Magnesium isotope fractionation during carbonate precipitation associated with bacteria and extracellular polymeric substances

Abstract

Magnesium (Mg) is abundant on Earth's surface aquatic environments and plays an important role during the precipitation of biogenic CaCO3 minerals. Moreover, Mg isotopes are increasingly used to study a number of bio-induced mineralization processes of calcite. In this study, Mg isotope signatures during calcite and aragonite precipitation in the presence of Curvibacter sp. HJ-1 and of their extracellular polymeric substances (EPS) were investigated and the evolution of mineralogical and liquid composition were tracked over time. The results showed that light Mg isotopes were preferentially incorporated into the precipitated solids in all the experiments with and without bacteria. Significant Mg isotope fractionation was associated with the transformation of amorphous calcium carbonate (ACC) to crystal carbonate (Δ26Mgcrystals-ACC = −0.9‰ and −0.7‰ in biotic and EPS experiments, respectively), and obvious Mg isotope fractionation was noted between solids and liquid (Δ26Mgsolid-liquid achieved −2.2‰ and −2.0‰ in biotic and EPS experiments, respectively). Besides, δ26Mg values were significantly correlated with pH, Mg content in liquid and solid, bacterial density and presence of EPS in biotic experiments. These findings indicated that strain HJ-1 had an effect on Mg isotope fractionation during calcite and aragonite precipitation. The difference in Mg isotope fractionation may be a new tool for understanding the biologically mediated effects on Mg-bearing carbonate precipitation, and serves as a useful alternative for aqueous Mg isotope, which is vital for reconstructing past environmental changes.