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Chapter 98 : Techniques for Studying Microbial Transformations of Metals and Radionuclides
This chapter provides an overview of the range of techniques that are available to help characterize the critical microbiological and geological factors underpinning metal-microbe interactions. These include techniques to study microbial cells, the mineral substrates that they live on, and the surrounding geochemical environment that together control the rate and extent of metal biotransformations. The chapter focuses on examining current techniques used in understanding the transformations of long-lived, redox-active radionuclides, such as uranium (U), technetium (Tc), neptunium (Np), and plutonium (Pu), in pure culture and laboratory simulation experiments and highlights the safe handling and measurement of these radionuclides. It highlights how the real need to understand the transformation behavior of radionuclides is balanced against the safe handling requirements for working with radiation. In summary, specialist radiochemistry laboratories are needed for handling all of the radionuclides mentioned above, with the possible exception of uranium. The long-lived radionuclides U, Tc, Np, and Pu are redox active, and all of these radionuclides have the potential to form less soluble species as reducing conditions develop. The chapter discusses two forms of microscopy based on conventional light and electron beams to study the fate of metals in geomicrobiological experiments. The focus here is on visualizing the interactions between “microbiological structures” and metals, while in a later section more general methods to assess mineralogical substrates, using a range of techniques, including microscopy are discussed.
Environmental scanning electron micrograph showing cells of Geobacter sulfurreducens growing attached to an iron oxide substrate (M. J. Wilkins, P. Wincott, J. R. Lloyd, and D. J. Vaughan, unpublished data).
Transmission electron micrograph of a thin section of Pseudomonas aeruginosa incubated with 1 mM LaCl3 for 15 min at room temperature. The image shows that the bacterial surfaces have reacted strongly with the La3+ metal ions.
Unstained whole mount of Shewanella putrefaciens CN32 showing extracellular iron minerals, secondary Fe minerals associated with the cell surface, and internal nanoscale Fe deposits.
(A) Cell envelope of frozen hydrated cryo-sectioned Escherichia coli K-12 viewed by TEM. The lipid asymmetry of the outer membrane (OM) is clearly visible, as are both bilayer faces of the periplasm (PM). (B) A micrograph of a thin section of freeze-substituted E. coli is included for comparison, showing metal decoration of reactive sites.
Diagram showing energy-dispersive X-ray powder diffraction data acquired using a synchrotron radiation source. A fine-particle precipitate of CuS formed by reacting copper and sulfide in aqueous solutions is seen to change over time (0 to 500 min) as the initially nearly amorphous sulfide precipitate becomes more crystalline. Diffraction peaks related to different parts of (or planes within) the structure are seen to emerge with time (L. N. Moyes, J. M. Charnock, R. A. D. Pattrick, and D. J. Vaughan, unpublished data).
Mössbauer spectrum (iron) acquired at room temperature for a fine-particle tidal flat sediment sample from the Wash area, England. Fitting of this spectrum confirmed that ferrous iron occurs in detrital clay minerals (doublet with large separation) and in pyrite (FeS2) (doublet with small separation) and that both ferrous and ferric iron occur in the magnetic sulfide greigite (Fe3S4) (small features at very high and very low velocities). There is also a substantial amount of ferric iron from clays and related phases (seen as another doublet with small separation). The sulfides have formed by reaction of bacterially generated sulfide (reduction of seawater sulfate) with iron released from detrital minerals. From the areas under the peaks, the relative percentages of the different types of iron can be determined. (Modified from reference 111 .)
STM data showing a 200-Å-by-200-Å image of the surface of magnetite at atomic resolution which has been exposed to pyridine ( 111 ) (top) and a scan along the line indicated in the top figure which shows that the large bright features in the image are individual pyridine molecules (bottom). (Modified from reference 24 .)
AFM image showing G. sulfurreducens on a stepped surface of goethite (Wilkins et al., unpublished data).
Schematic diagram showing the principles of XPS, AES, and X-ray emission spectroscopy (XES), which is also the basis of EPMA. (Redrawn from reference 118 .)
Concentrations of Tc, U, Np, and Pu used in geomicrobiological experiments and their corresponding ALIs a
Half-lives, decay modes, and predominant oxidation states of various isotopes of Tc, U, Np, and Pu