SOLUBLE ORGANIC-IRON(III) COMPLEXES ARE INTERMEDIATES IN THE REDUCTION OF IRON OXIDES BY SHEWANELLA PUTREFACIENS

Iron oxides and iron containing clays are efficient adsorbents of trace metals and other contaminants. The microbially-mediated reduction of these minerals in anoxic conditions can result in the concomitant release of adsorbed materials, which may have important environmental implications in the subsurface. Interestingly, the mechanisms of dissimilatory iron oxide reduction are not yet fully understood. For example, it remains uncertain how iron oxides can serve as terminal electron acceptors, despite their poor solubility in most natural waters. One of the proposed mechanisms to overcome this obstacle involves non-reductive dissolution of the oxides followed by reduction. In this process, the organism secretes an organic ligand which complexes and solubilizes ferric iron from the oxide surface to form organic-iron(III) compounds. These complexes eventually serve as terminal electron acceptors for microbial iron oxide reduction. Field studies in the sediments of Georgia have shown that in areas where iron reduction is prevalent, electrochemically labile soluble organic-iron(III) compounds are often present, suggesting that these complexes form during the dissolution of solid minerals. To determine if these complexes are a product of microbial iron reduction, we incubated a model iron reducing bacterium, Shewanella putrefaciens, with iron oxides of different chemical and physical composition. These experiments demonstrate that Shewanella putrefaciens produces endogenous organic ligands which solubilize iron oxides. Furthermore, a correlation between initial rates of organic-iron(III) and ferrous iron production indicates that the organic-iron(III) complexes are intermediates in the overall reduction of iron oxides. Our findings suggest that microbially-induced dissolution of iron oxides through an indigenous organic compound is an additional mechanism by which iron reducing bacteria can respire.