Adsorption of Hg2+/Cr6+ by metal-binding proteins heterologously expressed in Escherichia coli

Abstract

Background: Removal of heavy metals from water and soil is a pressing challenge in environmental engineering, and biosorption by microorganisms is considered as one of the most cost-effective methods. In this study, the metal-binding proteins MerR and ChrB derived from Cupriavidus metallidurans were separately expressed in Escherichia coli BL21 to construct adsorption strains. To improve the adsorption performance, surface display and codon optimization were carried out.

Results: In this study, we constructed 24 adsorption engineering strains for Hg²⁺ and Cr⁶⁺, utilizing different strategies. Among these engineering strains, the M'-002 and B-008 had the strongest heavy metal ion absorption ability. The M'-002 used the flexible linker and INPN to display the merR_opt at the surface of the E. coli BL21, whose maximal adsorption capacity reached 658.40 μmol/g cell dry weight under concentrations of 300 μM Hg²⁺. And the B-008 overexpressed the chrB in the intracellular, its maximal capacity was 46.84 μmol/g cell dry weight under concentrations 500 μM Cr⁶⁺. While in the case of mixed ions solution (including Pb²⁺, Cd²⁺, Cr⁶⁺ and Hg²⁺), the total amount of ions adsorbed by M'-002 and B-008 showed an increase of up to 1.14- and 4.09-folds, compared to the capacities in the single ion solution.

Conclusion: The construction and optimization of heavy metal adsorption strains were carried out in this work. A comparison of the adsorption behavior between single bacteria and mixed bacteria systems was investigated in both a single ion and a mixed ion environment. The Hg²⁺ absorption capacity is reached the highest reported to date with the engineered strain M'-002, which displayed the merR_opt at the surface of chassis cell, indicating the strain's potential for its application in practical environments.

Keywords: Bioremediation; Detoxification; Engineered E. coli; Heavy metals; Transcription protein.

Fig. 1

Construction and pre-selection of adsorption strains. a The profile of the artificial plasmids. b-d The influence of different elements and optimization methods on the ability of engineered strains to adsorb heavy metals. All data shown are the mean ± SEM of three independent experiments

Fig. 2

Effect of induction conditions on the heavy metal adsorption capacity of the engineered bacteria. a-c Mercury adsorption strains; d-e Chromium adsorption strains. All data shown are the mean ± SEM of three independent experiments

Fig. 3

Effect of Hg²⁺ or Cr⁶⁺ concentrations on the adsorption capacity of engineered strains. The colored lines indicate engineered strains, and the black lines indicate the wild-type BL21. a Mercury adsorption strains; b Chromium adsorption strains; c and d Mixture of strains M’-002 and B-008. All data shown are the mean ± SEM of three independent experiments

Fig. 4

Adsorption performance of engineered strains in mixed metal ion solutions containing Cr⁶⁺, Hg²⁺, Cd²⁺, and As³⁺ at different concentrations. a Mercury adsorption strains; b Chromium adsorption strains; c Mixture of strains M’-002 and B-008. The mixed metal ion concentrations were set to 160, 400, and 800 μM, respectively. All data shown are the mean ± SEM of three independent experiments

Fig. 5

Comparison of the adsorption rates of four engineered strains in the adsorption of single and mixed ions. The solid lines indicate adsorption of single ions, and the dashed lines indicate adsorption of mixed ions. a The mercury ion adsorption rates of M'-002 and M'-006 in single and mixed ion solutions; b The chromium ion adsorption rates of B'-002 and B-008 in single and mixed ion solutions