Superoxide mediated Mn(II) oxidation of an alkali-tolerant bacterium Pseudorhizobium flavum MXJ-1

Manganese-oxidizing microbes are capable of oxidizing Mn(II) to manganese oxides through direct enzymatic and indirect mechanisms. However, bacterial Mn(II) oxidation in alkaline environment remains unclear. This study isolated an alkali-tolerant bacterium Pseudorhizobium flavum MXJ-1 from marine surface sediments that can oxidize Mn(II) to biogenic manganese oxides (BioMnO_x). Characterization of BioMnO_x reveals that rod-shaped bacterial cells produce amorphous BioMnO_x containing mixed valences of Mn. The effects of different pH, carbon and nitrogen sources, metal ions on growth and Mn(II) oxidation activity of strain MXJ-1 were investigated. Results elucidate that strain MXJ-1 adapts to a broad range of pH from 5.0 to 10.0, achieves a maximum Mn(II) oxidation percentage of 69.41 %, but there is no significant correlation between biomass and Mn(II) oxidation. Cu(II) inhibited Mn(II) oxidation at concentration as low as 20 µmol/L, and Mn(II) oxidation decreased with Mn(II) levels increasing to 500 µmol/L. Genomic analysis identified two genes encoding animal heme peroxidases (AHPs) and three encoding dihydrolipoyl dehydrogenases (DLDHs), highlighting the potential role of superoxide in Mn(II) oxidation. Several alkali-tolerance genes, including those encoding Na⁺/H⁺ antiporter, were also identified, which probably associated with alkali-tolerance of MXJ-1. Superoxide detection by nitro blue tetrazolium (NBT) assay indicates it involved in Mn(II) oxidation. This study isolated a Mn(II)-oxidizing, alkali-tolerant bacterium, expanding insights into microbial Mn(II) oxidation in extreme environments.