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The cell surface polymers of microorganisms readily bind a variety of metal ions, which enables the organisms to immobilize potentially toxic metal ions before they encounter the plasma membrane. Under appropriate chemical conditions, bound metal ions can form a variety of minerals that may be of major geological importance. Many studies have shown the occurrence of metal binding and biomineralization in nature, but detailed knowledge of the underlying mechanisms is lacking. The microbial influence of this binding may be indirect, such as physicochemical influences on the solution chemistry, Eh, and pH; or direct, when it is determined by the type of organisms present, their energy metabolism, and the structural and chemical characteristics of the cell surface and extracellular polymers. Metal binding by bacterial cell surfaces has several implications in nuclear waste disposal including adsorption of soluble radionuclides. A detailed knowledge of the chemical mechanisms of metal interactions with the microbial cell surface will enhance our understanding of the geochemical environment within a nuclear waste disposal vault.Resume : Les polymkres de surface des microorganismes lient facilement une variktk d'ions mktalliques permettant ainsi B ces organismes d'immobiliser des ions mktalliques potentiellement toxiques avant qu'ils puissent atteindre la membrane plasmatique. Dans des conditions chimiques approprikes, ces ions mktalliques lies peuvent former une variktk de minCraux pouvant avoir une importance gkologique majeure. Plusieurs Ctudes ont dCjB dkmontrk que la liaison B des mCtaux et la biominkralisation se produisent dans la nature, mais on n'a pas une connaissance dktaillke des mkcanismes en cause. L'influence microbienne de cette liaison peut Ctre indirecte, comme des variations physicochimiques de la solution, Eh et pH, ou elle peut Ctre directe si elle est dkterminCe par le type de microorganismes prCsents, leur knergie mktabolique et les caractkristiques structurales et chimiques de la surface cellulaire et des polymkres extracellulaires. La liaison des mCtaux par la surface cellulaire bactkrienne a plusieurs implications pour I'Climination des dkchets nuclkaires, y compris l'adsorption des radionuclCides solubles. Une connaissance approfondie des mCcanismes chlmiques des interactions des mCtaux avec la surface des microorganismes va favoriser notre comprkhension de l'environnement geochimique i l'intkrieur d'une voute souterraine d'klimination des dkchets nuclCaires.

The concept of disposal of nuclear fuel waste in crystalline rock requires the effects of microbial action to be investigated. The Underground Research Laboratory excavated in a pluton of the Canadian Shield provides a unique opportunity to study these effects. Three biofilms kept moist by seepage through fractures in granitic rock faces of the Underground Research Laboratory have been examined. The biofilms contained a variety of gram-negative and gram-positive morphotypes held together by an organic extracellular matrix. Nutrient levels in the groundwater were low, but energy-dispersive X-ray spectroscopy has shown biogeochemical immobilization of several elements in the biofilms; some of these elements were concentrated from extremely dilute environmental concentrations, and all elements were chemically complexed together to form amorphous or crystalline fine-grained minerals. These were seen by transmission electron microscopy to be both associated with the surfaces of the bacteria and scattered throughout the extracellular matrix, suggesting their de novo development through bacterial surface-mediated nucleation. The biofilm consortia are thought to concentrate elements both by passive sorption and by energy metabolism. By Mossbauer spectroscopy and X-ray diffraction, one of the biofilms showed that iron was both oxidized and precipitated as ferrihydrite or hematite aerobically and reduced and precipitated as siderite anaerobically. We believe that some Archean banded-iron formations could have been formed in a manner similar to this, as it would explain the deposition of hematite and siderite in close proximity. This biogeochemical development of minerals may also affect the transport of material in waste disposal sites.

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