Bacterial Leaching of Metal Sulfides Proceeds by Two Indirect Mechanisms via Thiosulfate or via Polysulfides and Sulfur

The acid-insoluble metal sulfides FeS₂, MoS₂, and WS₂ are chemically attacked by iron(III) hexahydrate ions, generating thiosulfate, which is oxidized to sulfuric acid. Other metal sulfides are attacked by iron(III) ions and by protons, resulting in the formation of elemental sulfur via intermediary polysulfides. Sulfur is biooxidized to sulfuric acid. This explains leaching of metal sulfides by Thiobacillus thiooxidans.     

Microbial Populations Stimulated for Hexavalent Uranium Reduction in Uranium Mine Sediment

Uranium-contaminated sediment and water collected from an inactive uranium mine were incubated anaerobically with organic substrates. Stimulated microbial populations removed U almost entirely from solution within 1 month. X-ray absorption near-edge structure analysis showed that U(VI) was reduced to U(IV) during the incubation. Observations by transmission electron microscopy, selected area diffraction pattern analysis, and energy-dispersive X-ray spectroscopic analysis showed two distinct types of prokaryotic cells that precipitated only a U(IV) mineral uraninite (UO₂) or both uraninite and metal sulfides. Prokaryotic cells associated with uraninite and metal sulfides were inferred to be sulfate-reducing bacteria. Phylogenetic analysis of 16S ribosomal DNA obtained from the original and incubated sediments revealed that microbial populations were changed from microaerophilic Proteobacteria to anaerobic low-G+C gram-positive sporeforming bacteria by the incubation. Forty-two out of 94 clones from the incubated sediment were related to sulfate-reducing Desulfosporosinus spp., and 23 were related to fermentative Clostridium spp. The results suggest that, if in situ bioremediation were attempted in the uranium mine ponds, Desulfosporosinus spp. would be a major contributor to U(VI) and sulfate reduction and Clostridium spp. to U(VI) reduction.     

Differential Protein Expression during Growth of Acidithiobacillus ferrooxidans on Ferrous Iron, Sulfur Compounds, or Metal Sulfides

A set of proteins that changed their levels of synthesis during growth of Acidithiobacillus ferrooxidans ATCC 19859 on metal sulfides, thiosulfate, elemental sulfur, and ferrous iron was characterized by using two-dimensional polyacrylamide gel electrophoresis. N-terminal amino acid sequencing and mass spectrometry analysis of these proteins allowed their identification and the localization of the corresponding genes in the available genomic sequence of A. ferrooxidans ATCC 23270. The genomic context around several of these genes suggests their involvement in the energetic metabolism of A. ferrooxidans. Two groups of proteins could be distinguished. The first consisted of proteins highly upregulated by growth on sulfur compounds (and downregulated by growth on ferrous iron): a 44-kDa outer membrane protein, an exported 21-kDa putative thiosulfate sulfur transferase protein, a 33-kDa putative thiosulfate/sulfate binding protein, a 45-kDa putative capsule polysaccharide export protein, and a putative 16-kDa protein of unknown function. The second group of proteins comprised those downregulated by growth on sulfur (and upregulated by growth on ferrous iron): rusticyanin, a cytochrome c₅₅₂, a putative phosphate binding protein (PstS), the small and large subunits of ribulose biphosphate carboxylase, and a 30-kDa putative CbbQ protein, among others. The results suggest in general a separation of the iron and sulfur utilization pathways. Rusticyanin, in addition to being highly expressed on ferrous iron, was also newly synthesized, as determined by metabolic labeling, although at lower levels, during growth on sulfur compounds and iron-free metal sulfides. During growth on metal sulfides containing iron, such as pyrite and chalcopyrite, both proteins upregulated on ferrous iron and those upregulated on sulfur compounds were synthesized, indicating that the two energy-generating pathways are induced simultaneously depending on the kind and concentration of oxidizable substrates available.     

