REDUCTION OF HEXAVALENT CHROMIUM AND DETECTION OF ENZYMATIC ACTIVITY IN RHODOCOCCUS QINGSHENGII STRAIN SC26

Understanding bacterial detoxifying metabolic responses to hexavalent chromium (Cr(VI)) is important to clarify resistance mechanisms for improving the bioremediation of Cr(VI)-contaminated wastewater. The majority of resistance mechanisms for lowering chromium contamination relies on Cr(VI) reduction to its less toxic trivalent state (Cr(III)). Beside chromate reductase, also NADP-dependent oxidoreductase may transfer electrons facilitating the process of Cr(VI) reduction. The present study aimed at clarifying these mechanisms in the Cr(VI)-resistant Rhodococcus qingshengii strain SC26 (MIC 200 mg L-1). According to Jacob-Monods model, specific growth rates (, CFU ml-1 h-1) were 0.214, 0.177 and 0.155 at 25, 50 and 100 mg L-1 Cr(VI), respectively. Reduction rates (mg L-1 h-1) to Cr(III) were 0.353, 0.399 and 0.928 at 25, 50 and 100 mg L-1 Cr(VI), respectively, at 72 h incubation. Enzymatic activity of chromate reductase (U mg-1 of protein) was 288.08 in the presence of 50 mg L-1 Cr(VI) and 4.50 in absence of metal. Gene for chromate reductase was not detected by using Rhodococcus erythropolis primers previously designed. Beside chromate reductase, in strain SC26 NADP-dependent oxidoreductase gene expression (Livak method) was observed 7 times higher in the presence of 50 mg L-1 Cr(VI) than in its absence. When grown in real wastewater (100 mg L-1 Cr(VI)), strain SC26 maintained the ability to reduce Cr(VI) by 82% after 169 h incubation. Although strain SC26 is inhibited by Cr(VI), its ability to reduce the metal is enhanced due to over-expression of both chromate reductase and NADP-dependent oxidoreductase. The primary challenge lies in implementing the use of this strain SC26 in large-scale wastewater treatment. Acknowledgement. Research supported by Fondazione CARIPLO-Circular Economy 2020 project num. 1069-2020 “Heavy Metal Bio-recovery and Valorization-HMBV” https://sites.unimi.it/hmbv/. Melzi A. is awarded of a PhD fellowship by the University of Milan - Food Systems PhD Program.