Bioaugmentation of Cd(II) removal in high-salinity wastewater by engineered Escherichia coli harbouring EC20 and irrE genes

Wild strain generally can remove heavy metals but cannot tolerate high-salinity. In this study, Cd(II) adsorption protein EC20 on the cell surface and global regulator irrE in the cytoplasm of Escherichia coli were expressed to adsorb Cd(II) in high-salinity conditions. The maximum adsorption amount was 30.79 mg/g at 30.84 mg/L Cd(II) and 2% NaCl of E. coli (EC20/irrE), which was higher than E. coli (p23), E. coli (EC20) and E. coli (irrE). The adsorption kinetics fitted well with the Elovich model. And Redlich-Peterson model was used to describe the adsorption process successfully. Transcriptome data analysis showed that the overexpression of irrE in E. coli significantly increased high-salinity tolerance, because of trehalose accumulation and exopolysaccharide export. Subsequently, a bioaugmentation sequencing batch reactor was adopted for treating the actual Cd(II) contained high-salinity petrochemical wastewater. The results showed that the chemical oxygen demand and Cd(II) removal reached up to 96.17% and 97.60%, higher than the control group of 79.15% and 92.09%, under the influent chemical oxygen demand of 1715 mg/L, Cd(II) of 4.63 mg/L and salinity of 3.0% in bioaugmented system. 16S rRNA analysis revealed that the microbial community in the bioaugmented system adapted to the high-salinity and Cd(II) stress which proved the superiority of bioaugmentation. Finally, a mechanism for highsalinity tolerance in sequencing batch reactor was revealed and provides a basic foundation for the engineered strain to treat Cd(II) containing high-salinity wastewater. This study will promote the understanding of the effect of engineered strains bioaugmentation on the treatment of heavy metals and other pollutants in high-salinity wastewater.