Further reading in geomicrobiology

In the wetland rhizosphere, high densities of lithotrophic Fe(II)-oxidizing bacteria (FeOB) and a favorable environment (i.e., high Fe(II) availability and microaerobic conditions) suggest that these organisms are actively contributing to the formation of Fe plaque on plant roots. We manipulated the presence/absence of an Fe(II)-oxidizing bacterium (Sideroxydans paludicola, strain BrT) in axenic hydroponic microcosms containing the roots of intact Juncus effusus (soft rush) plants to determine if FeOB affected total rates of rhizosphere Fe(II) oxidation and Fe plaque accumulation. Our experimental data highlight the importance of both FeOB and plants in influencing short-term rates of rhizosphere Fe oxidation. Over time scales ca. 1 wk, the FeOB increased Fe(II) oxidation rates by 1.3 to 1.7 times relative to FeOB-free microcosms. Across multiple experimental trials, Fe(II) oxidation rates were significantly correlated with root biomass, reflecting the importance of radial O ₂ loss in supporting rhizosphere Fe(II) oxidation. Rates of root Fe(III) plaque accumulation (time scales: 3 to 6 wk) were ～ 70 to 83% lower than expected based on the short-term Fe(II) oxidation rates and were unaffected by the presence/absence of FeOB. Decreasing rates of Fe(II) oxidation and Fe(III) plaque accumulation with increasing time scales indicate changes in rates of Fe(II) diffusion and radial O ₂ loss, shifts in the location of Fe oxide accumulation, or temporal changes in the microbial community within the microcosms. The microcosms used herein replicated many of the environmental characteristics of wetland systems and allowed us to demonstrate that FeOB can stimulate rates of Fe(II) oxidation in the wetland rhizosphere, a finding that has implications for the biogeochemical cycling of carbon, metals, and nutrients in wetland ecosystems.     

Characterization of Neutrophilic Fe(II)-Oxidizing Bacteria Isolated from the Rhizosphere of Wetland Plants and Description of Ferritrophicum radicicola gen. nov. sp. nov., and Sideroxydans paludicola sp. nov.

Iron deposits (Fe plaque) on wetland plant roots contain abundant microbial populations, including Fe(II)-oxidizing bacteria (FeOB) that have not been cultured previously. In this study, 4 strains of Fe plaque-associated FeOB were isolated from 4 species of wetland plants. All 4 isolates grew in tight association with Fe-oxides, but did not form any identifiable Fe-oxide structures. All strains were obligate lithotrophic Fe(II)-oxidizers that were microaerobic, and were unable to use other inorganic or organic energy sources. One strain, BrT, was shown to fix ¹⁴ CO ₂ at a rate consistent with its requirement for total cell carbon. The doubling times for the strains varied between 9.5 and 15.8 hours. The fatty acid methyl ester (FAME) profiles of 2 strains, BrT and CCJ, revealed that 16:0, 15:1 isoG, and 14:0 were dominant fatty acids. Phylogenetic analysis of the 16S rRNA gene indicated that all the strains were Betaproteobacteria. Two of the strains, BrT and Br-1 belong to a new species, Sideroxydans paludicola; a third strain, LD-1, is related to Sideroxydans lithotrophicus, a recently described species of FeOB. The fourth isolate, Ferritrophicum radicicola, represented a new genus in a new order of Betaproteobacteria, the Ferritrophicales. There are no other cultured isolates in this order. A small subunit rRNA gene-based, cultivation-independent analysis of Typha latifolia collected from a wetland revealed terminal restriction fragment profiles (tRFLP) consistent with the presence of these bacteria in the rhizosphere. These novel organisms likely play an important role in Fe(II) oxidation kinetics and Fe cycling within many terrestrial and freshwater environments.     