Abiotic Reduction of 4-Chloronitrobenzene to 4-Chloroaniline in a Dissimilatory Iron-Reducing Enrichment Culture

4-chloronitrobenzene (4-Cl-NB) was rapidly reduced to 4-chloroaniline with half-lives of minutes in a dissimilatory Fe(III)-reducing enrichment culture. The initial pseudo-first-order rate constants at 25°C ranged from 0.11 to 0.19 per minute. The linear Arrhenius correlation in a temperature range of 6 to 85°C and the unchanged reactivity after pasteurization indicated that the nitroreduction occurred abiotically. A fine-grained black solid which was identified as poorly crystalline magnetite (Fe₃O₄) by X-ray diffraction accumulated in the enrichments. Magnetite produced by the Fe(III)-reducing bacterium Geobacter metallireducens GS-15 and synthetic magnetite also reduced 4-Cl-NB. These results suggest that the reduction of 4-Cl-NB by the enrichment material was a surface-mediated reaction by dissimilatory formed Fe(II) associated with magnetite.     

Effects of filamentous algal mats on sulfide invasion in eelgrass (Zostera marina)

The effect of filamentous algae invasion into Zostera marina meadows on water quality, sediment sulfur pools and sulfide invasion into plant tissues was studied experimentally. Sulfide invasion was assessed through analysis of sulfur isotopic composition (δ³⁴S) and total sulfur (TS) concentrations in plant tissues. The algal mats (5 and 10 cm thickness) depleted oxygen in the mats and increased the pools of sulfides in the sediments. Plants exposed to algal mats had δ³⁴S signals closer to the δ³⁴S of sediment sulfide, whereas plants with no mats present had δ³⁴S signals closer to the δ³⁴S of seawater sulfate, indicating a higher sulfide invasion in plants exposed to algal mats. The δ³⁴S varied between the plant tissues with the leaves having more positive δ³⁴S signals than roots and rhizomes, indicating that sulfide was invading into the roots and moved to the other tissues through the lacunae. TS concentrations were higher in plants exposed to algal mats suggesting that sulfur derived from sediment sulfide accumulated in the plants. F_sulfide showed that up to 50% of the sulfides in the plants were derived from sedimentary sulfides. The combined effect of water column anoxia in the lower parts of the meadow and high sulfide invasion into the plants lead to significantly reduced growth rates after 3 weeks and the below-ground tissues showed signs of degradation suggesting that algal mats invasion in to Zostera marina meadows can result in seagrass decline.     

Biogeochemical cycling of sulfur and iron in sediments of a south-east Asian mangrove,Phuket Island,Thailand

Benthic sulfate reduction and sediment pools of sulfur and iron were examined during January 1992 at 3 stations in the Ao Nam Bor mangrove, Phuket, Thailand. Patterns of sulfate reduction rates (0–53 cm) reflected differences in physical and biological conditions at the 3 stations, and highest rates were found at the vegetated site within the mangrove (Rhizophora apiculata) forest. Due to extended oxidation of mangrove sediments, a large portion of the added³⁵S-label was recovered in the chromium reducible pools (FeS₂ and S⁰) (41–91% of the reduced sulfur). Pyrite was the most important inorganic sulfur component, attaining pool sizes 50–100 times higher than acid volatile pools (FeS). HCl-extractable (0.5 M HCl) iron pools, including Fe(II)_HCl and Fe(III)_HCl, were generally low and Fe(III)_HCl was only present in the upper surface layers (0–5 cm). Maximum concentrations of dissolved Fe²⁺ (35–285 M) occurred just about the depth where dissolved H₂S accumulated. Furthermore Fe²⁺ and H₂S coexisted only where concentrations of both were low. There was an accumulation of organic sulfur in the deep sediment at 2 stations in the inner part of the mangrove. The reoxidation of reduced sulfides was rapid, and storage of sulfur was minor in the upper sediment layers, where factors like bioturbation, the presence of roots, or tidal mixing enhance oxidation processes.Author of correspondence.     

