Authigenic mineralization and detrital clay binding by freshwater biofilms: The Brahmani river, India

Epilithic biofilms, growing on submerged boulders, were collected upstream and downstream of sites of industrial discharge into the Brahmani River, Orissa State, India. Transmission electron microscopy (TEM) showed that the outer cell walls of attached bacteria in all samples were often encrusted with fine‐grained (<1 μm) inorganic precipitates. The density of mineralization ranged from a few epicellular grains to complete encrustation by clayey materials. Energy‐dispersive x‐ray spectroscopy (EDS) and selected‐area electron diffraction (SAED) indicated that the most abundant inorganic phase was a complex, poorly ordered, (Fe, Al)‐silicate of variable composition, containing minor amounts of potassium. No trace metals were detected in the authigenic precipitates. Bacterial cells were also found to entrap or adsorb detrital minerals such as kaolin, mica, quartz, iron oxide, and gibbsite onto their outer surfaces. Because epilithic microbial biofilms have a very large and highly reactive surface area, binding of major solutes and/or suspended detrital sediment will influence the chemical composition of the substrate‐water interface and, ultimately contribute to the makeup of the river bottom sediment.