Influence of Biogenic Fe(II) on Bacterial Crystalline Fe(III) Oxide Reduction

Bacterial crystalline Fe(III) oxide reduction has the potential to significantly influence the biogeochemistry of anaerobic sedimentary environments where crystalline Fe(III) oxides are abundant relative to poorly crystalline (amorphous) phases. A review of published data on solid-phase Fe(III) abundance and speciation indicates that crystalline Fe(III) oxides are frequently 2- to S 10-fold more abundant than amorphous Fe(III) oxides in shallow subsurface sediments not yet subjected to microbial Fe(III) oxide reduction activity. Incubation experiments with coastal plain aquifer sediments demonstrated that crystalline Fe(III) oxide reduction can contribute substantially to Fe(II) production in the presence of added electron donors and nutrients. Controls on crystalline Fe(III) oxide reduction are therefore an important consideration in relation to the biogeochemical impacts of bacterial Fe(III) oxide reduction in subsurface environments. In this paper, the influence of biogenic Fe(II) on bacterial reduction of crystalline Fe(III) oxides is reviewed and analyzed in light of new experiments conducted with the acetate-oxidizing, Fe(III)-reducing bacterium (FeRB) Geobacter metallireducens . Previous experiments with Shewanella algae strain BrY indicated that adsorption and/or surface precipitation of Fe(II) on Fe(III) oxide and FeRB cell surfaces is primarily responsible for cessation of goethite ( f -FeOOH) reduction activity after only a relatively small fraction (generally < 10%) of the oxide is reduced. Similar conclusions are drawn from analogous studies with G. metallireducens . Although accumulation of aqueous Fe(II) has the potential to impose thermodynamic constraints on the extent of crystalline Fe(III) oxide reduction, our data on bacterial goethite reduction suggest that this phenomenon cannot universally explain the low microbial reducibility of this mineral. Experiments examining the influence of exogenous Fe(II) (20 mM FeCl 2 ) on soluble Fe(III)-citrate reduction by G. metallireducens and S. algae showed that high concentrations of Fe(II) did not inhibit Fe(III)-citrate reduction by freshly grown cells, which indicates that surface-bound Fe(II) does not inhibit Fe(III) reduction through a classical end-product enzyme inhibition mechanism. However, prolonged exposure of G. metallireducens and S. algae cells to high concentrations of soluble Fe(II) did cause inhibition of soluble Fe(III) reduction. These findings, together with recent documentation of the formation of Fe(II) surface precipitates on FeRB in Fe(III)-citrate medium, provide further evidence for the impact of Fe(II) sorption by FeRB on enzymatic Fe(III) reduction. Two different, but not mutually exclusive, mechanisms whereby accumulation of Fe(II) coatings on Fe(III) oxide and FeRB surfaces may lead to inhibition of enzymatic Fe(III) oxide reduction activity (in the absence of soluble electron shuttles and/or Fe(III) chelators) are identified and discussed in relation to recent experimental work and theoretical considerations.     

Acid‐tolerant microaerophilic Fe(II)‐oxidizing bacteria promote Fe(III)‐accumulation in a fen

The ecological importance of Fe(II)‐oxidizing bacteria (FeOB) at circumneutral pH is often masked in the presence of O₂ where rapid chemical oxidation of Fe(II) predominates. This study addresses the abundance, diversity and activity of microaerophilic FeOB in an acidic fen (pH ～5) located in northern Bavaria, Germany. Mean O₂ penetration depth reached 16 cm where the highest dissolved Fe(II) concentrations (up to 140 µM) were present in soil water. Acid‐tolerant FeOB cultivated in gradient tubes were most abundant (10⁶ cells g^?1 peat) at the 10–20 cm depth interval. A stable enrichment culture was active at up to 29% O₂ saturation and Fe(III) accumulated 1.6 times faster than in abiotic controls. An acid‐tolerant, microaerophilic isolate (strain CL21) was obtained which was closely related to the neutrophilic, lithoautotrophic FeOB Sideroxydans lithotrophicus strain LD‐1. CL21 oxidized Fe(II) between pH 4 and 6.0, and produced nanoscale‐goethites with a clearly lower mean coherence length (7 nm) perpendicular to the (110) plane than those formed abiotically (10 nm). Our results suggest that an acid‐tolerant population of FeOB is thriving at redox interfaces formed by diffusion‐limited O₂ transport in acidic peatlands. Furthermore, this well‐adapted population is successfully competing with chemical oxidation and thereby playing an important role in the microbial iron cycle.     

