Interplay between iron species transformation and hydroxyl radicals production in soils and sediments during anoxic-oxic cycles

Iron (Fe) redox transformation in soils and sediments plays an essential role in the biogeochemical cycling of associated nutrients and elements. It is documented recently that hydroxyl radicals (center dot OH) can be produced upon oxygenation of Fe(II)-bearing minerals in soils and sediments. As center dot OH induces serious oxidative stress to microbes, it is likely that center dot OH produced upon Fe(II) oxygenation inactivates the coexisting dissimilatory iron-reducing bacteria and subsequently impacts Fe redox transformation. This hidden interplay between Fe species transformation and center dot OH production in soils and sediments during anoxic-oxic cycles remains unexplored. In this study, a field sediment was incubated with 1 x 10(9) CFU/mL Shewanella oneidensis strain MR-1 and treated for six anoxic-oxic cycles (117-h anoxic and 6-h oxic periods). Fe species transformation, center dot OH production and MR-1 activity variation were measured throughout the six cycles. Fe in the sediments prior to anoxic-oxic cycles mainly existed in Fe-bearing silicates (60-70%) and Fe (hydr)oxides (30-40%). For the anoxic periods, a portion of Fe(III) in sediments was either reduced to structural Fe(II) in (hydr)oxides and silicates or reductively dissolved to aqueous Fe2+. Total Fe(II) produced from Fe(III) bio-reduction increased from 2.46 to 8.24 mM in the first two anoxic periods, but decreased to 0.10 mM in the following four anoxic periods. For the oxic periods, Fe (II) was oxidized along with a production of center dot OH, and the cumulative center dot OH concentrations were around 31 mu M in the first three periods but decreased to 6.3 mu M in the subsequent three periods. MR-1 was inactivated by about 0.6 order of magnitude in the first two oxic periods, and the inactivation became insignificant for the later oxic periods. The six cycles of anoxic-oxic treatments promoted the transformation of Fe(III) (hydr)oxides into more amorphous phase. A strong interplay between Fe species transformation and center dot OH production occurred in anoxicoxic cycles. Fe(III) bio-reduction by MR-1 in anoxic periods determined the content and speciation of Fe(II), which governed center dot OH production in the subsequent oxic periods. The generated center dot OH affected MR-1 activity in oxic periods, which further controlled Fe(III) bio-reduction in anoxic periods. The hidden interplay identified herein improves our understanding of the cycling of Fe and associated elements under redox fluctuating environments.