Microbial iron reduction coupled to organic matter oxidation is
an important process in the biogeochemical cycling of iron in marine and
freshwater sediments, yet the physiological mechanism by which iron-reducing
bacteria reduce Fe(III) is poorly understood. In laboratory cultures, microbial
iron reduction is accelerated with amorphous iron oxides, compared to crystallized
iron oxides or clay minerals, partly because adsorption of biogenic Fe(II)
on crystalline iron oxides inhibits microbial iron reduction. The reduction
of clay minerals may be accelerated in the presence of organic ligands
because organic ligands simultaneously complex Fe(II) to prevent Fe(II)
adsorption. In addition, previous studies have shown that microbial
iron reduction is accelerated nearly two orders of magnitude if Fe(III)
is provided in a soluble organic form rather than amorphous ferrihydrite.
Thus, it is possible that organic ligands non-reductively dissolve Fe(III)
oxides in sediments to produce soluble organic-Fe(III), which is more readily
reduced by iron-reducing bacteria.
In this project, the mechanism of microbial reduction of iron oxides
is investigated in the laboratory with Shewanella putrefaciens,
a model organism well studied by microbiologists. The objective of this
project is to determine if iron is non-reductively solubilized during the
reduction process. To this end, microbial iron reduction is investigated
in batch reactors by voltammetry, a technique that allows the simultaneous
measurements of Fe(II) and soluble organic-Fe(III) complexes in situ.