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The structure of the manganese oxide on the sheath of the bacterium Leptothrix discophora: An XAFS study

Astrid Jürgensen^1,*, Joline R. Widmeyer², Robert A. Gordon¹, Leah I. Bendell-Young², Margo M. Moore² and E. Daryl Crozier^1,

¹ Department of Physics, Simon Fraser University, 8888 University Drive, Burnaby, B.C. V5A 1S6, Canada
² Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, B.C. V5A 1S6, Canada

Correspondence: E-mail: crozier{at}sfu.ca

In natural waters, manganese oxides (MnO_x) are important in mediating the bioavailability of trace metals such as Ni, Cu, Zn, Cd, and Pb, as these metals readily adsorb to the MnO_x surface. Manganese from a variety of anthropogenic sources usually enters the aquatic environment in dissolved form as Mn²⁺. It is subsequently oxidized under oxic and neutral (pH = 6–7) conditions. Often this oxidation is catalyzed by bacteria, such as Leptothrix discophora, as part of their natural metabolic process.

Mn K-edge X-ray Absorption Fine Structure Spectroscopy (XAFS) was used to investigate the local structure of manganese oxide on the sheath produced by the bacterium Leptothrix discophora SP-6. The features observed in the near edge region of the Mn K-edge spectrum indicate the presence of three oxidation states of manganese: Mn²⁺, Mn³⁺, and Mn⁴⁺. Fitting the experimental XAFS data identifies the bacterial MnO_x as being composed of single-layer microcrystals with layers similar to those occurring in Na-birnessite. Some MnO₆ octahedra might lie outside the layer plane, sharing corners with those in the layer plane. X-ray diffraction results for the same samples are consistent with the single-layer structure.

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