Chemistry Reference
In-Depth Information
nucleation and growth of the minerals and thus over the composition, size, habit, and
intracellular location of the minerals (Bazylinski and Frankel 2000a,b). These BCM
mineral particles are structurally well-ordered with a narrow size distribution and species-
specific, consistent, crystal habits. Because of these features, BCM processes are thought
to be under metabolic and genetic control. Because intra-vesicular conditions (e.g., pH,
Eh) are controlled by the organism, mineral formation is not as sensitive to external
environmental parameters as in BIM. BCM by bacteria is discussed later in this volume
(Bazylinski and Frankel 2003).
BIOLOGICALLY INDUCED MINERALIZATION
ON ORGANIC SURFACES
Because of the high surface to volume ratio of bacteria, cell surfaces and the surfaces
of exopolymers can be especially important in BIM processes. Negative charges on most
cell and exopolymer surfaces can result in binding of cations by non-specific electrostatic
interactions, effectively contributing to local supersaturation. Binding also helps stabilize
the surfaces of nascent mineral particles, decreasing the free energy barrier for critical,
crystal-nucleus formation. By this means, the rate of mineralization of amorphous to
crystalline mineral particles can become several orders of magnitude faster than inorganic
(i.e., without surface binding and nucleation) mineralization. In some cases this can result
in a mineral layer that covers the cell.
Two surface BIM processes, known as passive and active, have been distinguished
(Fortin and Beveridge 2000; Southam 2000). Passive mineralization refers to simple non-
specific binding of cations and recruitment of solution anions, resulting in surface
nucleation and growth of minerals. Active mineralization occurs by the direct redox
transformation of surface-bound metal ions, or by the formation of cationic or anionic by-
products of metabolic activities that form minerals on the bacterial surfaces.
Bacterial surface properties
Prokaryotes have various cell wall types whose chemistry determines the ionic
charges present on the surface of the organism. In the Domain Bacteria, there are two
general types of cell wall: gram-positive and gram-negative, the difference being the
cell's reaction to a staining procedure used in light microscopy. The gram-positive cell
wall is separated from the cytoplasm by a lipid/protein bilayer called the plasma or cell
membrane and consists mainly of peptidoglycan (murein) that is rich in carboxylate
groups that are responsible for the net negative charge of this structure (Beveridge and
Murray 1976, 1980). Peptidoglycan forms a 15-25 nm thick sheet (Beveridge 1981)
comprising multiple layers of repeating units of two sugar derivatives, N-
acetylglucosamine and N-acetylmuramic acid, and a small group of amino acids.
Peptidoglycan gives rigidity to the cell wall and its charged, multiple layers are mainly
responsible for mineral formation (Beveridge and Murray 1976; Fortin et al. 1997; Fortin
and Beveridge 2000). Additional components such as teichoic and/or teichuronic acids
can be bound into peptidoglycan (Beveridge 1981). These polymers contain phosphoryl
groups that further contribute to the net negative charge of the cell wall (Southam 2000).
The gram-negative cell wall is structurally more complex than, and differs from, the
gram-positive type in that it has a thinner peptidoglycan layer (about 3 nm thick) and
does not contain secondary polymers (Beveridge 1981). It is sandwiched between two
lipid/protein bilayers, the outer and the plasma (or cell) membranes, within the space
between the cell walls known as the periplasm. The outer membrane represents the cell's
outermost layer. Unlike the plasma membrane, the outer membrane is not solely
