Extracellular iron reduction is mediated in part by neutral red and hydrogenase in Escherichia coli

Abstract

Both microbial iron reduction and microbial reduction of anodes in fuel cells can occur by way of soluble electron mediators. To test whether neutral red (NR) mediates iron reduction, as it does anode reduction, by Escherichia coli, ferrous iron levels were monitored in anaerobic cultures grown with amorphous iron oxide. Ferrous iron levels were 19.4 times higher in cultures fermenting pyruvate in the presence of NR than in the absence of NR. NR did not stimulate iron reduction in cultures respiring with nitrate. To explore the mechanism of NR-mediated iron reduction, cell extracts of E. coli were used. Cell extract-NADH-NR mixtures had an enzymatic iron reduction rate almost 15-fold higher than the chemical NR-mediated iron reduction rate observed in controls with no cell extract. Hydrogen was consumed during stationary phase (in which iron reduction was detectable) especially in cultures containing both NR and iron oxide. An E. coli hypE mutant, with no hydrogenase activity, was also impaired in NR-mediated iron reduction activity. NR-mediated iron reduction rates by cell extracts were 1.5 to 2 times higher with hydrogen or formate as the electron source than with NADH. Our findings suggest that hydrogenase donates electrons to NR for extracellular iron reduction. This process appears to be analogous to those of iron reduction by bacteria that use soluble electron mediators (e.g., humic acids and 2,6-anthraquinone disulfonate) and of anode reduction by bacteria using soluble mediators (e.g., NR and thionin) in microbial fuel cells.

FIG. 1.

Hypothetical model depicting the role of NR in electron transfer reactions and electron translocation from metabolizing E. coli cells to an extracellular electron acceptor. NR enters the periplasm and cytoplasm by an unknown mechanism. NR is enzymatically reduced by hydrogenase and by other enzymes, such as dehydrogenases. NR then leaves the cell by an unknown mechanism and reduces extracellular iron or an electrode. NR_red, reduced NR; NR_ox, oxidized NR; OM, outermembrane; P, periplasm; CM, cytoplasmic membrane; C, cytoplasm; DH, dehydrogenase; XH, reduced cofactor; X⁺, oxidized cofactor; ?, possible transporter involved in NR transport.