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Biomineralisation by earthworms – an investigation into the stability and distribution of amorphous calcium carbonate

DOI: 10.1186/s12932-015-0019-z DOI Help

Authors: Mark Hodson (Environment Department, University of York) , Liane G Benning (School of Earth and Environment, University of Leeds) , Bea Demarchi (Departments of Chemistry and Archaeology, University of York) , Kirsty E H Penkman (Departments of Chemistry and Archaeology, University of York) , Juan Diego Rodriguez Blanco (University of Copenhagen) , Paul Schofield (Department of Earth Sciences, Natural History Museum) , Emma A A Versteegh (Department of Geography and Environmental Science, University of Reading)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Geochemical Transactions , VOL 16

State: Published (Approved)
Published: April 2015
Diamond Proposal Number(s): 9197 , 8989

Open Access Open Access

Abstract: Background: Many biominerals form from amorphous calcium carbonate (ACC), but this phase is highly unstable when synthesised in its pure form inorganically. Several species of earthworm secrete calcium carbonate granules which contain highly stable ACC. We analysed the milky fluid from which granules form and solid granules for amino acid (by liquid chromatography) and functional group (by Fourier transform infrared (FTIR) spectroscopy) compositions. Granule elemental composition was determined using inductively coupled plasma-optical emission spectroscopy (ICP-OES) and electron microprobe analysis (EMPA). Mass of ACC present in solid granules was quantified using FTIR and compared to granule elemental and amino acid compositions. Bulk analysis of granules was of powdered bulk material. Spatially resolved analysis was of thin sections of granules using synchrotron-based μ-FTIR and EMPA electron microprobe analysis. Results: The milky fluid from which granules form is amino acid-rich (≤ 136 ± 3 nmol mg−1 (n = 3; ± std dev) per individual amino acid); the CaCO3 phase present is ACC. Even four years after production, granules contain ACC. No correlation exists between mass of ACC present and granule elemental composition. Granule amino acid concentrations correlate well with ACC content (r ≥ 0.7, p ≤ 0.05) consistent with a role for amino acids (or the proteins they make up) in ACC stabilisation. Intra-granule variation in ACC (RSD = 16%) and amino acid concentration (RSD = 22–35%) was high for granules produced by the same earthworm. Maps of ACC distribution produced using synchrotron-based μ-FTIR mapping of granule thin sections and the relative intensity of the ν2: ν4 peak ratio, cluster analysis and component regression using ACC and calcite standards showed similar spatial distributions of likely ACC-rich and calcite-rich areas. We could not identify organic peaks in the μ-FTIR spectra and thus could not determine whether ACC-rich domains also had relatively high amino acid concentrations. No correlation exists between ACC distribution and elemental concentrations determined by EMPA. Conclusions: ACC present in earthworm CaCO3 granules is highly stable. Our results suggest a role for amino acids (or proteins) in this stability. We see no evidence for stabilisation of ACC by incorporation of inorganic components.

Journal Keywords: Calcite; Acc; Caco3; Ftir; Synchrotron; Amino Acids; Earthworms; Stability

Subject Areas: Chemistry, Biology and Bio-materials, Earth Science


Instruments: B22-Multimode InfraRed imaging And Microspectroscopy