previously he was at Sandia National Laboratories. His research concerns the energetics and kinetics of the reactions between minerals and aqueous solutions, and in recent work, he has determined rate coefficients for dissociation of Al−O bonds in various dissolved aluminum complexes.

Dissimilatory iron-reducing bacteria (DIRB) couple the oxidation of organic matter or H 2 to the reduction of iron oxides. The factors controlling the rate and extent of these reduction reactions and the resulting solid phases are complex and poorly understood. Batch experiments were conducted with amorphous hydrous ferric oxide (HFO) and the DIRB Shewanella putrefaciens, strain CN32, in well-defined aqueous solutions to investigate the reduction of HFO and formation of biogenic Fe(II) minerals. Lactate-HFO solutions buffered with either bicarbonate or 1,4-piperazinediethanesulfonic acid (PIPES) containing various combinations of phosphate and anthraquinone-2,6-disulfonate (AQDS), were inoculated with S. putrefaciens CN32. AQDS, a humic acid analog that can be reduced to dihydroanthraquinone by CN32, was included because of its ability to function as an electron shuttle during microbial iron reduction and as an indicator of pe. Iron reduction was measured with time, and the resulting solids were analyzed by X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED). In HCO 3 Ϫ buffered medium with AQDS, HFO was rapidly and extensively reduced, and the resulting solids were dominated by ferrous carbonate (siderite). Ferrous phosphate (vivianite) was also present in HCO 3 Ϫ medium containing P, and fine-grained magnetite was present as a minor phase in HCO 3 Ϫ medium with or without P. In the PIPESbuffered medium, the rate and extent of reduction was strongly influenced by AQDS and P. With AQDS, HFO was rapidly converted to highly crystalline magnetite whereas in its absence, magnetite mineralization was slower and the final material less crystalline. In PIPES with both P and AQDS, a green rust type compound [Fe (6-x) II Fe x III (OH) 12 ] xϩ [(A 2Ϫ) x/2 ⅐ yH 2 O] xϪ was the dominant solid phase formed; in the absence of AQDS a poorly crystalline product was observed. The measured pe and nature of the solids identified were consistent with thermodynamic considerations. The composition of aqueous media in which microbial iron reduction occurred strongly impacted the rate and extent of iron reduction and the nature of the reduced solids. This, in turn, can provide a feedback control mechanism on microbial metabolism. Hence, in sediments where geochemical conditions promote magnetite formation, two-thirds of the Fe(III) will be sequestered in a form that may not be available for anaerobic bacterial respiration.

Quantitative aspects of microbial crystalline iron(III) oxide reduction were examined using a dissimilatory iron(III) oxide-reducing bacterium (Shewanella alga strain BrY). The initial rate and long-term extent of reduction of a range of synthetic iron(III) oxides were linearly correlated with oxide surface area. Oxide reduction rates reached an asymptote at cell concentrations in excess of ≈1 × 109/m2 of oxide surface. Experiments with microbially reduced goethite that had been washed with pH 5 sodium acetate to remove adsorbed Fe(II) suggested that formation of a Fe(II) surface phase (adsorbed or precipitated) limited the extent of iron(III) oxide reduction. These results demonstrated explicitly that the rate and extent of microbial iron(III) oxide reduction is controlled by the surface area and site concentration of the solid phase. Strain BrY grew in media with synthetic goethite as the sole electron acceptor. The quantity of cells produced per micromole of goethite reduced (2.5 × 106) was comparable to that determined previously for growth of BrY and other dissimilatory Fe(III)-reducing bacteria coupled to amorphous iron(III) oxide reduction. BrY reduced a substantial fraction (8−18%) of the crystalline iron(III) oxide content of a variety of soil and subsurface materials, and several cultures containing these materials were transferred repeatedly with continued active Fe(III) reduction. These findings indicate that Fe(III)-reducing bacteria may be able to survive and produce significant quantities of Fe(II) in anaerobic soil and subsurface environments where crystalline iron(III) oxides (e.g., goethite) are the dominant forms of Fe(III) available for microbial reduction. Results suggest that the potential for cell growth and Fe(II) generation will be determined by the iron(III) oxide surface site concentration in the soil or sediment matrix.

John E McCarthy (1) is group leader for biological chemistry at the Environmental Sciences Division of Oak Ridge National Laboratory, which he joined in 1980 and where he has been directing a research project on colloid-facilitated transport for the Department of Energy since 1985. His research interests are in the fate and transport of contaminants and in the biological aspects of exposure to contaminants. Mc-Carthy holds a B. S. from Fordham University and a Ph.D. from the University of Rhode Island. John M. Zachara (r) is a technical group leader in the Geochemistry Section of Battelle ' s Pacific Northwest Laboratory, which he joined in 1979. His research interests include surface and colloid chemical reactions that control the migration of organic and inorganic contaminants in groundwaters and soils. Zuchara obtained his4.S. at Bucknell University and his Ph. D. from Washington State University.