Synthesis and characterization of a high spin d Fe(II) complex with the tridentate ligand dipyrido(2,3-a:3,2-j)phenazine (dpop)

A new bis-(N-tridentate) Fe(II) complex [Fe(dpop^′)₂](PF₆)₂ (dpop^′=dipyrido(2,3-a:3^′,2^′-j)phenazine) was prepared and studied. The magnetic moment of the solid was determined as μ=5.2-4.9 BM and in CH₃CN solution as μ=4.9 BM and indicate the high spin Fe(II) state. The electronic absorption spectrum displays a broad weak absorption MLCT transition at 602 nm (ε=3.8×10³ M⁻¹ cm⁻¹), consistent with CT absorptions of other Fe(II) HS complexes. The cyclic voltammogram of the complex shows an irreversible Fe^2+/3+ oxidation at +1.55 V and two dpop^′0/−1 centered reductions at −0.20 and −0.59 V versus Ag/AgCl.     

Dependence of In Situ Bacterial Fe(II)-Oxidation and Fe(III)-Precipitation on Sequential Reactive Transport

A study was conducted to determine in situ rates of Fe(II) oxidation and Fe(III) precipitation along a 5.0 m reach of a ferruginous groundwater discharge zone under two distinct conditions; (i) the natural state featuring abundant flocculent mats of bacteriogenic iron oxides (BIOS) produced by Fe(II)-oxidizing bacteria, and (ii) after a manual washout of the streambed to remove the microbial mat. Examination of mat samples by differential interference contrast light microscopy revealed tangled meshworks of filamentous Leptothrix sheaths and helical Gallionella stalks intermixed with fine-grained hydrous ferric oxide (HFO) precipitates. The greatest accumulation of BIOS mat was 1.0 m downstream of the groundwater spring. Redox potential (Eh) increased sharply from 200 mV to over 300 mV over the last 2.0 m of the reach. Similarly, dissolved oxygen increased from < 10% saturation to almost 100% saturation over the last 2.0 m of the reach, whereas pH increased from 6.4 to 7.3. Pseudo-first-order rate constants determined on the basis of analytical solutions to sequential partial differential advection-dispersion-reaction equations for the linear Fe(II)→Fe(III)→HFO reaction network yielded in situ Fe(II) oxidation rate constants (k_ox) of 1.70 ± 0.20 min^?1 in natural conditions and 0.48 ± 0.14 min^?1 after washout. Corresponding Fe(III)-precipitation rates (k_p) before and after washout were 3.45 ± 0.10 min^?1 and 0.90 ± 0.01 min^?1, respectively. These values for k_ox and k_p are higher than estimates obtained from closed batch microcosm and laboratory experiments, underscoring the crucial dependence of in situ Fe(II) oxidation and Fe(III) precipitation rates on advective and dispersive mass transport. The results also highlight the influence that BIOS microbial mats exert on the reaction kinetics of the multiple heterogeneous reactions contributing not only to Fe(II)/Fe(III) redox transformations in groundwater discharge zones, but also the precipitation of HFO.     

黄河滩地和稻田土中铁还原菌、不产氧光合细菌分布机制

【目的】探讨沿黄流域土壤中铁还原菌(ferric reducing bacteria, Fe RB)、不产氧光合细菌(anoxygenic phototrophic bacteria, An PB)的分布机制。【方法】以沿黄流域(原阳段)为研究对象,采集黄河滩地和稻田土样,利用16Sr RNA基因高通量测序和实时荧光定量分析技术,结合统计学分析,揭示Fe RB、An PB菌群结构、丰度和主要环境影响因子。【结果】二者中的优势Fe RB在科(属)水平为Hydrogenophilaceae(Thiobacillus)、 Bacillaceae(Bacillus)、 Clostridiaceae、Rhodobactereace(Rhodobacter)、 Geobacteraceae(Geobacter),优势An PB为Rhodobactereace(Rhodobacter)、 Chloroflexaceae(Chloronema)、 Acetobacteraceae(Roseomonas)。An PB中Rhodobacteraceae与Fe RB中Bacillaceae、 Clostri...     

