A soil-borne Mn(II)-oxidizing bacterium of Providencia sp. exploits a strategy of superoxide production coupled to hydrogen peroxide consumption to generate Mn oxides.

Bacterial non-enzymatic Mn(II) oxidation involving reactive oxygen species (ROS) (i.e., indirect oxidation), initially discovered from a marine alpha-proteobacterium, is believed to be of importance in controlling biogeochemical cycles. For soil-borne bacteria, however, evidence of indirect Mn(II) oxidation remains unclear. In this study, the indirect Mn(II) oxidation was evidenced in a soil-borne bacterium, Providencia sp. LLDRA6. First, with and without 50 mM of Mn(II) exposure for LLDRA6, 300 differentially expressed genes were found to be linked to Mn(II) exposure via transcriptome sequencing. Among them, an operon, responsible for phenylacetic acid catabolism, was sharply upregulated in transcription, drawing us a special attention, since its transcriptional upregulation has recently shown to be important for withstanding ROS. Next, a fluorometric probe, 2',7'-Dichlorofluorescin diacetate (DCFDA), was used to qualitatively detect ROS from cells, showing a distinct increase in fluorescence intensities of ROS during Mn(II) exposure. Furthermore, concentrations of superoxide and hydrogen peroxide from cells were detected, respectively, with and without Mn(II) exposure, exhibiting that when Mn(II) oxidation occurred, superoxide concentration significantly increased but hydrogen peroxide concentration significantly decreased. Particularly, superoxide produced by LLDRA6 was proven to be the oxidant for Mn(II) in the formation of Mn oxides. Finally, we predicted links between phenylacetic acid metabolism pathway and ROS during Mn(II) exposure, proposing that the excessive ROS, generated in response to Mn(II) exposure, transcriptionally activate phenylacetic acid catabolism presumably by increasing concentrations of highly reactive oxepins.