Hg(2+) accumulation by genetically engineered Escherichia coli in batch and continuous-flow systems: Construction, characterization, and applicability to Hg(2+) bioremediation of contaminated environments

Based on the characteristics of contaminated water, methods applicable for Hg{dollar}sp{lcub}2+{rcub}{dollar} cleanup should have selectivity in Hg{dollar}sp{lcub}2+{rcub}{dollar} removal, ability to remove Hg{dollar}sp{lcub}2+{rcub}{dollar} at low concentrations, and flexibility in dealing with variations of the ambient conditions. Selective Hg{dollar}sp{lcub}2+{rcub}{dollar} removal is necessary to recycle mercury for further use since it prevents metal contamination of the recycled mercury. To create an Hg{dollar}sp{lcub}2+{rcub}{dollar} cleanup system with the above properties, I constructed Escherichia coli strains by genetic engineering which can simultaneously express an Hg{dollar}sp{lcub}2+{rcub}{dollar} transport system and overexpress metallothionein as a fusion protein with glutathione S-transferase.; The best strain was tested for its ability to accumulate Hg{dollar}sp{lcub}2+{rcub}{dollar} at various concentrations, its selectivity for Hg{dollar}sp{lcub}2+{rcub},{dollar} and its resistance to ambient conditions. It is demonstrated that the strain is able to accumulate Hg{dollar}sp{lcub}2+{rcub}{dollar} at relatively low concentrations (0.2-4 mg Hg{dollar}sp{lcub}2+{rcub}{dollar}/L) in batch systems. The bioaccumulation can be empirically represented by the Langmuir equation. This Hg{dollar}sp{lcub}2+{rcub}{dollar}-bioaccumulating system appears to have more than a 10-fold and a 40-fold higher affinity for Hg{dollar}sp{lcub}2+{rcub}{dollar} than a biosorbent or an ion exchange resin, respectively. Furthermore, the cells can accumulate Hg{dollar}sp{lcub}2+{rcub}{dollar} from water without the addition of any nutrients or carbon sources. The Hg{dollar}sp{lcub}2+{rcub}{dollar}-bioaccumulating system is also resistant to ambient conditions, such as pH, ionic strength, and the presence of metal chelators or complexing agents (e.g., EDTA or cyanide), as well as being selective against other metals (e.g., Mg{dollar}sp{lcub}2+{rcub}{dollar} or Cd{dollar}sp{lcub}2+{rcub}).{dollar}; To test Hg{dollar}sp{lcub}2+{rcub}{dollar} bioaccumulation in a continuous flow mode, I constructed a hollow fiber bioreactor to retain the Hg{dollar}sp{lcub}2+{rcub}{dollar}-accumulating cells. After passage of Hg{dollar}sp{lcub}2+{rcub}{dollar}-laden influent through such a reactor, the level of Hg{dollar}sp{lcub}2+{rcub}{dollar} was reduced from 2 mg/L to about 5 {dollar}mu{dollar}Lg/L and the removed Hg{dollar}sp{lcub}2+{rcub}{dollar} was quantitatively recovered by the cells. A proposed mathematical model adequately described the bioreactor, providing a basis for its further optimization.