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

Mark E. Hodson*, Liane G. Benning, Bea Demarchi, Kirsty E. H. Penkman, Juan D. Rodriguez-Blanco, Paul F. Schofield, Emma A. A. Versteegh

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


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<sup>-1</sup> (n = 3; ± std dev) per individual amino acid); the CaCO<inf>3</inf> 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 ν<inf>2</inf>: ν<inf>4</inf> 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 CaCO<inf>3</inf> 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.

Original languageEnglish
JournalGeochemical Transactions
Issue number4
Early online date28 Apr 2015
Publication statusPublished - 2015

Bibliographical note

© 2015 Hodson et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver ( applies to the data made available in this article, unless otherwise stated.


  • ACC
  • Amino acids
  • CaCO<inf>3</inf>
  • Calcite
  • Earthworms
  • FTIR
  • Stability
  • Synchrotron

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