Cr(VI) reductase activity locates in the cytoplasm of Aeribacillus pallidus BK1, a novel Cr(VI)-reducing thermophile isolated from Tengchong geothermal region, China

Despite significant advances in remediation of Cr(VI) by microorganisms, studies on Cr(VI)-reducing thermophiles are still limited, which are highly desirable to be applied in Cr(VI) contaminated sites with high temperature. In this study, thermophilic Aeribacillus pallidus BK1, was isolated from Tengchong volcanic geothermal region China by virtue of microfluidics. The optimum temperature and pH value for the growth of Aeribacillus pallidus BK1 were found to be 60 degrees C and pH 7.5, respectively, and 400 mg/L was determined to be the minimal inhibition concentration of Cr(VI). A. pallidus BK1 exhibited superior heat-resistance which could withstand a high temperature up to 155 degrees C with maintained viability. The removal of Cr(VI) by A. pallidus BK1 was investigated, showing a removal efficiency of 98.34% and 86.87% Cr(VI) after 36 h with initial Cr(VI) concentrations of 20 mg/L and 100 mg/L, respectively, whereas 33.65% removal was achieved at 300 mg/L of Cr (VI). The removal of Cr(VI) by Aeribacillus pallidus BK1 was found to be absolutely through bioreduction, in the process of which Cr(VI) was transmembrane transported into the cytoplasm and then reduced by reductase, where sodium lactate was found to be the most favorable electron donor for the reduction of Cr(VI). FT-IR analyses confirmed that -COOH, -OH and -NH groups participated in the Cr(VI) transmembrane transportation, and TEM-EDX analyses indicated chromium were distributed as amorphous particles in the cytoplasm of Aeribacillus pallidus BK1. Moreover, XPS demonstrated that insoluble Cr(III) precipitates mainly coordinated with carboxyl, hydroxyl along with phenolic groups. Due to its thermophilic and extremely heat resistant properties, A. pallidus strain BK1 offered itself as a potential engineering bacteria for bioremediation of metal contaminated sites, especially in high temperature environments.