Solid phase speciation controls copper mobilisation from marine sediments by methanobactin.

Although marine environments represent huge reservoirs of the potent greenhouse gas methane, they currently contribute little to global net methane emissions. Most of the methane is oxidized by methanotrophs, minimizing escape to the atmosphere. Aerobic methanotrophs oxidize methane mostly via the copper (Cu)-bearing enzyme particulate methane monooxygenase (pMMO). Therefore, aerobic methane oxidation depends on sufficient Cu acquisition by methanotrophs. Because they require both oxygen and methane, aerobic methanotrophs reside at oxic-anoxic interfaces, often close to sulphidic zones where Cu bioavailability can be limited by poorly soluble Cu sulphide mineral phases. Under Cu-limiting conditions, certain aerobic methanotrophs exude Cu-binding ligands termed chalkophores, such as methanobactin (mb) exuded by Methylosinus trichosporium OB3b. Our main objective was to establish whether chalkophores can mobilise Cu from Cu sulphide-bearing marine sediments to enhance Cu bioavailability. Through a series of kinetic batch experiments, we investigated Cu mobilisation by mb from a set of well-characterized sulphidic marine sediments differing in sediment properties, including Cu content and phase distribution. Characterization of solid-phase Cu speciation included X-ray absorption spectroscopy and a targeted sequential extraction. Furthermore, in batch experiments, we investigated to what extent adsorption of metal-free mb and Cu-mb complexes to marine sediments constrains Cu mobilisation. Our results are the first to show that both solid phase Cu speciation and chalkophore adsorption can constrain methanotrophic Cu acquisition from marine sediments. Only for certain sediments did mb addition enhance dissolved Cu concentrations. Cu mobilisation by mb was not correlated to the total Cu content of the sediment, but was controlled by solid-phase Cu speciation. Cu was only mobilised from sediments containing a mono-Cu-sulphide (CuSx) phase. We also show that mb adsorption to sediments limits Cu acquisition by mb to less compact (surface) sediments. Therefore, in sulphidic sediments, mb-mediated Cu acquisition is presumably constrained to surface-sediment interfaces containing mono-Cu-sulphide phases.

Stability of Coumarins and Determination of the Net Iron Oxidation State of Iron-Coumarin Complexes: Implications for Examining Plant Iron Acquisition Mechanisms.

Coumarins are exuded into the soil environment by plant roots in response to iron (Fe) deficiency. Previous studies have shown that coumarins can increase the Fe solubility upon interaction with sparsely soluble Fe(III) (hydr)oxide. However, the chemical mechanisms of Fe(III) (hydr)oxide dissolution by coumarins remain unclear. The high redox instability of dissolved coumarins and the interference of coumarins in determining the Fe redox state hinder the quantitative and mechanistic investigation of coumarin-induced Fe mobilization. In this study, we investigated the oxidative stability of three coumarins that have been found in root exudates, esculetin, scopoletin, and fraxetin, over a broad pH range under oxic and anoxic conditions. Our results show that the oxidation of coumarins is irreversible under oxic conditions and that oxidative degradation rates increased with increasing pH under both oxic and anoxic conditions. However, the complexation of Fe protects coumarins from degradation in the circumneutral pH range even under oxic conditions. Furthermore, we observed that Ferrozine, which is commonly used for establishing Fe redox speciation, can facilitate the reduction of Fe(III) complexed by coumarins, even at circumneutral pH. Reduction rates increased with decreasing pH and were larger for fraxetin than for scopoletin and esculetin. Based on these observations, we optimized the Ferrozine method for determining the redox state of Fe complexed by coumarins. Understanding the stability of dissolved coumarins and using a precise analytical method to determine the redox state of Fe in the presence of coumarins are critical for investigating the mechanisms by which coumarins enhance the availability of Fe in the rhizosphere.

Ultrastructure of noise-induced cochlear synaptopathy.

