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 micr oorganisms 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 surface s, supersaturation of the proximal fluid with Fe3+, nucleation and precipit ation of a precursor ferric hydroxide phase on the cell surface, followed b y 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 po lymers or adsorbed iron, depending on their net charge. Over time, these hy drous precursors may dehydrate and convert to more stable crystalline phase s. Because microbial biofilms are expansive and highly reactive surfaces at the sediment-water interface, coupled with their ability to bind soluble c omponents and form solid inorganic phases, they should influence the chemic al composition of the overlying aqueous microenvironment, and ultimately co ntribute to the makeup of river bottom sediment. (C) 1999 Elsevier Science B.V. All rights reserved.