Sulfate-reducing bacteria and silica solubility: a possible mechanism for evaporite diagenesis and silica precipitation in banded iron formations

Abstract

Selective replacement of sulfate-evaporite minerals by silica and the precipitation of silica in association with sulfide mineral phases in banded iron formations may be mediated by the metabolic activities of sulfate-reducing bacteria. Hydrogen sulfide is known to be a product of this metabolism and is often called upon as a source of sulfur for metallic sulfides in sedimentary rocks. We report here on the influence that chemical changes induced by bacterial sulfate reduction have on silica solubility.Controlled in vitro growth experiments with Desulfovibrio desulfuricans and silica show (1) this organism can grow in silica concentrations as great as 400 ppm with no inhibition and (2) growth in the presence of silica yields a decrease in dissolved silica.Growth experiments with 80 ppm silica produced a lowering in dissolved silica from 80 ppm to 60 ppm, a 25% decrease, in just 30 h. Control experiments in the absence of cells resulted in no effective decrease in dissolved silica. The ability of sulfate-reducing bacteria to remove silica from solution may be related to local changes in pH and to hydrogen bonding of amorphous silica followed by polymerization to higher weight molecules.