METABOLIC SPATIAL VARIABILITY IN ELECTRODE-RESPIRING GEOBACTER SULFURREDUCENS BIOFILMS

In this study, we quantified electron transfer rates, depth profiles of electron donor, and biofilm structure of Geobacter sulfurreducens biofilms using an electrochemical-nuclear magnetic resonance microimaging biofilm reactor. Our goal was to determine whether electron donor limitations existed in electron transfer processes of electrode-respiring G. sulfurreducens biofilms. Cells near the top of the biofilms consumed acetate and were metabolically active; however, acetate concentration decreased to below detection within the top 100 microns of the biofilms. Additionally, porosity in the biofilms fell below 10% near the electrode surface, exacerbating exclusion of acetate from the lower regions. The dense biofilm matrix in the acetate-depleted zone acted as an electrical conduit passing electrons generated at the top of the biofilm to the electrode. To verify the distribution of cell metabolic activity, we used uranium as a redox-active probe for localizing electron transfer activity and X-ray absorption spectroscopy to determine the uranium oxidation state. Cells near the top reduced U^VI more actively than the cells near the base. High-resolution transmission electron microscopy images showed intact, healthy cells near the top and plasmolyzed cells near the base. Contrary to models proposed in the literature, which hypothesize that cells nearest the electrode surface are the most metabolically active because of a lower electron transfer resistance, our results suggest that electrical resistance through the biofilm does not restrict long-range electron transfer. Cells far from the electrode can respire across metabolically inactive cells, taking advantage of their extracellular infrastructure produced during the initial biofilm formation.

Exafs Studies and Simulations of Local Anisotropy in CO78CR22 Films

Extended x-ray absorption fine structure measurements of a Co78Cr22 film were performed using normal and glancing incident radiation in order to investigate, respectively, the in-plane and out-of-plane local structure and chemistry. The Fourier transformed EXAFS data of the in-plane and out-of-plane structures around the Co and Cr atoms illustrates the presence of an anisotropy. Analysis of the local environments around Co for the two sample orientations indicates the presence of Co-enriched clusters, while similar studies of the Cr environments indicate preferential ordering parallel to the film plane. Quantitative analysis of the higher order Fourier transform peaks shows a greater amount of disorder perpendicular to the film plane beyond that expected for a textured hcp film. These results are consistent with earlier reports of a high density of stacking faults or twinning planes perpendicular to the growth axis, supporting the interpretation that a platelet-like texturing exists within the columnar microstructure.

Deposition of uranium precipitates in dolomitic gravel fill

Uranium-containing precipitates have been observed in a dolomitic gravel fill near the Department of Energy (DOE) S-3 Ponds former waste disposal site as a result of exposure to acidic (pH 3.4) groundwater contaminated with U (33 mg L(-1)), Al3+ (900 mg L(-1)), and NO3- (14 000 mg L(-1)). The U containing precipitates fluoresce a bright green under ultraviolet (UV) short-wave light which identify U-rich coatings on the gravel. Scanning electron microscopy (SEM) microprobe analysis show U concentration ranges from 1.6-19.8% (average of 7%) within the coatings with higher concentrations at the interface of the dolomite fragments. X-ray absorption near edge structure spectroscopy (XANES) indicate that the U is hexavalent and extended X-ray absorption fine structure spectroscopy (EXAFS) shows that the uranyl is coordinated by carbonate. The exact nature of the uranyl carbonates are difficult to determine, but some are best described by a split K(+)-like shell similar to grimselite [K4Na(UO2)(CO3)3 x H2O] and other regions are better described by a single Ca(2+)-like shell similar to liebigite [Ca2(UO2)(CO3)3 x 11(H2O)] or andersonite [Na2CaUO2(CO3)3 x 6H2O]. The U precipitates are found in the form of white to light yellow cracked-formations as coatings on the dolomite gravel and as detached individual precipitates, and are associated with amorphous basalumnite [Al4(SO4)(OH)10 x 4H2O].

Hard X-ray micro(spectro)scopy: a powerful tool for the geomicrobiologists

During the past few decades, the use of electron microscopy approaches - many developed by Terry Beveridge - to probe the physiology of microorganisms has become a mainstay in fields including microbiology, human health, and geomicrobiology. Recent developments of third-generation synchrotron X-ray sources and X-ray-based microscopy approaches for studying microbial systems have proved their utility as complements to the very powerful approaches regularly employed by electron microscopists. In addition, in recent geomicrobiological studies, researchers have begun to take advantage of the strengths of each technique by using the superior spatial resolution of the electron microscope (relative to the X-ray microscope) and the superior elemental sensitivity of the X-ray microscope (relative to the electron microscope), along with the ability of the X-ray microscope to spatially probe the chemical speciation of elements. The benefits of integrating these two nanoprobes for investigating the same microenvironments within a geomicrobial system are far superior to those of independent studies separately employing each probe.

