The Formation of Bixbyite-Type Mn₂O₃ via Pyocyanin-Dependent Mn(II) Oxidation of Soil-Derived Pseudomonas aeruginosa

Abstract

Bacterial Mn(II) oxidation is believed to play a dominant role in accelerating the rate of Mn biomineralization in nature. Commonly, bacteria employ two ways involving Mn(II) oxidases and reactive oxygen species to oxidize Mn(II). In this study, a new strategy for bacterial Mn(II) oxidation involving the pyocyanin, a greenish blue phenazine pigment from Pseudomonas aeruginosa, was discovered. To begin with, a bacterial strain L3 was isolated from soils and identified as Pseudomonas aeruginosa, which exhibited the ability of Mn(II) oxidation. Next, the pyocyanin was purified from strain L3 cultures and proven to be involved in Mn(II) oxidation. Particularly, the oxidation of Mn(II) by pyocyanin was dependent on its ambient pH. In comparison with pH of 5 and 7, pyocyanin (the initial value of OD₃₈₇ was 0.56 at pH of 2) showed a stronger capability of oxidizing Mn(II) at pH of 9, reaching 144.03 μg L⁻¹ of Mn oxides after 108 h of Mn(II) oxidation, while pyocyanin ultimately produced 43.81 μg L⁻¹ at pH of 7 and 3.32 μg L⁻¹ at pH of 5, respectively. Further, strain L3 cultures were fractionated into three parts, i.e., the cell culture solution, fermentation supernatant, and cell suspension, and the Mn(II)-oxidizing activity was found to be distributed in the cell culture solution and fermentation supernatant, as evidenced by the formation of blackish glossy Mn oxides. Specifically, in the first half, the rate of Mn(II) oxidation by the fermentation supernatant was higher than that by the cell culture solution, whereas in the second half, the cell culture solution showed the much higher Mn(II)-oxidizing activity compared to the fermentation supernatant. Last but not least, the collective results from mineral characterization demonstrated that, the Mn oxides produced by P. aeruginosa strain L3, either by the cell culture solution or by the fermentation supernatant, were bixbyite-type Mn₂O₃ with poor crystallinity.

Keywords:

• Pseudomonas aeruginosa
• pyocyanin
• Mn(II) oxidation
• bixbyite-type Mn₂O₃

Acknowledgements

Dr. Ding Li sincerely thanks his classmates of Party School of the Hunan Committee of C.P.C. (Hunan academy of Governance), Lei Zhao, Ping Xiao, and Xin He for their constructive comments on the manuscript editing.

Authors’ contributions

DL conceived and designed the study. LX and FL retrieved the literature and drafted the manuscript. FT, SC, MH, LQ, YG, and HS participated in the study design and editing of the manuscript. All authors read and approved the final manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Data availability statement

The 16S rRNA gene sequence of P. aeruginosa L3 included in this study has been deposited at the GenBank with the accession number ON909190.

Additional information

Funding

This research was supported by the Key Research and Development Program of Hunan Province (2023NK2024), the National Natural Science Foundation of China (32171622 and 51774129), and the Science and Technology Innovation Program of Hunan Province (2022RC1128, 2023NK4295 and 2023JJ50169).