Acoustic overexposure can eliminate synapses between inner hair cells (IHCs) and auditory nerve fibers (ANFs), even if hair-cell function recovers. This synaptopathy has been extensively studied by confocal microscopy, however, understanding the nature and sequence of damage requires ultrastructural analysis. Here, we used focused ion-beam scanning electron microscopy to mill, image, segment and reconstruct ANF terminals in mice, 1 day and 1 week after synaptopathic exposure (8-16 kHz, 98 dB SPL). At both survivals, ANF terminals were normal in number, but 62% and 53%, respectively, lacked normal synaptic specializations. Most non-synapsing fibers (57% and 48% at 1 day and 1 week) remained in contact with an IHC and contained healthy-looking organelles. ANFs showed a transient increase in mitochondrial content (51%) and efferent innervation (34%) at 1 day. Fibers maintaining synaptic connections showed hypertrophy of pre-synaptic ribbons at both 1 day and 1 week. Non-synaptic fibers were lower in mitochondrial content and typically on the modiolar side of the IHC, where ANFs with high-thresholds and low spontaneous rates are normally found. Even 1 week post-exposure, many ANF terminals remained in IHC contact despite loss of synaptic specializations, thus, regeneration efforts at early post-exposure times should concentrate on synaptogenesis rather than neurite extension.

Rates and Mechanism of Vivianite Dissolution under Anoxic Conditions.

The iron phosphate mineral vivianite Fe(II)3(PO4)2·8H2O has emerged as a potential renewable P source. Although the importance of vivianite as a potential P sink in the global P cycle had previously been recognized, a mechanistic understanding of vivianite dissolution at the molecular level, critical to its potential application, is still elusive. The potential of vivianite as a P sink or source in natural or engineered systems is directly dependent on its dissolution kinetics under environmentally relevant conditions. To understand the thermodynamic and kinetic controls on bioavailability, the oxidation and dissolution processes of vivianite must be disentangled. In this study, we conducted controlled batch and flow-through experiments to quantitatively determine the dissolution rates and mechanisms of vivianite under anoxic conditions as a function of pH and temperature. Our results demonstrate that vivianite solubility and dissolution rates strongly decreased with increasing solution pH. Dissolution was nonstoichiometric at alkaline pH (>7). The rapid initial dissolution rate of vivianite is related to the solution saturation state, indicating a thermodynamic rather than a kinetic control. A defect-driven dissolution mechanism is proposed. Dissolution kinetics over pH 5-9 could be described with a rate law with a single rate constant and a reaction order of 0.61 with respect to {H+}: Rexp=36.0·e-1.41·pH·[1-e(0.2·ΔG/RT)]4.7 The activation energy of vivianite dissolution proved low (Ea = 20.3 kJ mol-1), suggesting hydrogen bridge dissociation as the rate-determining step.

Effect of Competing Metals and Humic Substances on Uranium Mobilization from Noncrystalline U(IV) Induced by Anthropogenic and Biogenic Ligands.

Anthropogenic and biogenic ligands may mobilize uranium (U) from tetravalent U (U(IV)) phases in the subsurface, especially from labile noncrystalline U(IV). The rate and extent of U(IV) mobilization are affected by geochemical processes. Competing metals and humic substances may play a decisive role in U mobilization by anthropogenic and biogenic ligands. A structurally diverse set of anthropogenic and biogenic ligands was selected for assessing the effect of the aforementioned processes on U mobilization from noncrystalline U(IV), including 2,6-pyridinedicarboxylic acid (DPA), citrate, N,N'-di(2-hydroxybenzyl)ethylene-diamine-N,N'-diacetic acid (HBED), and desferrioxamine B (DFOB). All experiments were performed under anoxic conditions at pH 7.0. The effect of competing metals (Ca, Fe(III), and Zn) on ligand-induced U mobilization depended on the particular metal-ligand combination ranging from nearly complete U mobilization inhibition (e.g., Ca-citrate) to no apparent inhibitory effects or acceleration of U mobilization (e.g., Fe(III)-citrate). Humic substances (Suwannee River humic acid and fulvic acid) were tested across a range of concentrations either separately or combined with the aforementioned ligands. Humic substances alone mobilized appreciable U and also enhanced U mobilization in the presence of anthropogenic or biogenic ligands. These findings illustrate the complex influence of competing metals and humic substances on U mobilization by anthropogenic and biogenic ligands in the environment.

Mercury Isotope Fractionation during Dark Abiotic Reduction of Hg(II) by Dissolved, Surface-Bound, and Structural Fe(II).

