Co-response of Fe-reducing/oxidizing bacteria and Fe species to the dynamic redox cycles of natural sediment.

Fe(III)-reducing bacteria (FRB) and Fe(II)-oxidizing bacteria (FOB) play essential roles in the biogeochemical cycling of iron (Fe). Although the redox transformation of Fe species mediated by FRB/FOB has been extensively studied, the co-responses of FRB and FOB and Fe species transformation in natural sediment under dynamic redox conditions are poorly known. This study explored the variations of potential FRB and FOB abundances and Fe species transformation in natural sediment during successive anoxic-oxic-anoxic-oxic-anoxic cycles. Compared with the pristine sediment sample, the FRB abundance increased 121-793% (initial: (2.6 ± 0.6) × 107 copies/g) in the anoxic stages, while it decreased by 38-64% in the oxic stages. The increase in FRB abundance was ascribed to energy gain of FRB from the reduction of the amorphous Fe(III) (Fe(III)am) and the crystalline Fe(III) (Fe(III)cry) to the aqueous Fe(II) (Fe(II)aq), the adsorbed Fe(II) (Fe(II)ad) and the amorphous Fe(II) (Fe(II)am), while the decrease was attributed to the oxidative stress caused by the reactive oxidant produced from the abiotic oxidation of Fe(II)aq, Fe(II)ad and Fe(II)am to Fe(III)am and Fe(III)cry. The FOB abundance decreased 38-44% (initial: (5 ± 1.8) × 107 copies/g) in the second and third anoxic stages, while slightly fluctuated in the oxic periods. This observation was contrary to the variation of FRB, which might be attributed to the strong resistance to oxidative stress of FOB and its ability to obtain energy under oxic conditions. Although the functions of FRB and FOB were impaired during anoxic-oxic cycles, the transformation of Fe(II)/Fe(III) was not immediately affected, which may be related to the residual reactivity of dead bacteria and the bio-availability of Fe(II)/Fe(III) species. In the anoxic-oxic alternation process, the iron cycle is mainly the mutual transformation between Fe(II)aq, Fe(II)ad, Fe(II)am and Fe(III)am, Fe(III)cry. This finding deepens our understanding of the biogeochemical cycling of Fe in the redox-dynamic environments.