Biochemistry. How iron activates O2.
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Catalysis↗
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Role of microbes in the smectite-to-illite reaction.
Temperature, pressure, and time have been thought to control the smectite-to-illite (S-I) reaction, an important diagenetic process used for petroleum exploration. We demonstrated that microorganisms can promote the S-I reaction by dissolving smectite through reduction of structural Fe(III) at room temperature and 1 atmosphere within 14 days. This reaction typically requires conditions of 300 degrees to 350 degrees C, 100 megapascals, and 4 to 5 months in the absence of microbial activity. These results challenge the conventional concept of the S-I reaction and of reaction kinetic models.
Aluminum Silicates↗
Redox stabilization of the atmosphere and oceans by phosphorus-limited marine productivity
Data from modern and ancient marine sediments demonstrate that burial of the limiting nutrient phosphorus is less efficient when bottom waters are low in oxygen. Mass-balance calculations using a coupled model of the biogeochemical cycles of carbon, phosphorus, oxygen, and iron indicate that the redox dependence of phosphorus burial in the oceans provides a powerful forcing mechanism for balancing production and consumption of atmospheric oxygen over geologic time. The oxygen-phosphorus coupling further guards against runaway ocean anoxia. Phosphorus-mediated redox stabilization of the atmosphere and oceans may have been crucial to the radiation of higher life forms during the Phanerozoic.
Atmosphere↗
Iron metabolism in eukaryotes: Mars and Venus at it again.
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Animals↗
O2 activation by nonheme iron complexes: A monomeric Fe(III)-Oxo complex derived from O2.
Iron species with terminal oxo ligands are implicated as key intermediates in several synthetic and biochemical catalytic cycles. However, there is a dearth of structural information regarding these types of complexes because their instability has precluded isolation under ambient conditions. The isolation and structural characterization of an iron(III) complex with a terminal oxo ligand, derived directly from dioxygen (O2), is reported. A stable structure resulted from placing the oxoiron unit within a synthetic cavity lined with hydrogen-bonding groups. The cavity creates a microenvironment around the iron center that aids in regulating O2 activation and stabilizing the oxoiron unit. These cavities share properties with the active sites of metalloproteins, where function is correlated strongly with site structure.
Anthracenes↗
Inhibitor-enhanced electron transfer: copper cytochrome c as a redox-inert probe of ternary complexes.
Copper-substituted cytochrome c (CuCc) has been used as a structurally faithful, redoxinert inhibitor to probe the mechanism of electron transfer (ET) between Cc molecules and cytochrome c peroxidase (CcP). This inhibitor enhances photoinduced ET quenching of the triplet excited state of a zinc-substituted protein (ZnCcP or ZnCc) by its iron(III) partner (Fe3+Cc or Fe3+CcP). These results show that CcP and Cc form a ternary complex in which one Cc molecule binds tightly at a surface domain of CcP having low ET reactivity, whereas the second Cc molecule binds weakly to the 1:1 complex at a second domain with markedly greater (approximately 10(3)) reactivity. These results also rule out the possibility that Cc bound at the second domain cooperatively enhances ET to Cc at the first domain. The multiphasic kinetics observed for the photoproduced ET intermediate do not reflect electron self-exchange between two Cc molecules within the ternary complex.
Cytochrome c Group↗
Divalent cation reversal of tetracycline-inhibited respiration of Klebsiella pneumoniae.
Magnesium and ferrous compounds reversed tetracycline inhibition of respiration in Klebsiella pneumoniae, and the two compounds together had greater reversing ability than either alone.
Drug Synergism↗
Ferric iron reduction by sulfur- and iron-oxidizing bacteria.
Acidophilic bacteria of the genera Thiobacillus and Sulfolobus are able to reduce ferric iron when growing on elemental sulfur as an energy source. It has been previously thought that ferric iron serves as a nonbiological oxidant in the formation of acid mine drainage and in the leaching of ores, but these results suggest that bacterial catalysis may play a significant role in the reactivity of ferric iron.
Acidithiobacillus thiooxidans↗
Evidence for microbial Fe(III) reduction in anoxic, mining-impacted lake sediments (Lake Coeur d'Alene, Idaho).
