Underestimation about the Contribution of Nitrate Reducers to Iron Cycling Indicated by <i>Enterobacter</i> Strain

Nitrate-reducing iron(II) oxidation (NRFO) has been intensively reported in various bacteria. Iron(II) oxidation is found to be involved in both enzymatic and chemical reactions in nitrate-reducing Fe(II)-oxidizing microorganisms (NRFOMs). However, little is known about the relative contribution of biotic and abiotic reactions to iron(II) oxidation for the common nitrate reducers during the NRFO process. In this study, the typical nitrate reducers, four Enterobacter strains E. hormaechei, E. tabaci, E. mori and E. asburiae, were utilized as the model microorganisms. The comparison of the kinetics of nitrate, iron(II) and nitrite and N2O production in setups with and without iron(II) indicates a mixture of enzymatic and abiotic oxidation of iron(II) in all four Enterobacter strains. It was estimated that 22−29% of total oxidized iron(II) was coupled to microbial nitrate reduction by E. hormaechei, E. tabaci, E. mori, and E. asburiae. Enterobacter strains displayed an metabolic inactivity with heavy iron(III) encrustation on the cell surface in the NRFOmedium during days of incubation. Moreover, both respiratory and periplasmic nitrate-reducing genes are encoded by genomes of Enterobacter strains, suggesting that cell encrustation may occur with periplasmic iron(III) oxide precipitation as well as the surface iron(II) mineral coating for nitrate reducers. Overall, this study clarified the potential role of nitrate reducers in the biochemical cycling of iron under anoxic conditions, in turn, re-shaping their activity during denitrification because of cell encrustation with iron(III) minerals