Biomass byproducts for the remediation of wastewaters contaminated with toxic metals

Pollution of the environment with toxic metals is widespread and often involves large volumes of wastewater. Remediation strategies must be designed to support high throughput while keeping costs to a minimum. Biosorption is presented as an alternative to traditional physicochemical means for removing toxic metals from wastewater. We have investigated the metal binding qualities of two biomass byproducts that are commercially available in quantity and at low cost, namely "spillage", a dried yeast and plant mixture from the production of ethanol from corn, and ground corn cobs used in animal feeds. The biomass materials effectively removed toxic metals, such as Cu, Cs, Mo, Ni, Pb, and Zn, even in the presence of competing metals likely to be found in sulfide mine tailing ponds. The effectiveness of these biosorbents was demonstrated using samples from the Berkeley Pit in Montana. Investigations included column chromatography and slurry systems, and linear distribution coefficients are presented. X-ray spectroscopy was used to identify the binding sites for metals adsorbed to the spillage material. The results of our experiments demonstrate that the biosorption of metals from wastewaters using biomass byproducts is a viable and cost-effective technology that should be included in process evaluations.

X-ray absorption fine structure spectroscopy determination of the binding mechanism of tetrahedral anions to self assembled monolayers on mesoporous support

X-ray absorption fine structure (XAFS) spectroscopy is used to investigate the chemical interaction between the end member [Cu(NH2)6] of self-assembled monolayers on mesoporous supports (SAMMS) and the tetrahedral anion SO4. The local structure about Cu indicates monodentate bonding between the SO4 anion and the SAMMS.

XAS investigations of Fe(VI)

Recent attention has been given to a reexamination of results from the early Viking missions to Mars that suggested the presence of one or more strong oxidants in Martian soil. Since Fe is one of the main constituents of the Martian surface and Fe(VI) is known to be a highly reactive, strong oxidant, we have made XANES and EXAFS measurements of Fe(II), Fe(III), Fe(IV), and Fe(VI) in solid and solution forms. Results from these studies indicate a preedge XANES feature from Fe(VI) samples similar to that commonly seen from Cr(VI) samples. Results of first shell analysis indicate a linear relationship between the Fe-O bondlength and Fe valence state.

XAFS determination of the bacterial cell wall functional groups responsible for complexation of Cd and U as a function of pH

Bacteria, which are ubiquitous in near-surface geologic systems, can affect the distribution and fate of metals in these systems through adsorption reactions between the metals and bacterial cell walls. Recently, Fein et al. (1997) developed a chemical equilibrium approach to quantify metal adsorption onto cell walls, treating the sorption as a surface complexation phenomenon. However, such models are based on circumstantial bulk adsorption evidence only, and the nature and mechanism of metal binding to cell walls for each metal system have not been determined spectroscopically. The results of XAFS measurements at the Cd K-edge and U L3-edge on Bacillus subtilis exposed to these elements show that, at low pH, U binds to phosphoryl groups while Cd binds to carboxyl functional groups.

Formation of sphalerite (ZnS) deposits in natural biofilms of sulfate-reducing bacteria

Abundant, micrometer-scale, spherical aggregates of 2- to 5-nanometer-diameter sphalerite (ZnS) particles formed within natural biofilms dominated by relatively aerotolerant sulfate-reducing bacteria of the family Desulfobacteriaceae. The biofilm zinc concentration is about 10(6) times that of associated groundwater (0.09 to 1.1 parts per million zinc). Sphalerite also concentrates arsenic (0.01 weight %) and selenium (0.004 weight %). The almost monomineralic product results from buffering of sulfide concentrations at low values by sphalerite precipitation. These results show how microbes control metal concentrations in groundwater- and wetland-based remediation systems and suggest biological routes for formation of some low-temperature ZnS deposits.

X-ray imaging and microspectroscopy of plants and fungi

X-ray fluorescence microscopy and microspectroscopy with micrometre spatial resolution and unprecedented capabilities for the study of biological and environmental samples are reported. These new capabilities are a result of both the combination of high-brilliance synchrotron radiation and high-performance X-ray microfocusing optics and the intrinsic advantages of X-rays for elemental mapping and chemical-state imaging. In this paper, these capabilities are illustrated by experimental results on hard X-ray phase-contrast imaging, X-ray fluorescence (XRF) imaging and microspectroscopy of mycorrhizal plant roots and fungi in their natural hydrated state. The XRF microprobe is demonstrated by the simultaneous mapping of the elemental distributions of P, S, K, Ca, Mn, Fe, Ni, Cu and Zn with a spatial resolution of approximately 1 x 3 micron and with an elemental sensitivity of approximately 500 p.p.b. Microspectroscopy with the same spatial resolution is demonstrated by recording near-edge X-ray absorption (XANES) spectra of Mn at a concentration of approximately 3 p.p.m.