Mining-impacted sediments of Lake Coeur d'Alene, Idaho, contain more than 10% metals on a dry weight basis, approximately 80% of which is iron. Since iron (hydr)oxides adsorb toxic, ore-associated elements, such as arsenic, iron (hydr)oxide reduction may in part control the mobility and bioavailability of these elements. Geochemical and microbiological data were collected to examine the ecological role of dissimilatory Fe(III)-reducing bacteria in this habitat. The concentration of mild-acid-extractable Fe(II) increased with sediment depth up to 50 g kg(-1), suggesting that iron reduction has occurred recently. The maximum concentrations of dissolved Fe(II) in interstitial water (41 mg liter(-1)) occurred 10 to 15 cm beneath the sediment-water interface, suggesting that sulfidogenesis may not be the predominant terminal electron-accepting process in this environment and that dissolved Fe(II) arises from biological reductive dissolution of iron (hydr)oxides. The concentration of sedimentary magnetite (Fe(3)O(4)), a common product of bacterial Fe(III) hydroxide reduction, was as much as 15.5 g kg(-1). Most-probable-number enrichment cultures revealed that the mean density of Fe(III)-reducing bacteria was 8.3 x 10(5) cells g (dry weight) of sediment(-1). Two new strains of dissimilatory Fe(III)-reducing bacteria were isolated from surface sediments. Collectively, the results of this study support the hypothesis that dissimilatory reduction of iron has been and continues to be an important biogeochemical process in the environment examined.
Culture Media↗
Bacterial reductive dissolution of crystalline Fe(III) oxide in continuous-flow column reactors.
Bacterial reductive dissolution of synthetic crystalline Fe(III) oxide-coated sand was studied in continuous-flow column reactors in comparison with parallel batch cultures. The cumulative amount of aqueous Fe(II) exported from the columns over a 6-month incubation period corresponded to (95.0 +/- 3.7)% (n = 3) of their original Fe(III) content. Wet-chemical analysis revealed that only (6.5 +/- 3.2)% of the initial Fe(III) content remained in the columns at the end of the experiment. The near-quantitative removal of Fe was visibly evidenced by extensive bleaching of color from the sand in the columns. In contrast to the column reactors, Fe(II) production quickly reached an asymptote in batch cultures, and only (13.0 +/- 2.2)% (n = 3) of the Fe(III) oxide content was reduced. Sustained bacterial-cell growth occurred in the column reactors, leading to the production and export of a quantity of cells 100-fold greater than that added during inoculation. Indirect estimates of cell growth, based on the quantity of Fe(III) reduced, suggest that only an approximate doubling of initial cell abundance was likely to have occurred in the batch cultures. Our results indicate that removal of biogenic Fe(II) via aqueous-phase transport in the column reactors decreased the passivating influence of surface-bound Fe(II) on oxide reduction activity, thereby allowing a dramatic increase in the extent of Fe(III) oxide reduction and associated bacterial growth. These findings have important implications for understanding the fate of organic and inorganic contaminants whose geochemical behavior is linked to Fe(III) oxide reduction.
Bacteria↗
Direct and Fe(II)-mediated reduction of technetium by Fe(III)-reducing bacteria.
The dissimilatory Fe(III)-reducing bacterium Geobacter sulfurreducens reduced and precipitated Tc(VII) by two mechanisms. Washed cell suspensions coupled the oxidation of hydrogen to enzymatic reduction of Tc(VII) to Tc(IV), leading to the precipitation of TcO(2) at the periphery of the cell. An indirect, Fe(II)-mediated mechanism was also identified. Acetate, although not utilized efficiently as an electron donor for direct cell-mediated reduction of technetium, supported the reduction of Fe(III), and the Fe(II) formed was able to transfer electrons abiotically to Tc(VII). Tc(VII) reduction was comparatively inefficient via this indirect mechanism when soluble Fe(III) citrate was supplied to the cultures but was enhanced in the presence of solid Fe(III) oxide. The rate of Tc(VII) reduction was optimal, however, when Fe(III) oxide reduction was stimulated by the addition of the humic analog and electron shuttle anthaquinone-2,6-disulfonate, leading to the rapid formation of the Fe(II)-bearing mineral magnetite. Under these conditions, Tc(VII) was reduced and precipitated abiotically on the nanocrystals of biogenic magnetite as TcO(2) and was removed from solution to concentrations below the limit of detection by scintillation counting. Cultures of Fe(III)-reducing bacteria enriched from radionuclide-contaminated sediment using Fe(III) oxide as an electron acceptor in the presence of 25 microM Tc(VII) contained a single Geobacter sp. detected by 16S ribosomal DNA analysis and were also able to reduce and precipitate the radionuclide via biogenic magnetite. Fe(III) reduction was stimulated in aquifer material, resulting in the formation of Fe(II)-containing minerals that were able to reduce and precipitate Tc(VII). These results suggest that Fe(III)-reducing bacteria may play an important role in immobilizing technetium in sediments via direct and indirect mechanisms.
