Formation of fine-grained metal and silicate precipitates on a bacterial surface ( Bacillus subtilis)

Abstract

The ability of the Gram-positive bacterium B. subtilis to bind and nucleate precipitates from silicate anions has been studied over 24 weeks in the presence of Fe and A1 at concentrations close to those levels in soils, and at slightly acid (5.5) and basic (8.0) pH. In all cases formation of silicate crystallites (quasi-crystalline precipitates) on the bacterial surfaces was observed. Bacterially-mediated minerals were more diverse in composition and morphology, less crystalline, smaller and (sometimes) more abundant than those that were abiotically formed. Fe pretreatment of the bacterial cells enhanced the binding of silicate at pH 8.0. Walls which were not pretreated with Fe, bound silicate more favourably at acid values. When heavy metals (Pb, Cd, Zn, Cr, Ni, Cu) were added to the mixture at pH 4.5, silicate retention was greatly favoured, giving greater retention of either Si or metals than was seen in abiotic controls. Experiments with only heavy metals showed a high affinity of the bacterial walls for the metals, even at low temperatures (4°C). It is postulated that a cationic bridging mechanism is involved in the binding of silicate anions by bacterial cell walls.