likewise been proposed to influence mineralization of nonskeletal carbonates. A recent study from our group found that nanomolar concentrations of acidic amino acids, peptides, and full proteins can accelerate calcite growth rate up to 25X through a relationship that correlates with hydrophilicity of the biomolecule (Elhadj et al. 2006, PNAS). The data showed that peptides promote growth by lowering the desolvation barrier of the cation. Because 1) cation incorporation is the rate-limiting step to growth and 2) Mg is more strongly solvated than Ca, we hypothesized that rate-modifying peptides would promote Mg incorporation relative to Ca, and thereby alter magnesium content of the calcite.

Here we demonstrate that a hydrophilic peptide, similar in composition to macromolecules isolated from sites of calcification, increases the magnesium content of calcite up to 3 mol%. Comparisons to previous studies that correlate Mg content of carbonate minerals with temperature show that the Mg enhancement due to peptides is equivalent to offsets of 7-14ºC. The insights also suggest a novel approach to tuning impurity levels in controlled materials synthesis and provide a physical basis for anecdotal evidence that organic chemistry modulates the mineralization of inorganic carbonates.