Extracellular Biomineralization of Gold by Delftia tsuruhatensis GX-3 Isolated from a Heavy Metal Contaminated Paddy Soil

Abstract

The use of microorganisms in geochemical gold recycling was considered recently. To explore the feasibility of gold biomineralization in this study, bacteria GX-3 were isolated from a heavy metal contaminated paddy soil in Guangxi province, China. This sulfur-reducing bacteria, Delftia tsuruhatensis, can survive in toxic Au3+ solution. Alkaline conditions (pH 8.0) and 35 °C were optimal for GX-3 cultivation. Nanoparticle formation was promoted by the coexistence of Fe3+. Field-emission scanning electron microscopy and transmission electron microscopy (TEM) images of GX-3 showed the presence of spherical particles, ranging from 50 to 100 and 200 to 500 nm in diameter, respectively, that stuck together and distributed around the surface of the bacteria as well as outside the cell. These nanoparticles were identified as gold according to the TEM–energy dispersive X-ray spectroscopy analysis. Here, bacterial extracellular solute-binding protein and/or porin should account for the gold nanoparticle formation and the detoxification in aqueous system as it transports electrons to Au3+ for nanoparticle precipitation. The ubiquitous nature of bacteria in near-surface environments and the rapid precipitation of gold by the sulfur-reducing bacterium suggest that these bacteria could be responsible for the wide bioaccumulation of gold in natural systems.