Mechanism of Cr (VI) reduction by Pichia guilliermondii ZJH-1

BACKGROUND: Chromium is one of the most used toxic heavy metals. A large amount of chromium waste is discharged into the environment every year, causing serious environmental pollution, especially the pollution of soil and water by hexavalent chromium. Eliminating hexavalent chromium is the primary challenge to achieve a pollution-free environment. OBJECTIVES: This study aims to understand the mechanism of Pichia guilliermondii’s reduction of hexavalent chromium through enzymatic characteristic, oxidative stress response, and reduction product. MATERIAL AND METHODS: The strain Pichia guilliermondii ZJH-1 was isolated and stored in our laboratory. The hexavalent chromium uses 1,5-diphenyl carbazide method (DPC) to measure. The UV spectrophotometer was used to measure the intracellular antioxidant enzyme activity, and the kit was used to measure the activity of catalase and glutathione reductase. The reduction products were analyzed by ultraviolet full-wavelength scanning and FTIR. RESULTS: The reduction of hexavalent chromium by ZJH-1 is accompanied by an increase in active oxygen and antioxidant levels. Chromate reductase mainly exists in the extracellular fluid, and the carboxyl, amide, hydroxide and other groups of the cell wall are involved in the bioremediation of Cr(VI) by complexing with Cr(VI) and Cr(III). After ZJH-1 was treated with different concentrations of Cr(VI), the expression of proteins with molecular weights of 15 kDa, 18 kDa, 35 kDa, 62 kDa, and 115 kDa increased significantly. This strain is the most suitable for chromate reductase (CChR). The optimum temperature is 40℃ and the optimum pH is 7.0. Cu2+ can enhance the activity of chromate reductase. At the optimum temperature and pH, the chromate reductase Km of this strain is 0.40 μmol and Vmax is 14.47 μmoL.L(-1)·min(-1). CONCLUSIONS: The bioremediation of Cr(VI) by Pichia guilliermondii ZJH-1 is attributable to the reduction product (Cr(III)) that can be removed in the precipitate and can be fixed on the cell surface and accumulated in the cell.