Shewanella oneidensis MR-1 is a model dissimilatory iron reducing bacterium and plays an important role in bioremediation and biogeochemical cycling of elements. Understanding the potential interaction between iron and nitrate respirations by S. oneidensis MR-1 is of great significance for environmental bioremediation. In this paper, the stimulating effect of nitrate on iron reduction by S. oneidensis MR-1 was reported, which was different from previous reports. Nitrate at environmentally relevant concentrations (e.g., <2 mM) significantly enhanced the reduction of iron oxides, while nitrate at a higher concentration was detrimental to such a reduction. In the absence of nitrate, mutants of Delta napB and Delta nrfA exhibited a more rapid iron oxides reduction than the wild type strain (WT). in the presence of 2 mM nitrate, Delta napB showed a promoted reduction ability, but Delta nrfA completely lost such a capability. Furthermore, after dosing 2 mM nitrate, both WT and Delta napB generated more ATP while Delta nrfA exhibited a reduced ATP production. This study demonstrates that nitrate respiration affected iron respiration of S. oneidensis MR-1 in a complex manner and its nitrate- and iron-respiration properties endowed this strain a great flexibility and adaptability to the fluctuation and availability of nitrate and iron. (C) 2015 Elsevier Ltd. All rights reserved.