Stable mercury (Hg) isotope ratios are an emerging tracer for biogeochemical transformations in environmental systems, but their application requires knowledge of isotopic enrichment factors for individual processes. We investigated Hg isotope fractionation during dark, abiotic reduction of Hg(II) by dissolved iron(Fe)(II), magnetite, and Fe(II) sorbed to boehmite or goethite by analyzing both the reactants and products of laboratory experiments. For homogeneous reduction of Hg(II) by dissolved Fe(II) in continuously purged reactors, the results followed a Rayleigh distillation model with enrichment factors of -2.20 ± 0.16‰ (ε202Hg) and 0.21 ± 0.02‰ (E199Hg). In closed system experiments, allowing reequilibration, the initial kinetic fractionation was overprinted by isotope exchange and followed a linear equilibrium model with -2.44 ± 0.17‰ (ε202Hg) and 0.34 ± 0.02‰ (E199Hg). Heterogeneous Hg(II) reduction by magnetite caused a smaller isotopic fractionation (-1.38 ± 0.07 and 0.13 ± 0.01‰), whereas the extent of isotopic fractionation of the sorbed Fe(II) experiments was similar to the kinetic homogeneous case. Small mass-independent fractionation of even-mass Hg isotopes with 0.02 ± 0.003‰ (E200Hg) and ≈ -0.02 ± 0.01‰ (E204Hg) was consistent with theoretical predictions for the nuclear volume effect. This study contributes significantly to the database of Hg isotope enrichment factors for specific processes. Our findings show that Hg(II) reduction by dissolved Fe(II) in open systems results in a kinetic MDF with a larger ε compared to other abiotic reduction pathways, and combining MDF with the observed MIF allows the distinction from photochemical or microbial Hg(II) reduction pathways.

An obligate intracellular bacterial pathogen forms a direct, interkingdom membrane contact site.

Interorganelle communication regulates cellular homeostasis through the formation of tightly-associated membrane contact sites 1-3. Prior work has identified several ways that intracellular pathogens alter contacts between eukaryotic membranes 4-6, but there is no existing evidence for contact sites spanning eukaryotic and prokaryotic membranes. Here, using a combination of live-cell microscopy and transmission and focused-ion-beam scanning electron microscopy, we demonstrate that the intracellular bacterial pathogen Rickettsia parkeri forms a direct membrane contact site between its bacterial outer membrane and the rough endoplasmic reticulum (ER), with tethers that are approximately 55 nm apart. Depletion of the ER-specific tethers VAPA and VAPB reduced the frequency of rickettsia-ER contacts, suggesting these interactions mimic organelle-ER contacts. Overall, our findings illuminate a direct, interkingdom membrane contact site uniquely mediated by rickettsia that seems to mimic traditional host MCSs.

The Potential Contribution of Hexavalent Chromium to the Carcinogenicity of Chrysotile Asbestos.

Chrysotile asbestos is a carcinogenic mineral that has abundantly been used in industrial and consumer applications. The carcinogenicity of the fibers is partly governed by reactive Fe surface sites that catalyze the generation of highly toxic hydroxyl radicals (HO•) from extracellular hydrogen peroxide (H2O2). Chrysotile also contains Cr, typically in the low mass permille range. In this study, we examined the leaching of Cr from fibers at the physiological lung pH of 7.4 in the presence and absence of H2O2. Furthermore, we investigated the potential of cells from typical asbestos-burdened tissues and cancers to take up Cr leached from chrysotile in PCR expression, immunoblot, and cellular Cr uptake experiments. Finally, the contribution of Cr to fiber-mediated H2O2 decomposition and HO• generation was studied. Chromium readily dissolved from chrysotile fibers in its genotoxic and carcinogenic hexavalent redox state upon oxidation by H2O2. Lung epithelial, mesothelial, lung carcinoma, and mesothelioma cells expressed membrane-bound Cr(VI) transporters and accumulated Cr up to 10-fold relative to the Cr(VI) concentration in the spiked medium. Conversely, anion transporter inhibitors decreased cellular Cr(VI) uptake up to 45-fold. Finally, chromium associated with chrysotile neither decomposed H2O2 nor contributed to fiber-mediated HO• generation. Altogether, our results support the hypothesis that Cr may leach from inhaled chrysotile in its hexavalent state and subsequently accumulate in cells of typically asbestos-burdened tissues, which could contribute to the carcinogenicity of chrysotile fibers. However, unlike Fe, Cr did not significantly contribute to the adverse radical production of chrysotile.

