The surfaces of epiphytic and episammic bacteria from the solute-rich waters of the Rio Solimões, Brazil, interacted with available cations in solution and provided major sites for iron deposition. Once bound to the bacteria, the iron served as nucleation sites for the formation and growth of fine-grained authigenic mineral phases. Because of progressive mineralization, the precipitates exhibited a wide range of morphologies, from amorphous "gel-like" to crystalline structures. Complete encrustation of some bacterial cells was observed. The most abundant mineral phase associated with the bacteria was a complex (Fe,Al) silicate with a variable composition. Energy-dispersive X-ray spectroscopy suggested that the gel-like structures were similar in composition to a chamositic clay, whereas the crystalline phases were increasingly kaolinitic. Bacteria collected from the solute-deficient waters of the Rio Negro remained conspicuously unmineralized, suggesting that metal sorption and biomineralization largely reflect the availability of dissolved metals in the water column. In a solute-rich river system, the fate of the metal-loaded bacteria has profound implications for the transfer of metals from the hydrosphere to the sediment. Through diagenesis, the bound metals may either be recycled to the overlying water column or become immobilized as stable mineral phases. If the latter occurs, microorganisms will have played an important role in metal deposition, low-temperature clay formation, and, invariably, mudstone diagenesis. This implies that in the geologic record, microorganisms may have been instrumental in the genesis of many sedimentary rocks of fluvial origin.