Isolation and Characterization of Environmental Bacteria Capable of Extracellular Biosorption of Mercury

Accumulation of toxic metals in the environment represents a public health and wildlife concern. Bacteria resistant to toxic metals constitute an attractive biomass for the development of systems to decontaminate soils, sediments, or waters. In particular, biosorption of metals within the bacterial cell wall or secreted extracellular polymeric substances (EPS) is an emerging process for the bioremediation of contaminated water. Here the isolation of bacteria from soil, effluents, and river sediments contaminated with toxic metals permitted the selection of seven bacterial isolates tolerant to mercury and associated with a mucoid phenotype indicative of the production of EPS. Inductively coupled plasma-optical emission spectroscopy and transmission electron microscopy in conjunction with X-ray energy dispersive spectrometry revealed that bacteria incubated in the presence of HgCl2 sequestered mercury extracellularly as spherical or amorphous deposits. Killed bacterial biomass incubated in the presence of HgCl2 also generated spherical extracellular mercury deposits, with a sequestration capacity (40 to 120 mg mercury per g [dry weight] of biomass) superior to that of live bacteria (1 to 2 mg mercury per g [dry weight] of biomass). The seven strains were shown to produce EPS, which were characterized by Fourier transform-infrared (FT-IR) spectroscopy and chemical analysis of neutral-carbohydrate, uronic acid, and protein contents. The results highlight the high potential of Hg-tolerant bacteria for applications in the bioremediation of mercury through biosorption onto the biomass surface or secreted EPS.