Control of Late Blight (Phytophthora capsici ) in Pepper Plant with a Compost Containing Multitude of Chitinase-producing Bacteria

Compost sustaining a multitude of chitinase-producing bacteria was evaluated in a greenhouse study as a soil amendment for the control of late blight (Phytophthora capsici L.) in pepper (Capsicum annuum L.). Microbial population and exogenous enzyme activity were measured in the rhizosphere and correlated to the growth and health of pepper plant. Rice straw was composted with and without a chitin source, after having been inoculated with an aliquot of coastal area soil containing a known titer of chitinase-producing bacteria. P. capsici inoculated plants cultivated in chitin compost-amended soil exhibited significantly higher root and shoot weights and lower root mortality than plants grown in pathogen-inoculated control compost. Chitinase and β-1,3-glucanase activities in rhizosphere of plants grown in chitin compost-amended soil were twice that seen in soil amended with control compost. Colony forming units of chitinase-producing bacteria isolated from rhizosphere of plants grown in chitin compost-amended soil were 10³ times as prevalent as bacteria in control compost. These results indicate that increasing the population of chitinase-producing bacteria and soil enzyme activities in rhizosphere by compost amendment could alleviate pathogenic effects of P. capsici.     

Microbial Communities Involved in Fe Reduction and Mobility During Soil Organic Matter (SOM) Mineralization in Two Contrasted Paddy Soils

Lowland rice fields of West Africa (Ivory Coast) and South Asia (Thailand) are affected by ferrous toxicity or salinity, respectively, and their soil waters contain large amounts of ferrous iron, depending on reducing irrigation condition and suggesting occurrence of bacterial reducing processes. To determine the involvement, dynamic and activities of bacterial communities in Fe(III) reduction and mobilization during anaerobic degradation and mineralization of soil organic matter (SOM), different experiments and analyses have been performed. Results demonstrated that the utilization of SOM as sole carbon, nutrient and energy sources favored the presence of large bacterial communities: facultative anaerobic and anaerobic bacteria, Fe(III)-reducing bacteria (FeRB) (fermentative and Fe respiring), sulfate reducing bacteria (SRB) which are involved in carbon, nitrogen, iron and sulfur cycling. The larger functional diversity is observed in the Ivory Coast paddy soils containing larger amounts of organic matter and sulfur compounds. These communities contained complementary populations (chemoorganotrophic, chemolitotrophic, aerobic, facultative anaerobic and anaerobic) that can be active at different steps of iron solubilization with simultaneous organic matter mineralization. Our results indicate that the pH controlled by bacterial activity, the nature much more than the content of organic matter, and consequently the structure and activity of bacterial communities influence significantly the availability and dynamic of iron in paddy fields which affect the soil quality.     

‘Candidatus ferrigenium straubiae’ sp. nov., ‘Candidatus ferrigenium bremense’ sp. nov., ‘Candidatus ferrigenium altingense’ sp. nov., are autotrophic Fe(II)-oxidizing bacteria of the family Gallionellaceae

