Enrichment and pure cultures of nitrate-reducing bacteria were shown to grow anaerobically with ferrous iron as the only electron donor or as the additional electron donor in the presence of acetate. The newly observed bacterial process may significantly contribute to ferric iron formation in the suboxic zone of aquatic sediments.

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Anaerobic dissimilatory ferric iron-reducing and ferrous iron-oxidizing bacteria gain energy through reduction or oxidation of iron minerals and presumably play an important role in catalyzing iron transformations in anoxic environments. Numerous ferric iron-reducing bacteria have been isolated from a great diversity of anoxic environments, including sediments, soils, deep terrestrial subsurfaces, and hot springs. In contrast, only few ferrous iron-oxidizing bacteria are known so far. At neutral pH, iron minerals are barely soluble, and the mechanisms of electron transfer to or from iron minerals are still only poorly understood. In natural habitats, humic substances may act as electron carriers for ferric iron-reducing bacteria. Also fermenting bacteria were shown to channel electrons to ferric iron via humic acids. Whether quinones or cytochromes released from cells act as electron transfer components in ferric iron reduction is still a matter of debate. Anaerobic ferrous iron-oxidizing phototrophic bacteria, on the other hand, appear to excrete complexing agents to prevent precipitation of ferric iron oxides at their cell surfaces. The present review evaluates recent findings on the physiology of ferric iron-reducing and ferrous iron-oxidizing bacteria with respect to their relevance to microbial iron transformations in nature.

Microbial anaerobic Fe(II) oxidation at neutral pH produces poorly soluble Fe(III) which is expected to bind to cell surfaces causing cell encrustation and potentially impeding cell metabolism. The challenge for Fe(II)-oxidizing prokaryotes therefore is to avoid encrustation with Fe(III). Using different microscopic techniques we tracked Fe(III) minerals at the cell surface and within cells of phylogenetically distinct phototrophic and nitrate-reducing Fe(II)-oxidizing bacteria. While some strains successfully prevented encrustation others precipitated Fe(III) minerals at the cell surface and in the periplasm. Our results indicate differences in the cellular mechanisms of Fe(II) oxidation, transport of Fe(II)/Fe(III) ions, and Fe(III) mineral precipitation.

In previous studies, three different strains (BrG1, BrG2, and BrG3) of ferrous iron-oxidizing, nitrate-reducing bacteria were obtained from freshwater sediments. All three strains were facultative anaerobes and utilized a variety of organic substrates and molecular hydrogen with nitrate as electron acceptor. In this study, analyses of 16S rDNA sequences showed that strain BrG1 was affiliated with the genus Acidovorax, strain BrG2 with the genus Aquabacterium, and strain BrG3 with the genus Thermomonas. Previously, bacteria similar to these three strains were detected with molecular techniques in MPN dilution series for ferrous iron-oxidizing, nitrate-reducing bacteria inoculated with different freshwater sediment samples. In the present study, further molecular analyses of these MPN cultures indicated that the ability to oxidize ferrous iron with nitrate is widespread amongst the Proteobacteria and may also be found among the Gram-positive bacteria with high GC content of DNA. Nitrate-reducing bacteria oxidized ferrous iron to poorly crystallized ferrihydrite that was suitable as an electron acceptor for ferric iron-reducing bacteria. Biologically produced ferrihydrite and synthetically produced ferrihydrite were both well suited as electron acceptors in MPN dilution cultures. Repeated anaerobic cycling of iron was shown in a coculture of ferrous iron-oxidizing bacteria and the ferric iron-reducing bacterium Geobacter bremensis. The results indicate that iron can be cycled between its oxidation states +II and +III by microbial activities in anoxic sediments. INTRODUCTIONIron is the fourth most abundant element in the Earth's crust, and in most soils and sediments it is the dominant redox-active metal (Cornell and Schwertmann 1996). Acidophilic or neutrophilic bacteria that gain energy from the oxidation of ferrous iron with oxygen as electron acceptor are widespread and have been studied for many decades (reviewed by Blake and Johnson 2000;Emerson 2000). In contrast, only a few years ago it was recognized that ferrous iron can be oxidized also under anoxic conditions either by phototrophic or by nitrate-reducing bacteria (Widdel et al. 1993;Straub, Benz, Schink, and Widdel 1996). Several strains of lithotrophic ferrous iron-oxidizing phototrophs from freshwater and marine sediments were isolated and characterized (Widdel et al. 1993;Ehrenreich and Widdel 1994;Heising and Schink 1998;Heising, Richter, Ludwig, and Schink 1999;Straub, Rainey, and Widdel 1999). Isolation of lithotrophic nitrate-reducing ferrous iron oxidizers turned out to be more difficult: Although this activity was first described with a lithotrophic enrichment culture (Straub et al. 1996), all strains isolated from this culture oxidized ferrous iron only in the presence of an organic cosubstrate (strains BrG1 and BrG3), or the organic cosubstrate accelerated the oxidation of ferrous iron significantly (strain BrG2). Also two other bacterial strains (strain HidR2 and Dechlorosoma suillum strain PS) oxidize ferrous iron with nitrate only in the p...