Influence of the sulfur species reactivity on biofilm conformation during pyrite colonization by Acidithiobacillus thiooxidans

Massive pyrite (FeS₂) electrodes were potentiostatically modified by means of variable oxidation pulse to induce formation of diverse surface sulfur species (S _n ^2?, S⁰). The evolution of reactivity of the resulting surfaces considers transition from passive (e.g., Fe_1?x S₂) to active sulfur species (e.g., Fe_1?x S_2?y , S⁰). Selected modified pyrite surfaces were incubated with cells of sulfur-oxidizing Acidithiobacillus thiooxidans for 24 h in a specific culture medium (pH 2). Abiotic control experiments were also performed to compare chemical and biological oxidation. After incubation, the attached cells density and their exopolysaccharides were analyzed by confocal laser scanning microscopy (CLMS) and atomic force microscopy (AFM) on bio-oxidized surfaces; additionally, S _n ^2?/S⁰ speciation was carried out on bio-oxidized and abiotic pyrite surfaces using Raman spectroscopy. Our results indicate an important correlation between the evolution of S _n ^2?/S⁰ surface species ratio and biofilm formation. Hence, pyrite surfaces with mainly passive-sulfur species were less colonized by A. thiooxidans as compared to surfaces with active sulfur species. These results provide knowledge that may contribute to establishing interfacial conditions that enhance or delay metal sulfide (MS) dissolution, as a function of the biofilm formed by sulfur-oxidizing bacteria.     

Dynamics of Iron Uptake and Fe3O4 Biomineralization during Aerobic and Microaerobic Growth of Magnetospirillum gryphiswaldense

Iron uptake and magnetite (Fe₃O₄) crystal formation could be studied in the microaerophilic magnetic bacterium Magnetospirillum gryphiswaldense by using a radioactive tracer method for iron transport and a differential light-scattering technique for magnetism. Magnetite formation occurred only in a narrow range of low oxygen concentration, i.e., 2 to 7 μM O₂ at 30°C. Magnetic cells stored up to 2% iron as magnetite crystals in intracytoplasmic vesicles. This extraordinary uptake of iron was coupled tightly to the biomineralization of up to 60 magnetite crystals with diameters of 42 to 45 nm.     

Sulfhydrolase Activity in Sediments of Wintergreen Lake, Kalamazoo County, Michigan

The hydrolysis of p-nitrophenyl sulfate, p-nitrocatechol sulfate, and [³⁵S]sodium dodecyl sulfate was examined in anoxic sediments of Wintergreen Lake, Michigan. Significant levels of sulfhydrolase activity were observed in littoral, transition, and profundal sediment samples. Rates of sulfate formation suggest that the sulfhydrolase system would represent a major source of sulfate within these sediments. Sulfate formed by ester sulfate hydrolysis can support dissimilatory sulfate reduction as shown by the incorporation of ³⁵S from labeled sodium dodecyl sulfate into H₂³⁵S. Sulfhydrolase activity varied with sediment depth, was greatest in the littoral zone, and was sensitive to the presence of oxygen. Estimations of ester sulfate concentrations in sediments revealed large quantities of ester sulfate (~30% of total sulfur). Both total sulfur and ester sulfate concentrations varied with the sediment type and were two to three orders of magnitude greater than the inorganic sulfur concentration.     

Effect of shading of Zostera marina (eelgrass) on sulfur cycling in sediments with contrasting organic matter and sulfide pools

Eelgrass Zostera marina was collected in spring and autumn from a light-saturated environment with low-organic sediments and a light-limited environment with organic-rich sediments in Denmark. The eelgrass and sediment responses to reduced light conditions were studied in 2-week shading experiments. Z. marina responded to reduced light conditions by decreasing growth rates and a loss of above-ground biomass. The spring plants were most sensitive to light reductions and the relative leaf elongation rates were reduced with up to 58% and the shoot densities with 33-36%. There was no difference in light response in relation to sediment organic matter contents. The sulfate reduction rates were reduced in the shaded low-organic sediments with up to 67%, whereas there was no effect of shading on rates in the organic-rich sediments. The lack of effect of shading in the organic-rich sediments was attributed to a limited coupling between Z. marina production and sediment bacterial carbon cycling. In contrast to the sulfate reduction rates, the pools of reduced sulfur were increased with up to 89% in the shaded, low-organic sediments, suggesting that the reoxidation of sulfides was reduced. Shading had no effect on the pools of sulfides in the organic-rich sediments due to much larger pools of sulfides. The enhanced sensitivity of spring plants to shading was probably due to a low above- to below-ground ratio compared to the autumn plants, which limited the plant-mediated oxidation of the sediments and thus the reoxidation of sulfides. The shaded plants were possibly more exposed to anoxic and sulfidic conditions affecting their growth and survival.     

