Crystallization of calcite from amorphous calcium carbonate: earthworms show the way

摘要

DARWIN (1881) was one of the first scientists to formally record that earthworms excrete calcium carbonate granules, with some up to 2 mm in diameter. However, since Darwin's studies they have received relatively little attention. The function of the granules remains a mystery, but may be related to regulation of Ca, CO_2, pH or some other as yet undiscovered metabolic process (Robertson, 1936; Piearce, 1972). There has recently been an increase in research activity focused on earthworm calcium carbonate granules driven by the work of Canti (Canti, 1998, 2007: Canti and Piearce, 2003) and largely relating to their potential use in archaeology. Canti determined granule production rates of 2.2 mg calcite day~(-1) worm~(-1) which translate into C processing rates of 60 kg C ha~(-1) y~(-1) via earthworm carbonate formation. These values indicate that granule production could play a small but significant role in the soil C cycle. Gago-Duport et al. (2008) have recently published the first detailed mineralogical study of the calcium carbonate granules and have traced their evolution from amorphous calcium carbonate (ACC) to calcite using scanning and transmission electron microscopy (SEM and THM). Fourier transform infrared (FTIR) spectro-scopy and X-ray diffraction (XRD). The aim of our complementary study has been to investigate the microstructures of the granules in order to determine the mechanisms by which they grow within the earthworm and subsequently crystallize to calcite.