Reductive Transformation Mechanism of Ferric Oxides in Hydromorphic Soils

In flooded soils and sediments the mechanism of Fe(III)-reduction has not yet been elucidated completely, although bacteria have been identified as the responsible agents. In model experiments the use of pure iron-reducing bacteria is required, if a distinction between the cause and consequence of their reductive activity on Fe(III)-oxides has to be made. In a nitrogen-free, synthetic glucose-Fe202-mineral salt medium (pH 7.1) the onset of Fe(II) formation and completely reduced conditions were obtained within 2-3 days, both with Bacillus polymyxa (nitrate reduc?ase positive = nit+) and Clostridium butyricum (nit-). The addition of NO3 or Mn02 suppressed iron-reduction almost completely in the case of B. polymyxa. With C. butyricum, only Mn02, but not nitrate, affected iron-reduction significantly. Nitrate remained unchanged throughout the incubation period although Fe(II) accumulated and completely reduced conditions were recorded. If bacteria and the Fe(III)-oxide particles were separated by a dialysis membrane, no Fe(II) could be detected although completely reduced conditions were recorded. Apparently, nitrate, Mn(IV) and Fe(III) are reduced directly and enzyme-specifically rather than indirectly as a consequence of reducing metabolites, the accumulation of ferrisiderophores and/or a lowered Eh. An intimate contact between cells and iron oxides is necessary for transfer of hydrogen from the bacterial metabolism to the external acceptors. The specificity of Fe(II)-formation could be observed in the course of the reduction of natural Fe(III)-oxides. If a mixture of amorphous and 59Fe-labelled crystalline Fe(III)-oxides was submitted to reduction, the former rather than the latter forms were used as electron acceptor in order to maintain the energy conserving reactions.