Iron(II) [Fe(II)] oxidation coupled to denitrification is recognized as an environmentally important process in many ecosystems. However, the Fe(II)-oxidizing bacteria (FeOB) dominating autotrophic nitrate-reducing Fe(II)-oxidizing enrichment cultures, affiliated with the family Gallionellaceae, remain poorly taxonomically defined due to lack of representative isolates. We describe the taxonomic classification of three novel FeOB based on metagenome-assembled genomes (MAGs) acquired from the autotrophic nitrate-reducing enrichment cultures KS, BP and AG. Phylogenetic analysis of nearly full-length 16S rRNA gene sequences demonstrated that these three FeOB were most closely affiliated to the genera Ferrigenium, Sideroxydans and Gallionella, with up to 96.5%, 95.4% and 96.2% 16S rRNA gene sequence identities to representative isolates of these genera, respectively. In addition, average amino acid identities (AAI) of the genomes compared to the most closely related genera revealed highest AAI with Ferrigenium kumadai An22 (76.35–76.74%), suggesting that the three FeOB are members of this genus. Phylogenetic analysis of conserved functional genes further supported that these FeOB represent three novel species of the genus Ferrigenium. Moreover, the three novel FeOB likely have characteristic features, performing partial denitrification coupled to Fe(II) oxidation and carbon fixation. Scanning electron microscopy of the enrichment cultures showed slightly curved rod-shaped cells, ranging from 0.2-0.7 μm in width and 0.5–2.3 μm in length. Based on the phylogenetic, genomic and physiological characteristics, we propose that these FeOB represent three novel species, ‘Candidatus Ferrigenium straubiae’ sp. nov., ‘Candidatus Ferrigenium bremense’ sp. nov. and ‘Candidatus Ferrigenium altingense’ sp. nov. that might have unique metabolic features among the genus Ferrigenium.     

Microbiological and Geochemical Characterization of Microbial Fe(III) Reduction in Salt Marsh Sediments

Population densities of anaerobic Fe(III)-reducing bacteria (FeRB) and aerobic heterotrophs were inversely correlated in the surficial (0-2 cm) layers of Sapelo Island, Georgia, salt marsh sediments. In surficial sediments where densities of aerobic heterotrophs were low, the density of culturable FeRB correlated positively with the concentration of amorphous Fe(III) oxyhydroxides extractable by ascorbate. High FeRB densities and a decrease with depth of ascorbate-extractable Fe(III) were observed in the upper 6 cm of a tidal creek core. Culturable sulfate-reducing bacteria (SRB) and SRB-targeted rRNA signals were also detected in the upper 6-cm depth. The disappearance of FeRB below 6 cm, however, coincided with a large increase in the abundance of SRB. Thus, when FeRB are not limited by the availability of readily reducible amorphous Fe(III) oxyhydroxides, FeRB may outcompete SRB for growth substrates. Shewanella putrefaciens- and Geobacteraceae-targeted rRNA signals were at or below detection limits in all sediment samples, indicating that these FeRB are not predominant members of the active FeRB populations. The ubiquitous presence of FeRB at the sites studied challenges the traditional view that dissimilatory Fe(III) reduction is not an important pathway of organic carbon oxidation in salt marsh sediments.     

Oxygen Dependency of Neutrophilic Fe(II) Oxidation by Leptothrix Differs from Abiotic Reaction

Neutrophilic Fe(II) oxidizing microorganisms are found in many natural environments. It has been hypothesized that, at low oxygen concentrations, microbial iron oxidation is favored over abiotic oxidation. Here, we compare the kinetics of abiotic Fe(II) oxidation to oxidation in the presence of the bacterium Leptothrix cholodnii Appels isolated from a wetland sediment. Rates of Fe(II) oxidation were determined in batch experiments at 20°C, pH 7 and oxygen concentrations between 3 and 120 μmol/l. The reaction progress in experiments with and without cells exhibited two distinct phases. During the initial phase, the oxygen dependency of microbial Fe(II) oxidation followed a Michaelis-Menten rate expression (K_M = 24.5 ± 10 μmol O₂/l, v_max = 1.8 ± 0.2 μmol Fe(II)/(l min) for 10⁸ cells/ml). In contrast, abiotic rates increased linearly with increasing oxygen concentrations. At similar oxygen concentrations, initial Fe(II) oxidation rates were faster in the experiments with bacteria. During the second phase, the accumulated iron oxides catalyzed further oxidative iron precipitation in both abiotic and microbial reaction systems. That is, abiotic oxidation also dominated the reaction progress in the presence of bacteria. In fact, in some experiments with bacteria, iron oxidation during the second phase proceeded slower than in the absence of bacteria, possibly due to an inhibitory effect of extracellular polymeric substances on the growth of Fe(III) oxides. Thus, our results suggest that the competitive advantage of microbial iron oxidation in low oxygen environments may be limited by the autocatalytic nature of abiotic Fe(III) oxide precipitation, unless the accumulation of Fe(III) oxides is prevented, for example, through a close coupling of Fe(II) oxidation and Fe(III) reduction.     

