FORMATION OF FINE-GRAINED METAL AND SILICATE PRECIPITATES ON A BACTERIAL SURFACE (BACILLUS-SUBTILIS)

The ability of the Gram-positive bacterium B. subtilis to bind and nuc leate precipitates from silicate anions has been studied over 24 weeks in the presence of Fe and Al 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 we re more diverse in composition and morphology, less crystalline, small er and (sometimes) more abundant than those that were abiotically form ed. Fe pretreatment of the bacterial cells enhanced the binding of sil icate at pH 8.0. Walls which were not pretreated with Fe, bound silica te 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 gr eatly favoured, giving greater retention of either Si or metals than w as seen in abiotic controls. Experiments with only heavy metals showed a high affinity of the bacterial walls for the metals, even at low te mperatures (4-degrees-C). It is postulated that a cationic briding mec hanism is involved in the binding of silicate anions by bacterial cell walls.