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, <i>Tetrahymena</i> cysteine synthetase 1 (TtCsa1) was overexpressed in <i>E. coli</i>. The tolerance of <i>E. coli</i>/pET-28a-<i>TtCSA1</i> to cadmium was enhanced by expressing TtCsa1. Upon addition of cysteine, <i>E. coli</i>/pET-28a-<i>TtCSA1</i> 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 <i>E. coli</i>/pET-28a-<i>TtCSA1</i> showed characteristic peaks corresponding to CdS QDs. Transmission Electron Microscopy (TEM) images confirmed that the formation of CdS QDs and their agglomeration in the <i>E. coli</i> cells. X-ray Diffraction Analysis (XRD) analysis further confirmed the presence of QDs and their crystalline nature. In rich medium, <i>E. coli</i>/pET-28a-<i>TtCSA1</i> 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, <i>E. coli</i>/pET-28a-<i>TtCSA1</i> 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 <i>E. coli</i> 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.