Continous-wave EPR study of Mn(II)-bearing calcites: comparison between inorganic calcite and biocalcite, obtained by biomineralisation from the bacterium Bacillus subtilis

The first multianalytical characterisation of a biogenic calcite, produced from bacterial activity is reported in this study. Microcrystalline calcite powders were obtained from the model bacterium Bacillus subtilis grown on a suitable precipitation medium. The powders were analysed, without further manipulation, by means of X-ray powder diffraction (XRPD) and by Electron Paramagnetic Resonance (EPR) spectroscopy. Both techniques reveals unusual parameters, attributed to the effects of the bioprecipitation of the mineral. Namely, XRPD pointed out an anomalous c0/a0 ratio, which were noticed before only in microcrystalline calcite samples grown by biomineralisation of seashells, and EPR spectroscopy evidenced an anomalously large distribution of the zero-field interaction, which allows to clearly distinguish bacterial and inorganic calcite. As a consequence, this study enables the coupled use of XRPD and EPR for identifying the traces of bacterial activity in fossile carbonate deposits. Perspectives in the reconstruction of palaeoclimate and in the role of bacterial calcite in the geochemical cycles of pollutants are fostered.