[Isolation and Identification of Iron-reducing Bacteria Enterobacter sp. B4 and Its Reduction Mechanism of Cr(Ⅵ)]

Cr(Ⅵ) is a highly toxic, strongly oxidizing, and carcinogenic heavy metal pollutant. Reducing Cr(Ⅵ) to the less toxic Cr(Ⅲ) is an effective method to remediate chromium pollution, but there are relatively few studies on the biological reduction of Cr(Ⅵ) under acidic conditions. In this study, an iron-reducing bacterium was isolated from the sediment of acid mine drainage (AMD). Its Cr(Ⅵ) reduction characteristics under different culture conditions were investigated, and the behavioral characteristics and promotion mechanism of Fe(Ⅲ)-mediated synergistic reduction of Cr(Ⅵ) were explored. The results identified the isolate as a facultative anaerobic bacterium belonging to the genus Enterobacter, designated as Enterobacter sp. B4. Under anaerobic conditions with 10 mg·L^-1 Cr(Ⅵ), the strain was capable of both growth and Cr(Ⅵ) reduction within the pH range of 3.0-9.0. The addition of Fe(Ⅲ) (1.5-4.5 g·L^-1) significantly enhanced the Cr(Ⅵ) reduction efficiency. At pH 5.0 and an initial Cr(Ⅵ) concentration of 20 mg·L^-1, the reduction efficiency increased by 30.9%, reaching 68.2%; at pH 7.0 and 40 mg·L^-1 Cr(Ⅵ), the efficiency improved by 17.64%, reaching 61.9%. After adding Fe(Ⅲ), the electron transfer system activity of the strain increased by 2.27 and 2.11 times under pH 5.0 and pH 7.0 conditions, respectively, indicating that it mainly reduces high-valent metals through electron transfer chains, while Fe(Ⅲ) acts as an electron shuttle between bacteria and Cr(Ⅵ). Key components of the electron transport chain, including NADH and cytochrome c, exhibited significant increases by 4.62 μmol·mg^-1 and 0.124 μg·mg^-1 (normalized by protein) under pH 5.0 following Fe(Ⅲ) supplementation and by 7.52 μmol·mg^-1 and 0.58 μg·mg^-1 under pH 7.0, respectively. Meanwhile, extracellular polymeric substances, which can also serve as electron transfer media, increased by 2-3-fold. The enhanced electron transport not only directly improved the reduction efficiency of Cr(Ⅵ) but also increased the output of ATP and improved the metabolic activity and resistance to Cr(Ⅵ) of cells.