桂林会仙喀斯特湿地芦苇群落土壤微生物数量动态分析

以广西桂林会仙喀斯特湿地典型芦苇植物群落为研究对象,于春、夏、秋、冬四个季节分别采集0~10cm,10~20 cm和20~30 cm不同层次的土壤样品,分析根际微生物与非根际微生物的数量特征及季节动态变化特点,探讨微生物数量对水热季节变化的响应规律。结果表明:不同季节的根际微生物与非根际微生物组成,均以细菌占绝对优势;微生物数量分布大小顺序为细菌放线菌真菌,细菌最高比例为96.62%,放线菌最高比例为35.38%,真菌的比例较低,最高仅为0.30%。细菌,真菌和放线菌的垂直变化明显,均随着土层的增加而呈现递减的趋势。不同土壤层次根际微生物与非根际微生物的季节变化一致,细菌数量表现为夏季秋季春季冬季,真菌数量表现为秋季夏季春季冬季,放线菌数量表现为秋季春季夏季冬季;细菌、放线菌、真菌的最大值分别为2.70×10~7、1.92×10~6、3.35×10~4cfu·g~-1,土壤微生物数量与土壤有机碳、全氮、全磷、全钾、速效氮、速效磷、速效钾等呈显著正相关。芦苇植物群落根际土壤微生物呈现出一定的根际效应,并与微生物数量、土壤深度、月平均降雨量和月平均气温变化等有关,而在冬季的根际效应则表现不显著。土壤养分含量是调节会仙喀斯特湿地土壤微生物数量变化的一个主要因素。     

Effect of fertilization on exudation,dehydrogenase activity,iron-reducing populations and Fe++ formation in the rhizosphere of rice (Oryza sativa L.) in relation to iron toxicity

Summary To explain the mechanism of iron toxicity, greenhouse and growth chamber (¹⁴CO₂ atmosphere) experiments were carried out. In pot experiments (with a typical iron-toxic soil and a fertile clay) we studied the effect of N, P, K and Ca+Mg fertilization (alone or in combination) on dehydrogenase activity, Fe⁺⁺ formation, and the populations of iron-reducing bacteria in the rhizosphere of rice IR22 and IR42. Fe uptake by the plants was measured at regular intervals. Dehydrogenase activity, the number of N₂-fixing iron-reducing bacteria, and the formation and uptake of Fe⁺⁺ decreased with increased supply of K, Ca, and Mg. This effect was clearer with IR22 (susceptible to iron toxicity) than with IR42 (releatively tolerant). Increased exudation and Fe uptake by IR36 at low nutrient and high Fe supply were recorded in a growth chamber experiment. Nutritional conditions, exudation rate (a measure of metabolic root leakage), the iron-reducing activity of the rhizosphere, and Fe⁺⁺ uptake by wetland rice appear to be clearly related. Iron toxicity is considered a physiological disorder caused by multiple nutritional soil stress rather than by a low pH and high Fe supply per sé.     