Copper mobilisation from Cu sulphide minerals by methanobactin: Effect of pH, oxygen and natural organic matter.

Aerobic methane oxidation (MOx) depends critically on the availability of copper (Cu) as a crucial component of the metal centre of particulate methane monooxygenase, one of the main enzymes involved in MOx. Some methanotrophs have developed Cu acquisition strategies, in which they exude Cu-binding ligands termed chalkophores under conditions of low Cu availability. A well-characterised chalkophore is methanobactin (mb), exuded by the microaerophilic methanotroph Methylosinus trichosporium OB3b. Aerobic methanotrophs generally reside close to environmental oxic-anoxic interfaces, where the formation of Cu sulphide phases can aggravate the limitation of bioavailable Cu due to their low solubility. The reactivity of chalkophores towards such Cu sulphide mineral phases has not yet been investigated. In this study, a combination of dissolution experiments and equilibrium modelling was used to examine the dissolution and solubility of bulk and nanoparticulate Cu sulphide minerals in the presence of mb as influenced by pH, oxygen and natural organic matter. In general, we show that mb is effective at increasing the dissolved Cu concentrations in the presence of a variety of Cu sulphide phases that may potentially limit Cu bioavailability. More Cu was mobilised per mole of mb from Cu sulphide nanoparticles compared with well-crystalline bulk covellite (CuS). In general, the efficacy of mb at mobilising Cu from Cu sulphides is pH-dependent. At lower pH, e.g. pH 5, mb was ineffective at solubilizing Cu. The presence of mb increased dissolved Cu concentrations between pH 7 and 8.5, where the solubility of all Cu sulphides is generally low, both in the presence and absence of oxygen. These results suggest that chalkophore-promoted Cu mobilisation from sulphide phases is an effective extracellular mechanism for increasing dissolved Cu concentrations at oxic-anoxic interfaces, particularly in the neutral to slightly alkaline pH range. This suggests that aerobic methanotrophs may be able to fulfil their Cu requirements via the exudation of mb in natural environments where the bioavailability of Cu is constrained by very stable Cu sulphide phases.

Ligand-Induced U Mobilization from Chemogenic Uraninite and Biogenic Noncrystalline U(IV) under Anoxic Conditions.

Microbial reduction of soluble hexavalent uranium (U(VI)) to sparingly soluble tetravalent uranium (U(IV)) has been explored as an in situ strategy to immobilize U. Organic ligands might pose a potential hindrance to the success of such remediation efforts. In the current study, a set of structurally diverse organic ligands were shown to enhance the dissolution of crystalline uraninite (UO2) for a wide range of ligand concentrations under anoxic conditions at pH 7.0. Comparisons were made to ligand-induced U mobilization from noncrystalline U(IV). For both U phases, aqueous U concentrations remained low in the absence of organic ligands (<25 nM for UO2; 300 nM for noncrystalline U(IV)). The tested organic ligands (2,6-pyridinedicarboxylic acid (DPA), desferrioxamine B (DFOB), N,N'-di(2-hydroxybenzyl)ethylene-diamine-N,N'-diacetic acid (HBED), and citrate) enhanced U mobilization to varying extents. Over 45 days, the ligands mobilized only up to 0.3% of the 370 µM UO2, while a much larger extent of the 300 µM of biomass-bound noncrystalline U(IV) was mobilized (up to 57%) within only 2 days (>500 times more U mobilization). This work shows the potential of numerous organic ligands present in the environment to mobilize both recalcitrant and labile U forms under anoxic conditions to hazardous levels and, in doing so, undermine the stability of immobilized U(IV) sources.

Soil-pH and cement influence the weathering kinetics of chrysotile asbestos in soils and its hydroxyl radical yield.

