Insights into the formation mechanism of vaterite mediated by a deep-sea bacterium Shewanella piezotolerans WP3

Microbially mediated carbonate mineralization plays a crucial role in biogeochemical cycling of carbon, as microbes can efficiently transform atmospheric CO2 and organic carbon into carbonate cements which represent a significant sink in the global carbon cycle. It is well established that diverse bacteria from various natural habitats can induce the precipitation of calcium carbonate, and an increasing number of bacteria were found to be able to mediate the mineralization of vaterite. However, the precise mechanisms for the formation of bacterial vaterite are not fully understood. To better understand the effect of bacterial activity on vaterite formation, Shewanella piezotolerans WP3 was selected as a model microbe to induce calcium carbonate mineralization. A combination of bacterial and biomimetic mineralization experiments was adopted. Different bacterial components including native cells, EPS-free cells, cell-bound EPS, soluble EPS, and small molecule organics were isolated from the cultures and used to influence calcium carbonate crystallization and growth. The identification and characterization of the mineralized products were done using field emission scanning electron microscopy (FESEM), X-ray powder diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), thermogravimetry and differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). Bacterial in situ mineralization experiments demonstrate that S. piezotolerans WP3 can not only promote the mineralization of calcium carbonate, but also mediate polymorph selection of vaterite. Biomimetic mineralization experiments involving individual bacterial components reveal that different bacterial components have different effects on calcium carbonate polymorphism, and low molecular-weight amino acids secreted by the bacteria play a dominant role on vaterite formation and stabilization. Our observations also suggest that soluble EPS can induce the formation of aragonite while bound EPS facilitate calcite erosion. Current results can provide a deeper insight into bacterially mediated mineralization of vaterite and calcium carbonate polymorphism problem, and help evaluate the significance of biogenic carbonate in the diagenetic carbonate sinks. (C) 2018 Elsevier Ltd. All rights reserved.