In this study we determined the composition and biogeochemistry of novel, brightly colored, white and orange microbial mats at the surface of a brine seep at the outer rim of the Chefren mud volcano. These mats were interspersed with one another, but their underlying sediment biogeochemistries differed considerably. Microscopy revealed that the white mats were granules composed of elemental S filaments, similar to those produced by the sulfide-oxidizing epsilonproteobacterium "Candidatus Arcobacter sulfidicus." Fluorescence in situ hybridization indicated that microorganisms targeted by a "Ca. Arcobacter sulfidicus"-specific oligonucleotide probe constituted up to 24% of the total the cells within these mats. Several 16S rRNA gene sequences from organisms closely related to "Ca. Arcobacter sulfidicus" were identified. In contrast, the orange mat consisted mostly of bright orange flakes composed of empty Fe(III) (hydr)oxide-coated microbial sheaths, similar to those produced by the neutrophilic Fe(II)-oxidizing betaproteobacterium Leptothrix ochracea. None of the 16S rRNA gene sequences obtained from these samples were closely related to sequences of known neutrophilic aerobic Fe(II)-oxidizing bacteria. The sediments below both types of mats showed relatively high sulfate reduction rates (300 nmol ⅐ cm ؊3 ⅐ day ؊1 ) partially fueled by the anaerobic oxidation of methane (10 to 20 nmol ⅐ cm ؊3 ⅐ day ؊1 ). Free sulfide produced below the white mat was depleted by sulfide oxidation within the mat itself. Below the orange mat free Fe(II) reached the surface layer and was depleted in part by microbial Fe(II) oxidation. Both mats and the sediments underneath them hosted very diverse microbial communities and contained mineral precipitates, most likely due to differences in fluid flow patterns.

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Two new strains of marine purple bacteria, N I T and N2T, were isolated from coastal sediment of the North Sea (Germany) with ferrous iron as the only electron donor for anoxygenic photosynthesis. The isolates are the first saltdependent, ferrous-iron-oxidizing purple bacteria characterized so far. Analysis of 165 rRNA gene sequences revealed an affiliation with the genus Rhodovulum, which until now comprises only marine species. The sequence similarity of both strains was 95.2 O/ O, and their closest relative was Rhodovulum adriaticum. Like all known Rhodovulum species, the new strains had ovoid to rod-shaped cells, contained bacteriochlorophyll a and carotenoids of the spheroidene series, and were able to oxidize sulfide and thiosulfate. Like Rhodovulum adriaticum, both strains were unable to assimilate sulfate; for growth they needed a reduced sulfur source, e.g. thiosulfate. In contrast to the new strains, none of the known Rhodovulum species tested was able to oxidize ferrous iron or iron sulfide. In growth experiments, strains N I T and NZT oxidized 65 and 95%, respectively, of the ferrous iron supplied. Electron diffraction analysis revealed ferrihydrite as the main product of ferrous iron oxidation. In addition, traces of magnetite were formed. Strains N I T (= DSM 12328T) and NZT ( = DSM 123293 are described as Rhodovulum iodosum sp. nov. and Rhodovulum robiginosum sp. nov., respectively.