Goethite-mediated enhancement of microbial Mn(II) oxidation and BaP degradation by Pseudomonas putida: The driving mechanism of extracellular superoxide radical.

Microbial functions and activities of Mn(II)-oxidizing microorganisms are influenced by minerals in the environment; however, the role of environmental minerals in mediating microbial Mn(II) oxidation and degradation of pollutants remains poorly understood. Here, the effect of a typical environmental mineral (goethite, Goe) on microbial Mn(II) oxidation by Pseudomonas putida MnB1 and degradation of benzo [a]pyrene (BaP) was systematically investigated. The addition of goethite significantly increased the bacterial oxidation rate of Mn(II) by MnB1 from 0.59 mg/L/h to 1.18 mg/L/h. Extracellular superoxide (O2•-) serves as the dominant driver of the biogenic oxidation of Mn(II) in the Goe-MnB1 composite system. Goethite enhances transmembrane transport, organic metabolism and oxidoreductase activity and increases the transfer activity of Complex I in the electron transport chain (ETC), while cytochrome c oxidase activity in the ETC was inhibited. The imbalance of electron transfer in ETC led to single-electron transfer at Complex III and contributed to the generation of extracellular superoxide. In addition, goethite that reacted with bacterial secrets also contributed to the enhanced generation of extracellular superoxide in the cell-free system. The Goe-MnB1 system exhibited an excellent BaP degradation efficiency during the biogenic oxidation process of Mn(II). The degradation rate of BaP reached 86.34% in 48 h, which was significantly higher than that of Goe-free system. These findings provide a deep understanding of molecular mechanism of Mn(II) oxidation and mechanistic insight into the remediation of BaP-contaminated environments.