BIOGENIC NANOPARTICLES OF TRANSITION METAL SULFIDES PRECIPITATED AT LOW TEMPERATURES: THE PROPOSED ROLE OF METABOLITES AND THE CELL MICRO-ENVIRONMENT

Transition metals (TMe) such as Zn, Ni, Co and Cu are vital trace nutrients required for optimal functioning of various geochemically-important enzymes. A major mechanism that mediates the mobility and bioavailability of these metals is through the formation of metal sulfide nanoclusters and nanoparticles in sulfide-rich zones that arise due to the activities of sulfate-reducing bacteria. Little is still known about the identity (phase and elemental composition), morphology and crystallinity of metal sulfide nanoparticles formed under such conditions, hindering accurate assessments on their solubility and reactivity, and subsequently their importance on the biogeochemical cycling of the elements. Using comparative experimental approach, we have for the first time systematically characterized metal sulfide nanoparticles precipitated from low temperature, anoxic aqueous solutions at the nano-scale through a combination of high-resolution transmission electron microscopy (HRTEM) coupled with selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDS). With the exception of Cu-sulfides, the biogenic minerals always display faster rates of transformation towards more crystalline phases relative to the abiogenic minerals. We propose that these phenomena arise due to a combination of (a) bacterial metabolites that promote cluster/particle attachments and/or (b) the unique chemistry (i.e., lower pH, higher TMe and sulfide concentrations) within the cell micro-environment that favors mineral growth and dissolution-reprecipitation reactions. The latter in particular may help to explain the detection of minerals in nature that are not predicted to form based on the bulk solution chemistry. We present a diagram detailing the characteristics of metal sulfide nanoparticles precipitated at different degrees of biological influence and within the range of initial aqueous TMe-to-Fe(II) ratios that are representative of natural environments. Our work is expected to contribute to the prediction of metal sulfide nanoparticles’ solubility and reactivity in various natural processes.