Bacterial clay authigenesis: a common biogeochemical process

Transmission electron microscopic (TEM) analyses of freshwater biofilms and bacterial cells, grown in experimental culture, have shown that these microorganisms are commonly associated with fine-grained (Fe, Al)-silicates of variable composition. The inorganic phases develop in a predictable manner, beginning with the adsorption of cationic iron to anionic cellular surfaces, supersaturation of the proximal fluid with Fe3+, nucleation and precipitation of a precursor ferric hydroxide phase on the cell surface, followed by reaction with dissolved silica and aluminum and eventually the growth of an amorphous clay-like phase. Alternatively, colloidal species of (Fe, Al)-silicate composition may react directly with either the anionic cellular polymers or adsorbed iron, depending on their net charge. Over time, these hydrous precursors may dehydrate and convert to more stable crystalline phases. Because microbial biofilms are expansive and highly reactive surfaces at the sediment–water interface, coupled with their ability to bind soluble components and form solid inorganic phases, they should influence the chemical composition of the overlying aqueous microenvironment, and ultimately contribute to the makeup of river bottom sediment.