Chrysotile asbestos is a toxic and carcinogenic mineral that has been used in a variety of industrial and consumer applications. Much of the fiber- and cement-containing asbestos waste has ended up in terrestrial environments. Chrysotile weathering in soils and the potential for natural attenuation have, however, hardly been examined yet. Here we explored how soil properties influence the dissolution rate of chrysotile, the release of the carcinogenic metals chromium and nickel, and the hydroxyl radical (HO•) generation by chrysotile fibers. Chrysotile dissolution rates in soil suspensions decreased with increasing soil-pH and were lower than reported rates in soil-free systems. Dissolved organic carbon did not markedly accelerate dissolution at circumneutral pH, whereas cement mixed with soil inhibited dissolution because of its alkalinity. The HO•-yield of incubated fibers in non-amended soils eventually decreased by 60-75%. The decline was fastest in an acidic podzol soil, yet was followed by a small rebound. Cement amendment induced the largest HO•-yield reduction (∼90%), presumably due to surface coating of the fibers. Overall, this work demonstrates that the potential for natural attenuation of chrysotile asbestos in soils critically depends on soil chemical parameters and the presence of cement in association with the fibers.

Catalytic effects of photogenerated Fe(II) on the ligand-controlled dissolution of Iron(hydr)oxides by EDTA and DFOB.

Low bioavailability of iron due to poor solubility of iron(hydr)oxides limits the growth of microorganisms and plants in soils and aquatic environments. Previous studies described accelerated dissolution of iron(hydr)oxides under continuous illumination, but did not distinguish between photoreductive dissolution and non-reductive processes in which photogenerated Fe(II) catalyzes ligand-controlled dissolution. Here we show that short illuminations (5-15 min) accelerate the dissolution of iron(hydr)oxides by ligands during subsequent dark periods under anoxic conditions. Suspensions of lepidocrocite (Lp) and goethite (Gt) (1.13 mM) with 50 µM EDTA or DFOB were illuminated with UV-A light of comparable intensity to sunlight (pH 7.0, bicarbonate-CO2 buffered solutions). During illumination, the rate of Fe(II) production was highest with Gt-EDTA; followed by Lp-EDTA > Lp-DFOB > Lp > Gt-DFOB > Gt. Under anoxic conditions, photochemically produced Fe(II) increased dissolution rates during subsequent dark periods by factors of 10-40 and dissolved Fe(III) reached 50 µM with DFOB and EDTA. Under oxic conditions, dissolution rates increased by factors of 3-5 only during illumination. With DFOB dissolved Fe(III) reached 35 µM after 10 h of illumination, while with EDTA it peaked at 15 µM and then decreased to below 2 µM. The observations are explained and discussed based on a kinetic model. The results suggest that in anoxic bottom water of ponds and lakes, or in microenvironments of algal blooms, short illuminations can dramatically increase the bioavailability of iron by Fe(II)-catalyzed ligand-controlled dissolution. In oxic environments, photostable ligands such as DFOB can maintain Fe(III) in solution during extended illumination.

Importance of oxidation products in coumarin-mediated Fe(hydr)oxide mineral dissolution.

Due to the low iron solubility in alkaline soils, plants have evolved different iron acquisition strategies, which are either based on ferric iron reduction (strategy I) or complexation by phytosiderophores (strategy II). Recently, a prominent role of coumarins for iron acquisition has been discovered, but details of the respective mechanism remain unclear. Since coumarins may act as iron-binding ligands but also as reductants, various reaction sequences are possible, resulting in different iron species and oxidized coumarins. In this context, it is often overlooked that oxidized coumarins are not just byproducts of iron(III) reduction, but may be actively involved in further steps of iron mobilization. In order to verify this active role of oxidized coumarins in Fe(hydr)oxide dissolution, we complemented iron dissolution data with data of single coumarins (esculetin, scopoletin, fraxetin) and their oxidation products, as a function of time, pH, and mineral (goethite, lepidocrocite). Our results demonstrate that there are four different routes for coumarin oxidation, leading to quinones, dimers, hydroxylated coumarins, demethylated coumarins, and combinations of these. The time-dependent species pattern differs with respect to mineral, pH, and coumarin molecule. Oxidized coumarins are often more reactive than the original coumarins, explaining unexpected iron mobilization by scopoletin, which is demethylated to esculetin. Also oxidative hydroxylation and dimerization increase the number of phenolic groups and yield new chelating properties. Several iron-species are identified for the three coumarins. Since oxidation reactions are initiated directly at mineral surfaces, they are often very effective-but this does not always result in more iron mobilization.

Genome wide transcriptomic analysis of the soil ammonia oxidizing archaeon Nitrososphaera viennensis upon exposure to copper limitation.