Fe(III) oxides protect fermenter–methanogen syntrophy against interruption by elemental sulfur via stiffening of Fe(II) sulfides produced by sulfur respiration

Thermosipho globiformans (rod-shaped thermophilic fermenter) and Methanocaldococcus jannaschii (coccal hyperthermophilic hydrogenotrophic methanogen) established H₂-mediated syntrophy at 68 °C, forming exopolysaccharide-based aggregates. Electron microscopy showed that the syntrophic partners connected to each other directly or via intercellular bridges made from flagella, which facilitated transfer of H₂. Elemental sulfur (S⁰) interrupted syntrophy; polysulfides abiotically formed from S⁰ intercepted electrons that were otherwise transferred to H⁺ to produce H₂, resulting in the generation of sulfide (sulfur respiration). However, Fe(III) oxides significantly reduced the interruption by S⁰, accompanied by stiffening of Fe(II) sulfides produced by the reduction of Fe(III) oxides with the sulfur respiration-generated sulfide. Sea sand replacing Fe(III) oxides failed to generate stiffening or protect the syntrophy. Several experimental results indicated that the stiffening of Fe(II) sulfides shielded the liquid from S⁰, resulting in methane production in the liquid. Field-emission scanning electron microscopy showed that the stiffened Fe(II) sulfides formed a network of spiny structures in which the microorganisms were buried. The individual fermenter rods likely produced Fe(II) sulfides on their surface and became local centers of a core of spiny structures, and the connection of these cores formed the network, which was macroscopically recognized as stiffening.     

Sandwich‐Like Ultrathin TiS2 Nanosheets Confined within N,S Codoped Porous Carbon as an Effective Polysulfide Promoter in Lithium‐Sulfur Batteries

As the lightest member of transition metal dichalcogenides, 2D titanium disulfide (2D TiS₂) nanosheets are attractive for energy storage and conversion. However, reliable and controllable synthesis of single‐ to few‐layered TiS₂ nanosheets is challenging due to the strong tendency of stacking and oxidation of ultrathin TiS₂ nanosheets. This study reports for the first time the successful conversion of Ti₃C₂T_x MXene to sandwich‐like ultrathin TiS₂ nanosheets confined by N, S co‐doped porous carbon (TiS₂@NSC) via an in situ polydopamine‐assisted sulfuration process. When used as a sulfur host in lithium–sulfur batteries, TiS₂@NSC shows both high trapping capability for lithium polysulfides (LiPSs), and remarkable electrocatalytic activity for LiPSs reduction and lithium sulfide oxidation. A freestanding sulfur cathode integrating TiS₂@NSC with cotton‐derived carbon fibers delivers a high areal capacity of 5.9 mAh cm^?2 after 100 cycles at 0.1 C with a low electrolyte/sulfur ratio and a high sulfur loading of 7.7 mg cm^?2, placing TiS₂@NSC one of the best LiPSs adsorbents and sulfur conversion catalysts reported to date. The developed nanospace‐confined strategy will shed light on the rational design and structural engineering of metal sulfides based nanoarchitectures for diverse applications.     

Acidithiobacillus ferrooxidans and its potential application

The widely distributed Acidithiobacillus ferrooxidans (A. ferrooxidans) lives in extremely acidic conditions by fixing CO₂ and nitrogen, and by obtaining energy from Fe²⁺ oxidation with either downhill or uphill electron transfer pathway and from reduced sulfur oxidation. A. ferrooxidans exists as different genomovars and its genome size is 2.89–4.18 Mb. The chemotactic movement of A. ferrooxidans is regulated by quorum sensing. A. ferrooxidans shows weak magnetotaxis due to formation of 15–70 nm magnetite magnetosomes with surface functional groups. The room- and low-temperature magnetic features of A. ferrooxidans are different from other magnetotactic bacteria. A. ferrooxidans has potential for removing sulfur from solids and gases, metals recycling from metal-bearing ores, electric wastes and sludge, biochemical production synthesizing, and metal workpiece machining.

