Nitrogen removal characteristics and Cr(VI) tolerance mechanisms of heterotrophic nitrifying bacterium Pseudomonas putida strain LX1

Abstract

To solve the severe inhibition effect of nitrification process by heavy metals, necessitating the discovery of heavy metals-resistant nitrifying bacteria. In this study, a highly efficient bacterium, Pseudomonas putida strain LX1, capable of heterotrophic nitrification–aerobic denitrification (HN-AD) and resistant to hexavalent chromium (Cr(VI)) was isolated from activated sludge. Strain LX1 exhibited excellent growth activity and achieved efficient NH4+-N removal rates (94.07%–83.36 %) under 0–25 mg/L Cr(VI) stress. The heterotrophic nitrification ability of strain LX1 was superior to that of aerobic denitrification, and there was no accumulation of nitrite or nitrate during HN-AD process. Moreover, Three-dimensional excitation-emission matrix and Fourier-transform infrared spectroscopy analysis indicated that strain LX1 secreted more extracellular polymeric substances with negatively charged functional groups to bind Cr(VI) and alleviate its toxicity. Enzymatic analysis showed that strain LX1 countered the oxidative damage of reactive oxygen species by promoting the activities of oxidative dismutase and catalase. Furthermore, transcriptomic analyses revealed that the upregulation of genes associated with heavy metal substrate transporter proteins and antioxidant systems in strain LX1 contributed to the resistance against the high biotoxicity caused by Cr(VI). This study is important for understanding the heavy metal response mechanisms of HN-AD strains and its potential application in practical wastewater treatment.