Ammonia-oxidizing archaea (AOA) are widespread in nature and are involved in nitrification, an essential process in the global nitrogen cycle. The enzymes for ammonia oxidation and electron transport rely heavily on copper (Cu), which can be limited in nature. In this study the model soil archaeon Nitrososphaera viennensis was investigated via transcriptomic analysis to gain insight regarding possible Cu uptake mechanisms and compensation strategies when Cu becomes limiting. Upon Cu limitation, N. viennensis exhibited impaired nitrite production and thus growth, which was paralleled by downregulation of ammonia oxidation, electron transport, carbon fixation, nucleotide, and lipid biosynthesis pathway genes. Under Cu-limitation, 1547 out of 3180 detected genes were differentially expressed, with 784 genes upregulated and 763 downregulated. The most highly upregulated genes encoded proteins with a possible role in Cu binding and uptake, such as the Cu chelator and transporter CopC/D, disulfide bond oxidoreductase D (dsbD), and multicopper oxidases. While this response differs from the marine strain Nitrosopumilus maritimus, conserved sequence motifs in some of the Cu-responsive genes suggest conserved transcriptional regulation in terrestrial AOA. This study provides possible gene regulation and energy conservation mechanisms linked to Cu bioavailability and presents the first model for Cu uptake by a soil AOA.

Three-dimensional ultrastructure of Plasmodium falciparum throughout cytokinesis.

New techniques for obtaining electron microscopy data through the cell volume are being increasingly utilized to answer cell biologic questions. Here, we present a three-dimensional atlas of Plasmodium falciparum ultrastructure throughout parasite cell division. Multiple wild type schizonts at different stages of segmentation, or budding, were imaged and rendered, and the 3D structure of their organelles and daughter cells are shown. Our high-resolution volume electron microscopy both confirms previously described features in 3D and adds new layers to our understanding of Plasmodium nuclear division. Interestingly, we demonstrate asynchrony of the final nuclear division, a process that had previously been reported as synchronous. Use of volume electron microscopy techniques for biological imaging is gaining prominence, and there is much we can learn from applying them to answer questions about Plasmodium cell biology. We provide this resource to encourage readers to consider adding these techniques to their cell biology toolbox.

Copper limiting threshold in the terrestrial ammonia oxidizing archaeon Nitrososphaera viennensis.

Ammonia oxidizing archaea (AOA) inhabiting soils have a central role in the global nitrogen cycle. Copper (Cu) is central to many enzymes in AOA including ammonia monooxygenase (AMO), the enzyme involved in the first step of ammonia oxidation. This study explored the physiological response of the AOA soil isolate, Nitrososphaera viennensis (EN76T) to Cu-limiting conditions in order to approach its limiting threshold under laboratory conditions. The chelator TETA (1,4,8,11-tetraazacyclotetradecane N, N', N″, N‴-tetraacetic acid hydrochloride hydrate) with selective affinity for Cu2+ was used to lower bioavailable Cu2+ in culture experiments as predicted by thermodynamic speciation calculations. Results show that N. viennensis is Cu-limited at concentrations ≤10-15 mol L-1 free Cu2+ compared to standard conditions (10-12 mol L-1). This Cu2+ limiting threshold is similar to pure cultures of denitrifying bacteria and other AOA and AOB inhabiting soils, freshwaters and sewage (<10-16 mol L-1), and lower than pure cultures of the marine AOA Nitrosopumilus maritimus (<10-12.7 mol L-1), which also possesses a high amount of Cu-dependent enzymes.

Identifying the reactive sites of hydrogen peroxide decomposition and hydroxyl radical formation on chrysotile asbestos surfaces.

