A quantitative comparison of the Cd binding mechanism to Gram‐positive (Bacillus subtilis) and Gram‐negative bacteria (Shewanella oneidensis) is presented. At pH 6.0, EXAFS data for the Gram‐positive bacteria were modeled using carboxyl and phosphoryl sites only. However, additional sulfide sites were required to model the spectrum from the Gram‐negative bacteria under similar experimental conditions. Cd binding to a bacterial consortium at the same pH value, sampled from natural river water, was modeled using the models developed for the individual Gram‐positive and Gram‐negative bacterial strains.
Despite being thermodynamically less stable, small ferrous colloids (60 nm to 3 μm in diameter) remain an important component of the biogeochemical cycle at the Earth's surface, yet their composition and structure and the reasons for their persistence are still poorly understood. Here we use X-ray-based Fe L-edge and carbon K-edge spectromicroscopy to address the speciation and organic–mineral associations of ferrous, ferric, and Fe-poor particles collected from sampling sites in both marine and freshwater environments. We show that Fe(II)-rich phases are prevalent throughout different aquatic regimes yet exhibit a high degree of chemical heterogeneity. Furthermore, we show that Fe-rich particles show strong associations with organic carbon. The observed association of Fe(II) particles with carboxamide functional groups suggests a possible microbial role in the preservation of Fe(II). These finding have significant implications for the behavior of Fe(II) colloids in oxygenated waters, and their role in different aquatic biogeochemical processes.
This study investigates the complexation environments of aqueous Pb and Cd in the presence of the trihydroxamate microbial siderophore, desferrioxamine-B (DFO-B) as a function of pH. Complexation of aqueous Pb and Cd with DFO-B was predicted using equilibrium speciation calculation. Synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy at Pb L(III) edge and Cd K edge was used to characterize Pb and Cd−DFO-B complexes at pH values predicted to best represent each of the metal−siderophore complexes. Pb was not found to be complexed measurably by DFO-B at pH 3.0, but was complexed by all three hydroxamate groups to form a totally "caged'" hexadentate structure at pH 7.5–9.0. At the intermediate pH value (pH 4.8), a mixture of Pb−DFOB complexes involving binding of the metal through one and two hydroxamate groups was observed. Cd, on the other hand, remained as hydrated Cd2+ at pH 5.0, occurred as a mixture of Cd−DFOB and inorganic species at pH 8.0, and was bound by three hydroxamate groups from DFO-B at pH 9.0. Overall, the solution species observed with EXAFS were consistent with those predicted thermodynamically. However, Pb speciation at higher pH values differed from that predicted and suggests that published constants underestimate the binding constant for complexation of Pb with all three hydroxamate groups of the DFO-B ligand. This molecular-level understanding of metal−siderophore solution coordination provides physical evidence for complexes of Pb and Cd with DFO-B, and is an important first step toward understanding processes at the microbial− and/or mineral−water interface in the presence of siderophores.
Bacteria are ubiquitous in a wide-range of low temperature aqueous systems, and can strongly affect the distribution and transport of metals and radionuclides in the environment. However, the role of metal adsorption onto bacteria, via the reactive cell wall functional groups, has been largely overlooked. Previous macroscale metal sorption, and XAS studies have shown that carboxyl and phosphoryl functional groups to be the important metal binding groups on bacterial cell walls and the sulfhydryl groups were not considered. The goal of our investigation was to evaluate the density of the sulfhydryl sites on different bacterial cell membranes that are common to soil systems, the binding affinities of these reactive groups towards Hg, and how this binding modifies the speciation of Hg in the natural waters.
Abstract Aberration‐corrected (AC) STEM, AC TEM and in situ X‐ray absorption fine structure spectroscopy (XAFS) were used to characterize the Pt clusters present on a 0.35 wt % Pt on γ‐alumina support after reduction in hydrogen at 700 °C. STEM high‐angle annular dark field imaging shows that cluster formation takes place at temperatures up to approximately 350 °C, and this is followed by gradual growth in cluster size for heat treatments in hydrogen up to 700 °C. The STEM data show that after 700 °C reduction the Pt clusters are present in a narrow size distribution centered at 0.88 nm, and using a method involving a redistribution of the Pt atoms using a high electron dosage in the STEM, it is shown that the clusters are present in two‐dimensional morphology. This conclusion is verified using intensity line scans. The in situ extended X‐ray absorption fine structure data are in good agreement with these observations. High‐resolution AC–TEM, which uses a broad coherent electron beam, and can thus offer advantages relative to STEM for structure determination of fine clusters, supported by image simulations of through‐focus series, were used to analyze the structures of Pt particles. The structures determined by using AC–TEM are consistent with STEM and EXAFS data in having a flat two‐dimensional morphology. Comparison of AC–STEM and AC TEM data for the same 700 °C reduced sample suggests that parallel‐beam TEM mode of imaging may be advantageous because of the less pronounced beam‐induced structural rearrangements that occur when imaging with a fine STEM probe.
Bacteria are common and play an important role in controlling the speciation, biological toxicity and cycling of metals and contaminants in the environment.The species of microbes and their abundance vary significantly from one natural environment to the other and this modifies the role that the microorganisms may have on the metal chemistry in the environment.However, in all bacterial species, the cell surface chemistry is one key variable that influences the exposure of organism to metal, metal uptake and electronic state transformation by cells, which in turn influences the metal speciation.These bacterial cell-metal interactions have been studied using bulk wet chemistry or spectroscopy methods (e.g.infrared and UV-vis spectroscopy), or using ex-situ high resolution electron microscopy (e.g.SEM, TEM) methods.However, recent developments in synchrotron X-ray spectroscopy and imaging techniques and the development of 3rd generation synchrotron sources enabled the direct examination of reactions on cell membranes at spatial resolution closer to 10 nm.In this presentation, we will go over the application of X-ray imaging and spectromicroscopy methods in studying the cell membrane functional group composition (e.g.proteins, carboxyls) and their distribution in different bacterial cells, and how this information complements the infrared and other spectral information.In addition, we will present how X-ray scattering (XANES and EXAFS spectroscopy methods) and wet chemistry and fluorescence spectroscopy can be used to explore the bcaterial cell interactions with aqueous contaminants such as Hg and Zn.These studies showed that metal interactions with cell membranes vary significantly with the metal concentration and bacterial species.A summary of these studies, and how the new developments of synchrotron methods can be applied to explore the bacterial cell surface interactions will be presented.
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