Competitive reduction of nitrate and iron oxides by Shewanella putrefaciens 200 under anoxic conditions

While the competitive reduction of nitrate and iron oxides has been documented previously, the detailed kinetics and the influence of iron oxides’ properties remain unclear. In this study, the kinetics for the microbial reduction of nitrate and iron oxides by a dissimilatory iron reducing bacterium Shewanella putrefaciens 200 (SP200) were investigated in artificial groundwater medium containing nitrate and various synthetic iron oxides. All iron oxides exhibited the inhibition of nitrate reduction by SP200, and the reduction of those iron oxides by SP200 was simultaneously inhibited, as well. The effects of crystalline α-Fe2O3 indicated that the extent of nitrate reduction inhibition decreased gradually in the presence of increasing crystalline hematite. The competitive effects were also confirmed by the cyclic voltammetry analysis and spectral analysis of cytochromes in whole cells of SP200. The competition between nitrate and ferric reduction might be ascribed to (1) the competition of Fe (III) and nitrate as coexisting electron acceptors for iron-reducing bacteria; (2) the slow reaction of cell-sorbed Fe(II) with nitrate compared with the direct reduction of nitrate by SP200. This study would provide useful information concerning the redox cycles of N in various iron-bearing environments.