Iron cycling at corroding carbon steel surfaces

Surfaces of carbon steel (CS) exposed to mixed cultures of iron-oxidizing bacteria (FeOB) and dissimilatory iron-reducing bacteria (FeRB) in seawater media under aerobic conditions were rougher than surfaces of CS exposed to pure cultures of either type of microorganism. The roughened surface, demonstrated by profilometry, is an indication of loss of metal from the surface. In the presence of CS, aerobically grown FeOB produced tight, twisted helical stalks encrusted with iron oxides. When CS was exposed anaerobically in the presence of FeRB, some surface oxides were removed. However, when the same FeOB and FeRB were grown together in an aerobic medium, FeOB stalks were less encrusted with iron oxides and appeared less tightly coiled. These observations suggest that iron oxides on the stalks were reduced and solubilized by the FeRB. Roughened surfaces of CS and denuded stalks were replicated with culture combinations of different species of FeOB and FeRB under three experimental conditions. Measurements of electrochemical polarization resistance established different rates of corrosion of CS in aerobic and anaerobic media, but could not differentiate rate differences between sterile controls and inoculated exposures for a given bulk concentration of dissolved oxygen. Similarly, total iron in the electrolyte could not be used to differentiate treatments. The experiments demonstrate the potential for iron cycling (oxidation and reduction) on corroding CS in aerobic seawater media.     

Reduction of Fe(III) (Hydr)oxides with Known Thermodynamic Stability by Geobacter metallireducens

Sulfate-reducing and methanogenic microorganisms become inactive when the concentration of the electron donors drops below a threshold set by the minimum Gibbs free energy required for the bacterial metabolism to be maintained. Thus, their activity is thermodynamically controlled. In this paper we study if the activity of dissimilatory Fe(III) reducing bacteria is also limited by the thermodynamics of the reaction. We synthesized five Fe (III) (hydr)oxides (FHOs) of moderate stability and determined the solubility product (log K _SO (?39.1)-(?41.8)), in order to calculate their standard free energy of formation. K _SO values, estimated from the particle size did not correspond with experimentally determined ones. HCO₃ ^? and PIPES-buffered media, containing 45 mM FHO and either 1, 10, or 100 mM acetate were inoculated with Geobacter metallireducens. At the end of bacterial reduction, the Gibbs free energy of the reaction showed significant differences between the different FHOs. The termination of the bacterial activity was consequently not triggered thermodynamically. However, the non-dissolved Fe(II) (HCl-soluble minus soluble Fe(II)) showed an excellent correlation with the surface of the FHOs (15 μmol m^?2). It is therefore likely that the termination of the reaction was caused by blocking of the FHO surface with insoluble Fe(II), as has been previously reported in the literature. The ecological significance of both thermodynamic limitation and surface availability limitation is discussed for FHOs of different K _SO in environments with approximately neutral pH.     

异化铁还原细菌LQ25还原重金属Cr (VI)的特性研究

[背景] 异化铁还原细菌能够在还原Fe （III）的同时将毒性较大的Cr （VI）还原成毒性较小的Cr （III）,解决铬污染的问题。[目的] 基于丁酸梭菌（Clostridium butyricum） LQ25异化铁还原过程制备生物磁铁矿,开展异化铁还原细菌还原Cr （VI）的特性研究。[方法] 构建以氢氧化铁为电子受体和葡萄糖为电子供体的异化铁培养体系。菌株LQ25培养结束时制备生物磁铁矿。设置不同初始Cr （VI）浓度（5、10、15、25和30 mg/L）,分别测定菌株LQ25对Cr （VI）还原效率以及生物磁铁矿对Cr （VI）的还原效率。[结果] 菌株LQ25在设置的Cr （VI）浓度范围内都能良好生长。当Cr （VI）浓度为15 mg/L时,在异化铁培养条件下,菌株LQ25对Cr （VI）的还原率为63.45%±5.13%,生物磁铁矿对Cr （VI）的还原率为87.73%±9.12%,相比菌株还原Cr （VI）的效率提高38%。pH变化能影响生物磁铁矿对Cr （VI）的还原率,当pH 2.0时,生物磁铁矿对Cr （VI）的还原率最高,几乎达到100%。电子显微镜观察发现生物磁铁矿表面有许多孔隙,X-射线衍射图谱显示生物磁铁矿中Fe （II）的存在形式是Fe （OH）₂。[结论] 基于异化铁还原细菌制备生物磁铁矿可用于还原Cr （VI）,这是一种有效去除Cr （VI）的途径。     