A number of species of Gram-negative bacteria can use insoluble minerals of Fe(III) and Mn(IV) as extracellular respiratory electron acceptors. In some species of Shewanella, deca-heme electron transfer proteins lie at the extracellular face of the outer membrane (OM), where they can interact with insoluble substrates. To reduce extracellular substrates, these redox proteins must be charged by the inner membrane/periplasmic electron transfer system. Here, we present a spectro-potentiometric characterization of a trans-OM icosa-heme complex, MtrCAB, and demonstrate its capacity to move electrons across a lipid bilayer after incorporation into proteoliposomes. We also show that a stable MtrAB subcomplex can assemble in the absence of MtrC; an MtrBC subcomplex is not assembled in the absence of MtrA; and MtrA is only associated to the membrane in cells when MtrB is present. We propose a model for the modular organization of the MtrCAB complex in which MtrC is an extracellular element that mediates electron transfer to extracellular substrates and MtrB is a trans-OM spanning ␤-barrel protein that serves as a sheath, within which MtrA and MtrC exchange electrons. We have identified the MtrAB module in a range of bacterial phyla, suggesting that it is widely used in electron exchange with the extracellular environment.cytochrome-c ͉ iron respiration ͉ protein film voltammetry ͉ electron paramagnetic resonance ͉ Shewanella

Respiration of metal (hydr)oxides by

facilities within the weapons complex. The inventory of chemicals and mixtures was used to identify generic chemical mixtures to be used by DOE'S Subsurface Science Program in basic research on the subsurface geochemical and microbiological behavior of mixed contaminants (DOE 1990a and b). The generic mixtures contain specific radionuclides, metals, organic ligands, organic solvents, fuel hydrocarbons, and polychlorinated biphenyls (PCBs) in various binary and ternary combinations. The mixtures are representative of in-ground contaminant associations at DOE facilities that are likely to exhibit complex geochemical behavior as a result of intercontaminant reactions and/or microbiologic activity stimulated by organic substances. Use of the generic mixtures will focus research on important mixed contaminants that are likely to be long-term problems at DOE sites and that will require cleanup or remediation.The report provides information on the frequency of associations among different chemicals and compound classes at DOE waste sites that require remediation. For example, radionuclides such as uranium, plutonium, strontium, and cobalt were found, in some cases, to be disposed of with organic substances (e.g., organic acids, complexing agents, and solvents) that could influence radionuclide geochemical behavior and subsurface transport. Knowledge of the types of chemicals that coexist in waste sites is important to remediation for various reasons:. The efficiency of many biotic and abiotic treatment processes for soil and ground-water contaminants is affected by the presence of co-contaminants.m Multiple contaminant species may be treated simultaneously and more effectively by specific aboveground or in-ground techniques if the nature of the contaminant association is understood in advance.rn Certain types of chemical mixtures may require special precautions or the development of new remediation strategies or techniques.rn In-ground remediation activities may selectively mobilize certain mixtures of chemical constituents to air or ground water, thereby increasing environmental risk; or some mixtures may be stabilized, thus reducing environmental impact.iiiThe report provides quantitative information on the frequency of occurrence of binary, ternary, and higher order contaminant mixtures in the 91 waste sites. This quantitative information may be used to refine or guide the development of new aboveground and in situ remediation strategies that can be used throughout the weapons complex.Scientists T waste sites at 18 U.S. Department of Energy (DOE) facilities. The review was conducted to identify (1) inorganic and organic contaminants found within soil and ground water at DOE waste sites, ( 2 ) their concentration ranges, and (3) their frequency of occurrence as single compounds and as binary, ternary, quaternary, and higher order contaminant mixtures. Fuel hydrocarbons, chlorinated hydrocarbons, radionuclides, metals, inorganic anions, and ketones were the contaminant classes most frequently measured in the ground at...

Dissimilatory metal reducing bacteria (DMRB) catalyze the reduction of Fe(III) to Fe(II) in anoxic soils, sediments, and groundwater. Two-line ferrihydrite is a bioavailable Fe(III) oxide form that is exploited by DMRB as a terminal electron acceptor. A wide variety of biomineralization products result from the interaction of DMRB with 2-line ferrihydrite. Here we describe the state of knowledge on the biotransformation of synthetic 2-line ferrihydrite by laboratory cultures of DMRB using select published data and new experimental results. A facultative DMRB is emphasized (Shewanella putrefaciens) upon which most of this work has been performed. Key factors controlling the identity of the secondary mineral suite are evaluated including medium composition, electron donor and acceptor concentrations, ferrihydrite aging/recrystallization status, sorbed ions, and co-associated crystalline Fe(III) oxides. It is shown that crystalline ferric (goethite, hematite, lepidocrocite), ferrous (siderite, vivianite), and mixed valence (magnetite, green rust) iron solids are formed in anoxic, circumneutral DMRB incubations. Some products are well rationalized based on thermodynamic considerations, but others appear to result from kinetic pathways driven by ions that inhibit interfacial electron transfer or the precipitation of select phases. The primary factor controlling the nature of the secondary mineral suite appears to be the Fe(II) supply rate and magnitude, and its surface reaction with the residual oxide and other sorbed ions. The common observation of end-product mineral mixtures that are not at global equilibrium indicates that microenvironments surrounding respiring DMRB cells or the reaction-path trajectory (over Eh-pH space) may in uence the identity of the nal biomineralization suite.