     

Chemolithotrophic growth of the phototrophic sulfur bacterium Thiocapsa roseopersicina

Chemotrophic growth capacities of the purple sulfur bacterium Thiocapsa roseopersicina strain M1 were studied in continuous culture under thiosulfate limitation.Pigment synthesis was completely inhibited upon a shift from anaerobic to semi-aerobic conditions (52 μM O₂) in the light, but no active breakdown occurred. During the transient state, the cells grew in a mixed photo- and chemolithotrophic mode; the specific respiration rate gradually increased with a concomitant drop in the bacteriochlorophyll a content. Photolithotrophically grown cells have the ability to respire. It was concluded that photosynthesis and respiration compete for electrons, but that photosynthesis is preferred under electron donor-limiting conditions, when the cells still contain large amounts of pigments. Eventually, a fully chemolithotrophic steady state was attained.The chemolithotropic growth of T. roseopersicina was studied in the dark under semiaerobic conditions at various dilution rates. The maximum specific growth rate was 68% of the maximum attainable growth rate under photolithotrophic conditions. The growth affinity for thiosulfate was high (K_m = 1.5 μM). The yield on thiosulfate under chemolithotrophic conditions exceeded that of thiobacilli. Oxygen uptake was studied in short-term experiments. It was shown that respiration in T. roseopersicina has a K_m of approx. 1 μM O₂. the ecological importance for T. roseopersicina of chemolithotrophic growth and pigment content is discussed with respect to the occurrence of T. roseopersicina in laminated microbial ecosystems and its possible competition with colorless sulfur bacteria.     

Distinguishable root plaque on root surface of Potamogeton crispus grown in two sediments with different nutrient status

The properties of plaques were different on the root surface of Potamogeton crispus planted in sediments from two different shallow lakes. Lake Tangxunhu sediment, with low pH, contained low organic matter, whereas Lake Yuehu sediment, with high pH, had high calcium deposits mixed with high organic matter. The contents of mineral elements in sediment of Lake Tangxunhu was lower than that of Lake Yuehu, except for iron (Fe) content, but the contents of mineral elements extracted by sodium dithionite–sodium citrate–sodium bicarbonate (DCB) from root plaques were higher in Lake Tangxunhu than those in Lake Yuehu, except for Fe. These element distributions on P. crispus root plaques were characterized by scanning electron microscope combined with energy-dispersive X-ray spectrometer and were consistent with the contents of mineral elements in sediment. The root plaque of P. crispus planted in Lake Tangxunhu sediment mainly contained silicon (Si) and Fe, and the content of Si was greater than Fe, which may be contributed to the formation of poly-silicic-ferric in the natural conditions. However, the root plaque of P. crispus planted in the sediment with higher calcium content of Lake Yuehu was rich in Fe, Si, phosphorus (P), and calcium (Ca). Due to oxygen secretion by plant roots, the root plaque has more Fe₃(PO₄)₂ and a certain amount of Ca₃(PO₄)₂. The ratio of magnesium (Mn) to Fe extracted by DCB from root plaque in Lake Tangxunhu sediment was 0.031 and 0.010 in Lake Yuehu sediment. In Lake Tangxunhu sediment, lower content of organic matter results in weak reducibility. Enhanced oxidation ability by oxygen secretion of P. crispus root could oxidize low-valent Fe and Mn into iron–manganese oxide, which leads to formation of iron–manganese plaque on the root surface. However, this case is different in Lake Yuehu sediment, where Fe and Mn can be reduced in high organic sediment and low-valent Mn can precipitate in the sediment in which pH is >8. Thus, low-valent Fe in Lake Yuehu sediment moves to the root surface of P. crispus, where it oxidizes into Fe oxide, i.e., Fe plaque.     

Heat Transfer in MHD Mixed Convection Flow of a Ferrofluid along a Vertical Channel

This study investigated heat transfer in magnetohydrodynamic (MHD) mixed convection flow of ferrofluid along a vertical channel. The channel with non-uniform wall temperatures was taken in a vertical direction with transverse magnetic field. Water with nanoparticles of magnetite (Fe ₃ O ₄) was selected as a conventional base fluid. In addition, non-magnetic (Al ₂ O ₃) aluminium oxide nanoparticles were also used. Comparison between magnetic and magnetite nanoparticles were also conducted. Fluid motion was originated due to buoyancy force together with applied pressure gradient. The problem was modelled in terms of partial differential equations with physical boundary conditions. Analytical solutions were obtained for velocity and temperature. Graphical results were plotted and discussed. It was found that temperature and velocity of ferrofluids depend strongly on viscosity and thermal conductivity together with magnetic field. The results of the present study when compared concurred with published work.     

