MINERALS ASSOCIATED WITH BIOFILMS OCCURRING ON EXPOSED ROCK IN A GRANITIC UNDERGROUND RESEARCH LABORATORY

The concept of disposal of nuclear fuel waste in crystalline rock requ ires the effects of microbial action to be investigated. The Undergrou nd Research Laboratory excavated in a pluton of the Canadian Shield pr ovides a unique opportunity to study these effects, Three biofilms kep t moist by seepage through fractures in granitic rock faces of the Und erground Research Laboratory have been examined. The biofilms containe d a variety of gram-negative and gram-positive morphotypes held togeth er by an organic extracellular matrix. Nutrient levels in the groundwa ter were low, but energy dispersive X-ray spectroscopy has shown bioge ochemical immobilization of several elements in the biofilms; some of these elements were concentrated from extremely dilute environmental c oncentrations, and all elements were chemically complexed together to form amorphous or crystalline fine-grained minerals. These were seen b y transmission electron microscopy to be both associated with the surf aces of the bacteria and scattered throughout the extracellular matrix , suggesting their de novo development through bacterial surface-media ted nucleation. The biofilm consortia are thought to concentrate eleme nts both by passive sorption and by energy metabolism. By Mossbauer sp ectroscopy and X-ray diffraction, one of the biofilms showed that iron was both oxidized and precipitated as ferrihydrite or hematite aerobi cally and reduced and precipitated as siderite anaerobically. We belie ve that some Archean banded-iron formations could have been formed in a manner similar to this, as it would explain the deposition of hemati te and siderite in close proximity. This biogeochemical development of minerals may also affect the transport of material in waste disposal sites.