Portraying manganese biofilms via a merger of EPR spectroscopy and cathodic polarization

Abstract

Microbial biofilms on stainless steel surfaces exposed to water from a freshwater pond were dominated by manganese-oxidizing bacteria, as initially diagnosed by microscopy and elemental analysis. The application of electron paramagnetic resonance (EPR) spectroscopy revealed conspicuous sextet (six-line) patterns that intensified with immersion time, implying the gradual accumulation of Mn(II) in the biofilms. Correspondingly, cathodic polarization designated the manganese oxide (MnO_x) reduction peak in the form of a distinctive ‘nose’, which grew increasingly more negative with biofilm growth. The progressive expansion of cathodic current densities and the concurrent area-under-the-curve also allowed the quantification of microbially mediated MnO_x deposition_. Furthermore, the merger of EPR and cathodic polarization techniques yielded key insights, in tandem with Mn speciation data, into the pathways of microbial manganese transformations in biofilms, besides providing meaningful interpretations of prevailing literature. Accordingly, the natural freshwater biofilm was inferred as one supporting a complete manganese cycle encompassing multiple redox states.

Keywords:

• Microbial biofilms
• natural freshwater
• stainless steel
• manganese oxide
• EPR spectroscopy
• cathodic kinetics
• electrochemical polarization

Acknowledgements

The authors thank the Director, CSIR–Central Electrochemical Research Institute, Karaikudi (India) for encouragement and support. CSIR–HRDG, New Delhi is gratefully acknowledged for the award of a senior research fellowship (SRF) to A.L.K. The authors are especially thankful to Dr Brenda Little for reviewing an earlier version of this manuscript and for inviting M.E. to present this work at the CORCON Conference in Mumbai (India) in September 2017. The authors also owe their profound gratitude to Prof. Stephen C. Dexter for his permission to present unpublished data from work performed by M.E. while the latter worked at the University of Delaware. This paper benefited considerably from helpful comments and criticisms of the anonymous reviewers.

Disclosure statement

The authors declare no competing financial interests concerning this work.