Culture Media↗
Suboxic deposition of ferric iron by bacteria in opposing gradients of Fe(II) and oxygen at circumneutral pH.
The influence of lithotrophic Fe(II)-oxidizing bacteria on patterns of ferric oxide deposition in opposing gradients of Fe(II) and O(2) was examined at submillimeter resolution by use of an O(2) microelectrode and diffusion microprobes for iron. In cultures inoculated with lithotrophic Fe(II)-oxidizing bacteria, the majority of Fe(III) deposition occurred below the depth of O(2) penetration. In contrast, Fe(III) deposition in abiotic control cultures occurred entirely within the aerobic zone. The diffusion microprobes revealed the formation of soluble or colloidal Fe(III) compounds during biological Fe(II) oxidation. The presence of mobile Fe(III) in diffusion probes from live cultures was verified by washing the probes in anoxic water, which removed ca. 70% of the Fe(III) content of probes from live cultures but did not alter the Fe(III) content of probes from abiotic controls. Measurements of the amount of Fe(III) oxide deposited in the medium versus the probes indicated that ca. 90% of the Fe(III) deposited in live cultures was formed biologically. Our findings show that bacterial Fe(II) oxidation is likely to generate reactive Fe(III) compounds that can be immediately available for use as electron acceptors for anaerobic respiration and that biological Fe(II) oxidation may thereby promote rapid microscale Fe redox cycling at aerobic-anaerobic interfaces.
Aerobiosis↗
Characterization of microbial communities removing nitrogen oxides from flue gas: the BioDeNOx process.
BioDeNOx is an integrated physicochemical and biological process for the removal of nitrogen oxides (NOx) from flue gases. In this process, the flue gas is purged through a scrubber containing a solution of Fe(II)EDTA2-, which binds the NOx to form an Fe(II)EDTA.NO2- complex. Subsequently, this complex is reduced in the bioreactor to dinitrogen by microbial denitrification. Fe(II)EDTA2-, which is oxidized to Fe(III)EDTA- by oxygen in the flue gas, is regenerated by microbial iron reduction. In this study, the microbial communities of both lab- and pilot-scale reactors were studied using culture-dependent and -independent approaches. A pure bacterial strain, KT-1, closely affiliated by 16S rRNA analysis to the gram-positive denitrifying bacterium Bacillus azotoformans, was obtained. DNA-DNA homology of the isolate with the type strain was 89%, indicating that strain KT-1 belongs to the species B. azotoformans. Strain KT-1 reduces Fe(II)EDTA.NO2- complex to N2 using ethanol, acetate, and Fe(II)EDTA2- as electron donors. It does not reduce Fe(III)EDTA-. Denaturing gradient gel electrophoresis analysis of PCR-amplified 16S rRNA gene fragments showed the presence of bacteria closely affiliated with members of the phylum Deferribacteres, an Fe(III)-reducing group of bacteria. Fluorescent in situ hybridization with oligonucleotide probes designed for strain KT-1 and members of the phylum Deferribacteres showed that the latter were more dominant in both reactors.
Bacillus↗
Effect of oxidation rate and Fe(II) state on microbial nitrate-dependent Fe(III) mineral formation.
A nitrate-dependent Fe(II)-oxidizing bacterium was isolated and used to evaluate whether Fe(II) chemical form or oxidation rate had an effect on the mineralogy of biogenic Fe(III) (hydr)oxides resulting from nitrate-dependent Fe(II) oxidation. The isolate (designated FW33AN) had 99% 16S rRNA sequence similarity to Klebsiella oxytoca. FW33AN produced Fe(III) (hydr)oxides by oxidation of soluble Fe(II) [Fe(II)sol] or FeS under nitrate-reducing conditions. Based on X-ray diffraction (XRD) analysis, Fe(III) (hydr)oxide produced by oxidation of FeS was shown to be amorphous, while oxidation of Fe(II)sol yielded goethite. The rate of Fe(II) oxidation was then manipulated by incubating various cell concentrations of FW33AN with Fe(II)sol and nitrate. Characterization of products revealed that as Fe(II) oxidation rates slowed, a stronger goethite signal was observed by XRD and a larger proportion of Fe(III) was in the crystalline fraction. Since the mineralogy of Fe(III) (hydr)oxides may control the extent of subsequent Fe(III) reduction, the variables we identify here may have an effect on the biogeochemical cycling of Fe in anoxic ecosystems.
