MINERAL PRECIPITATION BY EPILITHIC BIOFILMS IN THE SPEED RIVER, ONTARIO, CANADA

Epilithic microbial communities, ubiquitously found in biofilms on sub merged granite, limestone, and sandstone, as well as on the concrete s upport pillars of bridges, were examined in the Speed River, Ontario, Canada. Transmission electron microscopy showed that attached bacteria (on all substrata) were highly mineralized, ranging from Fe-rich caps ular material to fine-grained (<1 mu m) authigenic (primary) mineral p recipitates. The authigenic grains exhibited a wide range of morpholog ies, from amorphous gel-like phases to crystalline structures. Energy- dispersive X-ray spectroscopy indicated that the most abundant mineral associated with epilithic bacteria was a complex (Fe, Al) silicate of variable composition. The gel-like phases were similar in composition to a chamositic clay, whereas the crystalline structures were more si liceous and had compositions between those of glauconite and kaolinite . The consistent formation of (Fe, Al) silicates by all bacterial popu lations, regardless of substratum lithology, implies that biomineraliz ation was a surface process associated with the anionic nature of the cell wall. The adsorption of dissolved constituents from the aqueous e nvironment contributed significantly to the mineral formation process. In this regard, it appears that epilithic microbial biofilms dominate the reactivity of the rock-water interface and may determine the type of minerals formed, which will ultimately become part of the riverbed sediment. Because rivers typically contain high concentrations of dis solved iron, silicon, and aluminum, these findings provide a unique in sight into biogeochemical activities that are potentially widespread i n natural waters.