BACKGROUND: Fibrous chrysotile has been the most commonly applied asbestos mineral in a range of technical applications. However, it is toxic and carcinogenic upon inhalation. The chemical reactivity of chrysotile fiber surfaces contributes to its adverse health effects by catalyzing the formation of highly reactive hydroxyl radicals (HO•) from H2O2. In this Haber-Weiss cycle, Fe on the fiber surface acts as a catalyst: Fe3+ decomposes H2O2 to reductants that reduce surface Fe3+ to Fe2+, which is back-oxidized by H2O2 (Fenton-oxidation) to yield HO•. Chrysotile contains three structural Fe species: ferrous and ferric octahedral Fe and ferric tetrahedral Fe (Fe3+tet). Also, external Fe may adsorb or precipitate onto fiber surfaces. The goal of this study was to identify the Fe species on chrysotile surfaces that catalyze H2O2 decomposition and HO• generation. RESULTS: We demonstrate that at the physiological pH 7.4 Fe3+tet on chrysotile surfaces substantially contributes to H2O2 decomposition and is the key structural Fe species catalyzing HO• generation. After depleting Fe from fiber surfaces, a remnant fiber-related H2O2 decomposition mode was identified, which may involve magnetite impurities, remnant Fe or substituted redox-active transition metals other than Fe. Fe (hydr)oxide precipitates on chrysotile surfaces also contributed to H2O2 decomposition, but were per mole Fe substantially less efficient than surface Fe3+tet. Fe added to chrysotile fibers increased HO• generation only when it became incorporated and tetrahedrally coordinated into vacancy sites in the Si layer. CONCLUSIONS: Our results suggest that at the physiological pH 7.4, oxidative stress caused by chrysotile fibers largely results from radicals produced in the Haber-Weiss cycle that is catalyzed by Fe3+tet. The catalytic role of Fe3+tet in radical generation may also apply to other pathogenic silicates in which Fe3+tet is substituted, e.g. quartz, amphiboles and zeolites. However, even if these pathogenic minerals do not contain Fe, our results suggest that the mere presence of vacancy sites may pose a risk, as incorporation of external Fe into a tetrahedral coordination environment can lead to HO• generation.

Linking Isotope Exchange with Fe(II)-Catalyzed Dissolution of Iron(hydr)oxides in the Presence of the Bacterial Siderophore Desferrioxamine-B.

Dissolution of Fe(III) phases is a key process in making iron available to biota and in the mobilization of associated trace elements. Recently, we have demonstrated that submicromolar concentrations of Fe(II) significantly accelerate rates of ligand-controlled dissolution of Fe(III) (hydr)oxides at circumneutral pH. Here, we extend this work by studying isotope exchange and dissolution with lepidocrocite (Lp) and goethite (Gt) in the presence of 20 or 50 µM desferrioxamine-B (DFOB). Experiments with Lp at pH 7.0 were conducted in carbonate-buffered suspensions to mimic environmental conditions. We applied a simple empirical model to determine dissolution rates and a more complex kinetic model that accounts for the observed isotope exchange and catalytic effect of Fe(II). The fate of added tracer 57Fe(II) was strongly dependent on the order of addition of 57Fe(II) and ligand. When DFOB was added first, tracer 57Fe remained in solution. When 57Fe(II) was added first, isotope exchange between surface and solution could be observed at pH 6.0 but not at pH 7.0 and 8.5 where 57Fe(II) was almost completely adsorbed. During dissolution of Lp with DFOB, ratios of released 56Fe and 57Fe were largely independent of DFOB concentrations. In the absence of DFOB, addition of phenanthroline 30 min after tracer 57Fe desorbed predominantly 56Fe(II), indicating that electron transfer from adsorbed 57Fe to 56Fe of the Lp surface occurs on a time scale of minutes to hours. In contrast, comparable experiments with Gt desorbed predominantly 57Fe(II), suggesting a longer time scale for electron transfer on the Gt surface. Our results show that addition of 1-5 µM Fe(II) leads to dynamic charge transfer between dissolved and adsorbed species and to isotope exchange at the surface, with the dissolution of Lp by ligands accelerated by up to 60-fold.

An essential contractile ring protein controls cell division in Plasmodium falciparum.

During the blood stage of human malaria, Plasmodium falciparum parasites divide by schizogony-a process wherein components for several daughter cells are produced within a common cytoplasm and then segmentation, a synchronized cytokinesis, produces individual invasive daughters. The basal complex is hypothesized to be required for segmentation, acting as a contractile ring to establish daughter cell boundaries. Here we identify an essential component of the basal complex which we name PfCINCH. Using three-dimensional reconstructions of parasites at electron microscopy resolution, we show that while parasite organelles form and divide normally, PfCINCH-deficient parasites develop inviable conjoined daughters that contain components for multiple cells. Through biochemical evaluation of the PfCINCH-containing complex, we discover multiple previously undescribed basal complex proteins. Therefore, this work provides genetic evidence that the basal complex is required for precise segmentation and lays the groundwork for a mechanistic understanding of how the parasite contractile ring drives cell division.