Biologically Induced Mineralization by Bacteria - Biomineralization - page 96

most active at the sites of initial nucleation on the outer surface of the cell. Complete
mineralization of the cell surface may eventually occur producing hollow minerals the size
and shape of a bacterial cell (Southam 2000) (Fig. 1). It is interesting that nonliving cells
may also form minerals in this way and, in one study, living cells of Bacillus subtilis bound
less metal ions than nonliving cells (Urrutia et al. 1992). In this case, the membrane-
induced proton motive force reduces the metal binding ability of the cell wall, most likely
through competition of protons with metal ions for anionic wall sites.
There are a large number of examples of bacterial BIM resulting from active
mineralization and the formation of reactive by-products. Some cyanobacteria precipitate
a number of different minerals that result from the uptake of bicarbonate from solution
and the release of hydroxyl anions. This causes an increase in the local pH of the cell.
The S layer in some species (e.g., Synechococcus spp.) is the site of nucleation of gypsum
(CaSO 4 •2H 2 O) in weak light. However, during photosynthesis, an increase in pH at the S
layer causes precipitation of calcite (CaCO 3 ) (Schultze-Lam et al. 1992, Fortin and
Beveridge 2000). Cyanobacteria can also promote the precipitation of Fe and Mn oxides
by increasing the pH and raising the O 2 concentration through oxygenic photosynthesis
(Fortin and Beveridge 2000). The formation of iron sulfides by sulfate-reducing bacteria
is also an excellent example of active mineralization from the formation of sulfide.
IRON AND MANGANESE MINERALIZATION PROCESSES
Biogenic iron and manganese minerals are particularly common products (Table 1)
of BIM processes because of the relatively high concentrations of these elements in the
earth's crust (4 th and the 12 th most abundant elements, respectively). Of these minerals,
magnetite and maghemite are especially significant in geology because of their
contribution to the magnetism of sediments. We will, therefore, emphasize BIM of iron
minerals, especially magnetite. Our discussion is organized in terms of the major
metabolic processes that cause deposition or dissolution of iron minerals, including metal
oxidation and reduction, and sulfate oxidation and metal sulfide reduction.
Figure 1. Unstained ultrathin section transmission electron micrograph of a “bacterial fossil” from a
sulfate-reducing consortium. The cell has lysed but iron sulfide mineral encrustation has preserved the
cell envelope. Figure kindly supplied by W. Stanley and G. Southam.