Toward Bioremediation of Methylmercury Using Silica Encapsulated Escherichia coli Harboring the mer Operon

Abstract

Mercury is a highly toxic heavy metal and the ability of the neurotoxin methylmercury to biomagnify in the food chain is a serious concern for both public and environmental health globally. Because thousands of tons of mercury are released into the environment each year, remediation strategies are urgently needed and prompted this study. To facilitate remediation of both organic and inorganic forms of mercury, Escherichia coli was engineered to harbor a subset of genes (merRTPAB) from the mercury resistance operon. Protein products of the mer operon enable transport of mercury into the cell, cleavage of organic C-Hg bonds, and subsequent reduction of ionic mercury to the less toxic elemental form, Hg(0). E. coli containing merRTPAB was then encapsulated in silica beads resulting in a biological-based filtration material. Performing encapsulation in aerated mineral oil yielded silica beads that were smooth, spherical, and similar in diameter. Following encapsulation, E. coli containing merRTPAB retained the ability to degrade methylmercury and performed similarly to non-encapsulated cells. Due to the versatility of both the engineered mercury resistant strain and silica bead technology, this study provides a strong foundation for use of the resulting biological-based filtration material for methylmercury remediation.

Fig 1. Representative zone of inhibition from filter disc assays for mercury(II) chloride resistance for A) E. coli pDU1358 B) E. coli pBBRBB::mer and C) E. coli pBBRBB.

Filter discs in each image are identical, (6 mm in diameter).

Fig 2. Scanning Electron Microscopy images of encapsulation silica sol-gel microbeads containing E. coli pBBRBB::mer.

A) Representative image depicting the smooth, spherical shape of silica microbeads following encapsulation in aerated mineral oil. Scale bar represents 200 μm B) Image of engineered E. coli pBBRBB::mer cells within encapsulation beads. Scale bar represents 5 μm.

Fig 3. Degradation of methylmercury chloride by A) Non-encapsulated (open symbols) and B) Encapsulated (closed symbols) E. coli pBBRBB::mer (circles) and E. coli pBBRBB (squares).

Degradation of methylmercury chloride in abiotic medium (open triangle) and sorption by abiotic beads (closed triangle) were included as controls. Data presented is for experiments performed at least in triplicate with error bars represented as SEM.

Fig 4. Process for utilizing silica-encapsulated E. coli pBBRBB::mer as a bioremediation catalyst in flow-through systems.

E. coli containing pBBRBB::mer are encapsulated in silica beads using sol-gel technology and catalyze the cleavage of organic C-Hg bonds of mercury species and subsequent reduction of Hg(II) to Hg(0). Resulting Hg(0) is then captured downstream by an activated charcoal filter.