Culture Media↗
Inhibition of NO3- and NO2- reduction by microbial Fe(III) reduction: evidence of a reaction between NO2- and cell surface-bound Fe2+.
A recent study (D. C. Cooper, F. W. Picardal, A. Schimmelmann, and A. J. Coby, Appl. Environ. Microbiol. 69:3517-3525, 2003) has shown that NO(3)(-) and NO(2)(-) (NO(x)(-)) reduction by Shewanella putrefaciens 200 is inhibited in the presence of goethite. The hypothetical mechanism offered to explain this finding involved the formation of a Fe(III) (hydr)oxide coating on the cell via the surface-catalyzed, abiotic reaction between Fe(2+) and NO(2)(-). This coating could then inhibit reduction of NO(x)(-) by physically blocking transport into the cell. Although the data in the previous study were consistent with such an explanation, the hypothesis was largely speculative. In the current work, this hypothesis was tested and its environmental significance explored through a number of experiments. The inhibition of approximately 3 mM NO(3)(-) reduction was observed during reduction of a variety of Fe(III) (hydr)oxides, including goethite, hematite, and an iron-bearing, natural sediment. Inhibition of oxygen and fumarate reduction was observed following treatment of cells with Fe(2+) and NO(2)(-), demonstrating that utilization of other soluble electron acceptors could also be inhibited. Previous adsorption of Fe(2+) onto Paracoccus denitrificans inhibited NO(x)(-) reduction, showing that Fe(II) can reduce rates of soluble electron acceptor utilization by non-iron-reducing bacteria. NO(2)(-) was chemically reduced to N(2)O by goethite or cell-sorbed Fe(2+), but not at appreciable rates by aqueous Fe(2+). Transmission and scanning electron microscopy showed an electron-dense, Fe-enriched coating on cells treated with Fe(2+) and NO(2)(-). The formation and effects of such coatings underscore the complexity of the biogeochemical reactions that occur in the subsurface.
Ferric Compounds↗
Iron acquisition systems of Listeria monocytogenes.
The uptake of iron by Listeria monocytogenes was studied. The microorganism was found to bind both 59Fe(II) and [59Fe3+]citrate. In contrast, L. monocytogenes was unable to acquire iron from [59Fe3+]ferroxamine or [59Fe3+]EDTA or as 59FeCl3. The data suggest that iron is acquired principally as iron(II) and that a citrate-inducible iron uptake system is also operative.
Chlorides↗
Contribution of TonB- and Feo-mediated iron uptake to growth of Salmonella typhimurium in the mouse.
We examined the role of iron(II) and iron(III) uptake, mediated by FeoB and TonB, respectively, in infection of the mouse by Salmonella typhimurium. The S. typhimurium feoB gene, encoding a homolog of an Escherichia coli cytoplasmic membrane iron(II) permease, was cloned, and a mutant was generated by allelic exchange. In addition, an S. typhimurium tonB mutant was constructed. Together these two mutations inactivate all known iron uptake systems of S. typhimurium. We examined the abilities of these mutants to grow in vitro and in different compartments of the host. Mutants in feoB were outcompeted by the wild type during mixed colonization of the mouse intestine, but the feoB mutation did not attenuate S. typhimurium for oral or intraperitoneal infection of mice. The tonB mutation attenuated S. typhimurium for infection of mice by the intragastric route but not the intraperitoneal route, and the mutant was recovered in lower numbers from the Peyer's patches and mesenteric lymph nodes than the wild type. These results indicate that TonB-mediated iron uptake contributes to colonization of the Peyer's patches and mesenteric lymph nodes but not the liver and spleen of the mouse. The tonB feoB double mutant, given intraperitoneally, was able to infect the liver and spleen at wild-type doses, indicating that additional iron acquisition systems are used during growth at systemic sites of infection.
Animals↗
Iron storage in Mycoplasma capricolum.
Considerable quantities or iron were incorporated into the Mycoplasma capricolum cell membrane. Mossbauer studies showed that the iron is in a form which becomes magnetically ordered at low temperatures. The iron-enriched cells contained membrane-bound electron-dense particles of about 6.0 nm in diameter.
Cell Membrane↗