PRECIPITATION OF CARBONATE MINERALS BY MICROORGANISMS - IMPLICATIONS FOR SILICATE WEATHERING AND THE GLOBAL CARBON-DIOXIDE BUDGET

Direct light and electron microscopic studies show that cyanobacterial cells serve as nucleation sites for carbonate mineral precipitation in a variety of fresh to saline-alkaline lakes on the Cariboo Plateau in central British Columbia, Canada, and in mineralized crusts on weathered basalt in Iceland. The carbonate minerals found in association with the cyanobacteria were extremely fine-grained, and invariably occurred on the external surfaces of the cells. Carbonate mineralogy was variable, ranging from calcite to magnesite, depending on differences in lake and groundwater chemistry (i.e., saturation state of the water with respect to individual carbonate minerals). In microcosm experiments, phototrophic cyanobacterial growth increased alkalinity and the degree of oversaturation with respect to calcite. Calculated values for the saturation state of calcite and magnesite in Cariboo Plateau natural waters exhibited two distinct trends, with (1) high magnesite saturation values in areas where the weathering of magnesium olivine-rich basalt bedrock determines water chemistry and (2) high calcite saturation values where bedrock is a mix of basic lava flows, limestone argillite, and chert. Similar calculations for Iceland show that cold surface waters are generally oversaturated with respect to calcite, as expected for the weathering of calcium plagioclase-rich lava. These observations of microbial carbonate precipitation in the Caribou Plateau region of British Columbia, Canada, and on the Budarhaun lava plain in Iceland suggest that weathering of silicate minerals in bedrock is biogeochemically coupled to the deposition. of carbonate minerals by microorganisms. This process may provide a sink for atmospheric carbon dioxide in terrestrial environments.