Iron-Oxidizing Bacteria Are Associated with Ferric Hydroxide Precipitates (Fe-Plaque) on the Roots of Wetland Plants

The presence of Fe-oxidizing bacteria in the rhizosphere of four different species of wetland plants was investigated in a diverse wetland environment that had Fe(II) concentrations ranging from tens to hundreds of micromoles per liter and a pH range of 3.5 to 6.8. Enrichments for neutrophilic, putatively lithotrophic Fe-oxidizing bacteria were successful on roots from all four species; acidophilic Fe-oxidizing bacteria were enriched only on roots from plants whose root systems were exposed to soil solutions with a pH of <4. In Sagittaria australis there was a positive correlation (P < 0.01) between cell numbers and the total amount of Fe present; the same correlation was not found for Leersia oryzoides. These results present the first evidence for culturable Fe-oxidizing bacteria associated with Fe-plaque in the rhizosphere.     

Efficient aerobic oxidation of hydrocarbons promoted by high-spin nonheme Fe(II) complexes without any reductant

Fe(II)-tris(2-pyridylmethyl)amine complexes, Fe(II)-tpa, having different co-existing anions, [Fe(tpa)(MeCN)₂](ClO₄)₂ (1), [Fe(tpa)(MeCN)₂](CF₃SO₃)₂ (2) and [Fe(tpa)Cl₂] (3), were prepared. Effective magnetic moments (evaluated by the Evans method) revealed that while 1-3 in acetone and 3 in acetonitrile (MeCN) have a high-spin Fe(II) ion at 298 K, the Fe(II) ions of 1 and 2 are in the low-spin state in MeCN. The aerobic oxidation of 1-3 was monitored by UV-Vis spectral changes in acetone or MeCN under air at 298 K. Only the high-spin Fe(II)-tpa complexes were oxidized with rate constants of k_obs = 0.1-1.3 h⁻¹, while 1 and 2 were stable in MeCN. The aerobic oxidation of 1 or 2 in acetone was greatly accelerated in the presence of pure, peroxide-free cyclohexene (1000 equiv.) and yielded a large amount of oxidized products; 2-cyclohexe-1-ol (A) and 2-cyclohexene-1-one (K) (A + K: 23 940% yield based on Fe; A/K = 0.3), and cyclohexene oxide (810%). Besides cyclohexene, aerobic oxidation of norbornene, cyclooctene, ethylbenzene, and cumene proceeded in the presence of 1 in acetone at 348 K without any reductant. Essential factors in the reaction are high-spin Fe(II) ion and labile coordination sites, both of which are required to generate Fe(II)-superoxo species as active species for the H-atom abstraction of hydrocarbons.     

嗜中性微好氧铁氧化菌研究进展

在弱酸至近中性微氧条件下,嗜中性微好氧铁氧化菌能够通过依赖氧气的呼吸机制将二价亚铁氧化成三价铁,并获得生长所需能量。这一生物铁氧化过程的主要产物之一是无定形羟基氧化铁——一种异化铁还原作用(铁呼吸)的理想底物,故可加速铁元素在氧化还原分界层的地质循环。有关嗜中性微好氧铁氧化菌的记载可追溯到19世纪30年代,但对其生理、生态与系统发育学的研究自20世纪90年代中期才取得显著进展,主要得益于专性铁氧化菌新种、属的成功培养与分离。已知微好氧铁氧化菌广泛分布于弱酸及近中性富铁地下水、湿地和深海等环境,其参与调控的铁氧化过程对铁及其他元素(如碳、氮、磷、锰和砷等)的生物地球化学循环具有重要意义。这类古老微生物在金属成矿、地壳演变、全球气候变化及其它生源要素地球化学过程中的作用研究已逐渐受到关注,正成为地质与环境微生物学领域的研究热点。主要总结国外近15a对嗜中性微好氧铁氧化菌的研究进展,包括其代谢机理、种类和分布、生态学研究方法和技术、以及细菌铁氧化作用的实际应用和环境意义等,并对今后研究方向提出展望。