Leptothrix discophora: How bacterial MnO2 oxidises iron

Tanja Eggerichs1, Oliver Opel1, Tobias Otte1, Wolfgang Ruck1
1 Institute of Sustainable and Environmental Chemistry, Leuphana University of Lüneburg

P 31 in Coupling between iron biogeochemistry and cycles of other elements


Occurring in wells, pipework, and in natural groundwaters, the oxidation of ferrous iron is a partly abiotic-, partly biotic-induced process. One of the most frequent species of neutrophilic iron oxidising bacteria is Leptothrix discophora. It is known to be able to oxidise Fe(II) as well as Mn(II).

The adverse effects of ferric iron depositions are well known, but how Leptothrix benefits from this feature is still not fully understood. The elimination of H2O2 by MnO2 [1] or the protective effect from high, inhibitory concentrations of Mn(II) and Fe(II) within the sheaths [2] have been discussed as possible advantages.

We assume that the iron oxidation through Leptothrix discophora is a purely abiotic process: iron is oxidised by the simultaneous reduction of actively oxidised Mn(IV).

Fe(II) can be oxidised by Mn(IV). Oxidation experiments have been carried out in artificial groundwater, comparing the iron oxidation rates of biotic and abtioic produced Mn(IV). As a reference, the influence of bacterial cells without Mn(IV) depositions on the iron oxidation has been measured.

L. discophora SS1, a sheathless strain and L. discophora SP6, a sheathed strain have been grown in a complex and L. discophora SP6 only in a minimal medium with and without the addition of Mn(II). The cells have been harvested in stationary phase by centrifugation. The bacteria or abiotic MnO2 precipitates have been added to a buffered, artificial groundwater. Samples from the stirred experimental reactor have been taken regularly to determine Mn(II), Mn(IV) and Fe(II) concentrations by ICP-OES in filtered and unfiltered samples and ferrozine assay. Oxidation rates have been calculated for all sets of experiments.

L. discophora cells grown without the addition of Mn(II) do not accelerate the Fe(II) oxidation rate, compared to oxidation rates in artificial groundwater without the addition of cells or Mn(IV).

Comparing iron oxidation in artificial groundwater with Mn(IV) from all sources (L. discophora SP6 and SS1 in minimal and complex medium, abiotic MnO2) lead to very similar oxidation rates. Mn(IV) particles are hardly incorporated in sheaths or covered with EPS. All biotically produced Mn(IV) can serve as an oxidating agent for iron oxidation.

Leptothrix can oxidise the freshly reduced Mn again and lead, therefore, to constant iron oxidation even in low Mn containing systems.

[1] G. Dubinina, Mikrobiologiia 1978, 47, 783–789.

[2] S. Rogers, J. Anderson, J Bacteriol 1976, 126, 264–271.

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last modified 2013-01-30