Stereoselective oxidation of sulfides by cloned naphthalene dioxygenase

Washed-cell preparations of recombinant Escherichia coli JM109(pDTG141), engineered to express the naphthalene dioxygenase (NDO) gene from Pseudomonas sp. NCIB 9816-4, have been used to biooxidise a range of aryl alkyl-, dialkyl- and bicyclic sulfides. A series of 16 phenyl alkyl sulfides was oxidised to equivalent sulfoxides, typically with moderate to high (>90%) yield and high enantioselectivity (>85% ee), the (S)-enantiomer being the predominant product, with little if any further oxidation. The addition of some electron-donating or electron-withdrawing groups to the phenyl ring decreased yield and/or stereoselectivity of the NDO-catalysed biotransformation, whereas increasing the size of the alkyl chain (nC₃H₇, iC₃H₇ and nC₄H₉) resulted in a notable inversion in selectivity to yield (R)-series sulfoxides (>74% ee) as the predominant products. The addition of one or more methylene groups between the phenyl ring and sulfur atom resulted in notable reductions in both the yield and stereoselectivity of the (S)-predominant sulfoxidations. With the exception of cyclohexyl- and n-hexyl methyl sulfide which both gave (S)-sulfoxides with good stereoselectivity and yield, other dialkyl- and bicyclic sulfides were poor substrates for sulfoxidation by NDO. Both the close agreement with data obtained using purified NDO and the absence of stereoselective sulfoxidation in equivalent controls with the E. coli JM109 host support the contribution made by the cloned NDO carried on the pDTG141 plasmid.     

Sulfur accumulation in eelgrass (Zostera marina) and effect of sulfur on eelgrass growth

Eelgrass (Zostera marina) was grown under exposure to high levels of sediment sulfides to examine their ability to reoxidize sulfides intruding into the plants. The plants were kept under full light (control and high sulfide level) and at 10% of light saturation (high sulfide level) for 3 weeks and growth and accumulation of elemental sulfur (S⁰) in the plants were examined. The growth rate was reduced with ∼75% in the low light treatment, whereas there was no significant difference between the rates at full light saturation. S⁰ was accumulating in the below-ground structures of the plants exposed to high sulfide concentrations with highest concentration in the youngest roots and oldest internodes. There was no accumulation of S⁰ in the leaves, suggesting that the intruding sulfides were reoxidized in the below-ground structures before reaching the leaves. The accumulation of S⁰ was higher in the roots of the low light treatment (up to two times) suggesting a larger intrusion of sulfides. These plants also appeared highly affected by the treatment with rotting meristems and increased mortality after the 3-week growth period. These results are the first to show an accumulation of sulfur compounds internally in seagrasses as a result of reoxidation of sulfides. The reoxidation is facilitated by the internal transport of oxygen and is an example of the advantage of the internal lacunae system in seagrasses.     

Precipitation of greigite and pyrite induced by Thermococcales: an advantage to live in Fe- and S-rich environments?

Thermococcales, a major order of archaea inhabiting the iron- and sulfur-rich anaerobic parts of hydrothermal deep-sea vents, have been shown to rapidly produce abundant quantities of pyrite FeS₂ in iron–sulfur-rich fluids at 85°C, suggesting that they may contribute to the formation of ‘low temperature’ FeS₂ in their ecosystem. We show that this process operates in Thermococcus kodakarensis only when zero-valent sulfur is directly available as intracellular sulfur vesicles. Whether in the presence or absence of zero-valent sulfur, significant amounts of Fe₃S₄ greigite nanocrystals are formed extracellularly. We also show that mineralization of iron sulfides induces massive cell mortality but that concomitantly with the formation of greigite and/or pyrite, a new generation of cells can grow. This phenomenon is observed for Fe concentrations of 5 mM but not higher suggesting that above a threshold in the iron pulse all cells are lysed. We hypothesize that iron sulfides precipitation on former cell materials might induce the release of nutrients in the mineralization medium further used by a fraction of surviving non-mineralized cells allowing production of new alive cells. This suggests that biologically induced mineralization of iron-sulfides could be part of a survival strategy employed by Thermococcales to cope with mineralizing high-temperature hydrothermal environments.