Abstract
The bioreduction of Fe(III) oxides by dissimilatory iron-reducing bacteria may result in the formation of a suite of Fe(II)-bearing secondary minerals, including magnetite (a mixed Fe(II)/Fe(III) oxide), siderite (Fe(II) carbonate), vivianite (Fe(II) phosphate), chukanovite (ferrous hydroxy carbonate), and green rusts (mixed Fe(II)/Fe(III) hydroxides). In an effort to better understand the factors controlling the formation of specific Fe(II)-bearing secondary minerals, we examined the effects of Fe(III) oxide mineralogy, phosphate concentration, and the availability of an electron shuttle (9,10-anthraquinone-2,6-disulfonate, AQDS) on the bioreduction of a series of Fe(III) oxides (akaganeite, feroxyhyte, ferric green rust, ferrihydrite, goethite, hematite, and lepidocrocite) by Shewanella putrefaciens CN32, and the resulting formation of secondary minerals, as determined by X-ray diffraction, Mössbauer spectroscopy, and scanning electron microscopy. The overall extent of Fe(II) production was highly dependent on the type of Fe(III) oxide provided. With the exception of hematite, AQDS enhanced the rate of Fe(II) production; however, the presence of AQDS did not always lead to an increase in the overall extent of Fe(II) production and did not affect the types of Fe(II)-bearing secondary minerals that formed. The effects of the presence of phosphate on the rate and extent of Fe(II) production were variable among the Fe(III) oxides, but in general, the highest loadings of phosphate resulted in decreased rates of Fe(II) production, but ultimately higher levels of Fe(II) than in the absence of phosphate. In addition, phosphate concentration had a pronounced effect on the types of secondary minerals that formed; magnetite and chukanovite formed at phosphate concentrations of ≤1 mM (ferrihydrite), <~100 µM (lepidocrocite), 500 µM (feroxyhyte and ferric green rust), while green rust, or green rust and vivianite, formed at phosphate concentrations of 10 mM (ferrihydrite), ≥100 µM (lepidocrocite), and 5 mM (feroxyhyte and ferric green rust). These results further demonstrate that the bioreduction of Fe(III) oxides, and accompanying Fe(II)-bearing secondary mineral formation, is controlled by a complex interplay of mineralogical, geochemical, and microbiological factors.
Highlights
Iron(III) oxides—a term which we use to include formal Fe oxides, oxyhydroxides, and hydroxides—are common constituents of soils and sediments and are present in a variety of mineralogical forms, including ferrihydrite, goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), feroxyhyte (δ0 -FeOOH), hematite (α-Fe2 O3 ), and maghemite (γ-Fe2 O3 )
We examine the effects of Fe(III) oxide mineralogy and the presence of phosphate on the bioreduction of hematite, goethite, maghemite, ferrihydrite, lepidocrocite, feroxyhyte, and ferric green rust by the Shewanella putrefaciens strain CN32, an IRB isolated from subsurface sediment [84], and the subsequent formation of secondary minerals using X-ray diffraction (XRD), 57 Fe
The bioreduction of Fe(III) oxides by S. putrefaciens CN32 resulted in the formation of siderite, magnetite, chukanovite, green rust, or vivianite, depending on the experimental conditions
Summary
Iron(III) oxides—a term which we use to include formal Fe oxides, oxyhydroxides, and hydroxides—are common constituents of soils and sediments and are present in a variety of mineralogical forms, including ferrihydrite, goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), feroxyhyte (δ0 -FeOOH), hematite (α-Fe2 O3 ), and maghemite (γ-Fe2 O3 ). The biogeochemistry of Fe in most aquatic and terrestrial environments is driven largely by microbial activity, and the presence of Fe(II) in near surface suboxic and anoxic environments is typically the result of the activity of iron(III)-reducing bacteria (IRB) and archaea These phylogenetically diverse microorganisms couple the oxidation of an electron donor (organic compounds or molecular hydrogen, H2 ) to the reduction of. We examine the effects of Fe(III) oxide mineralogy (in the presence and absence of an electron shuttle) and the presence of phosphate on the bioreduction of hematite, goethite, maghemite, ferrihydrite, lepidocrocite, feroxyhyte, and ferric green rust by the Shewanella putrefaciens strain CN32, an IRB isolated from subsurface sediment [84], and the subsequent formation of secondary minerals using X-ray diffraction (XRD), 57 Fe. Mössbauer spectroscopy, and scanning electron microscopy (SEM)
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