Overexpression of Tetrahymena Cysteine Synthetase 1 Promotes Cadmium Removal by Biosynthesizing Cadmium Sulfide Quantum Dots in Escherichia coli

Heavy metal cadmium causes significant contamination in aquatic ecosystems. The biomineralization of cadmium represents a vital biological mechanism for handling cadmium stress in diverse microorganisms. To improve the biomineralization capacity of cadmium by microorganisms in aquatic environments, Tetrahymena cysteine synthetase 1 (TtCsa1) was overexpressed in E. coli. The tolerance of E. coli/pET-28a-TtCSA1 to cadmium was enhanced by expressing TtCsa1. Upon addition of cysteine, E. coli/pET-28a-TtCSA1 generated more H2S, which reacted with Cd2+ to form CdS quantum dots (QDs), resulting in a stronger fluorescence signal. The UV-visible absorption and fluorescence spectra of the culture supernatant of E. coli/pET-28a-TtCSA1 showed characteristic peaks corresponding to CdS QDs. Transmission Electron Microscopy (TEM) images confirmed that the formation of CdS QDs and their agglomeration in the E. coli cells. X-ray Diffraction Analysis (XRD) analysis further confirmed the presence of QDs and their crystalline nature. In rich medium, E. coli/pET-28a-TtCSA1 achieved removal rates of 99.5%, 98.2%, 56.5%, and 49.4%, respectively, for Cd2+ concentrations of 0.15, 0.3, 0.45, and 0.6 mM within 48 h. In simulated wastewater, E. coli/pET-28a-TtCSA1 achieved removal rates of 99.4%, 94.3%, 90.1%, and 89.8%, respectively, for Cd2+ concentrations of 0.3, 0.45, 0.6, and 0.75 mM within 12 h. These results demonstrate that overexpressing TtCsa1 in E. coli can significantly enhance its ability to biomineralize Cd2+ in rich medium and simulated wastewater, which has potential applications in bioremediation of aquatic environments contaminated with heavy metals.