Study on the oxidative stress and transcriptional level in Cr(VI) and Hg(II) reducing strain Acinetobacter indicus yy-1 isolated from chromium-contaminated soil

Abstract

The bioreduction of Cr(VI) and Hg(II) has become a hot topic in the field of heavy metals bioremediation. However, the mechanism of antioxidant stress in Cr(VI) and Hg(II) reducing bacteria is still not clear. In this work, a novel Cr(VI) and Hg(II) reducing strain Acinetobacter indicus yy-1, was isolated from chromium landfill at a chromate factory, which was used to investigate the mechanism of antioxidant stress during the Cr(VI) and Hg(II) reduction process. The results demonstrated that the removal of Cr(VI) and Hg(II) by A. indicus yy-1 from solution was through reduction rather than biosorption. The reduction rates of Cr(VI) and Hg(II) by resting cells reached 59.71% and 31.73% at 24 h with initial concentration of 10 mg L−1, respectively. X-ray photoelectron spectroscopy (XPS) analysis further showed that Cr(III) and Hg(0) were mainly the Cr(VI)- and Hg(II)-reduced productions, respectively. Results of physiological assays showed Hg(II) was more toxic to A. indicus yy-1 than Cr(VI), and the activities of antioxidant enzymes (SOD and CAT) were significantly increased in A. indicus yy-1 for relieving the oxidative stress. The transcriptional level of genes related to Cr(VI) and Hg(II) reductases and antioxidant enzymes were up-regulated, indicating that the reductases have participated in the reduction of Cr(VI) and Hg(II), and SOD and CAT served as the vital antioxidant enzymes for defending the oxidative stress. This work provides a deep insight into the mechanism of antioxidant stress in Cr(VI) and Hg(II) reducing bacteria, which helps seek the highly resistant heavy metal reducing bacteria.