Inhibition of NO3- and NO2- reduction by microbial Fe(III) reduction:: Evidence of a reaction between NO2- and cell surface-bound Fe2+

A recent study (D. C. Cooper, F. W. Picardal, A. Schimmelmann, and A.J. Coby, Appl. Environ. Microbiol. 69:3517-3525, 2003) has shown that NO3- and NO2- (NOx-) reduction by Shewanella putrefaciens 200 is inhibited in the presence of goethite. The hypothetical mechanism offered to explain this finding involved the formation of a Fe(III) (hydr)oxide coating on the cell via the surface-catalyzed, abiotic reaction between Fe2+ and NO2-. This coating could then inhibit reduction of NOx- by physically blocking transport into the cell. Although the data in the previous study were consistent with such an explanation, the hypothesis was largely speculative. In the current work, this hypothesis was tested and its environmental significance explored through a number of experiments. The inhibition of similar to 3 mM NO3- reduction was observed during reduction of a variety of Fe(III) (hydr)oxides, including goethite, hematite, and an iron-bearing, natural sediment. Inhibition of oxygen and fumarate reduction was observed following treatment of cells with Fe2+ and NO2-, demonstrating that utilization of other soluble electron acceptors could also be inhibited. Previous adsorption of Fe2+ onto Paracoccus denitrificans inhibited NOx- reduction, showing that Fe(II) can reduce rates of soluble electron acceptor utilization by non-iron-reducing bacteria. NO2- was chemically reduced to N2O by goethite or cell-sorbed Fe2+, but not at appreciable rates by aqueous Fe2+. Transmission and scanning electron microscopy showed an electron-dense, Fe-enriched coating on cells treated with Fe2+ and NO2-. The formation and effects of such coatings underscore the complexity of the biogeochemical reactions that occur in the subsurface.