I just learned about a really intriguing study about how Shewanella accesses iron(III) from solid phase iron oxide.  The study used a novel electrochemical method to detect Fe(III) complexed with an organic ligand  secreted by Shewanella putrefaciens strain 200.  Controls showed that this ligand is secreted by Shewanella and not an artifact of dead cells or growth medium effects. The authors also discuss that the ligand is not the hydroxamate-type siderophore that Shewanella putrefaciens (strain 200) is known to secrete under aerobic conditions only.  It will be very interesting to see what the nature of this Fe(III) ligand is.

Shewanella putrefaciens produces an Fe(III)-solubilizing organic ligand during anaerobic respiration on insoluble Fe(III) oxides

Author(s): Taillefert, M (Taillefert, Martial); Beckler, JS (Beckler, Jordon S.); Carey, E (Carey, Elizabeth); Burns, JL (Burns, Justin L.); Fennessey, CM (Fennessey, Christine M.); DiChristina, TJ (DiChristina, Thomas J.)

Source: JOURNAL OF INORGANIC BIOCHEMISTRY, 101 (11-12): 1760-1767 NOV 2007

Abstract: The mechanism of Fe(III) reduction was investigated using voltarnmetric techniques in anaerobic incubations of Shewanella putrefaciens strain 200 supplemented with Fe(III) citrate or a suite of Fe(III) oxides as terminal electron acceptor. Results indicate that organic complexes of Fe(III) are produced during the reduction of Fe(III) at rates that correlate with the reactivity of the Fe(III) phase and bacterial cell density. Anaerobic Fe(III) solubilization activity is detected with either Fe(III) oxides or Fe(III) citrate, suggesting that the organic ligand produced is strong enough to destabilize Fe(III) from soluble or solid Fe(III) substrates. Results also demonstrate that Fe(III) oxide dissolution is not controlled by the intrinsic chemical reactivity of the Fe(III) oxides. Instead, the chemical reaction between the endogenous organic ligand is only affected by the number of reactive surface sites available to S. putrefaciens. This report describes the first application of voltarnmetric techniques to demonstrate production of soluble organic-Fe(III) complexes by any Fe(III)-reducing microorganism and is the first report of a Fe(Ill)-solubilizing ligand generated by a metal-reducing member of the genus Shewanella. (c) 2007 Elsevier Inc. All rights reserved.