Chemistry Reference
In-Depth Information
constructed of phospholipid and its outer face contains lipopolysaccharide (LPS) which is
highly anionic. LPS consists of
O
-polysaccharide, the core polysaccharide, and lipid A.
The
O
-sidechain can extend up to 40 nm away from the core polysaccharide which is
attached to lipid A. Lipid A contains several strongly hydrophobic fatty acid chains that
cement the LPS into the outer membrane bilayer. The core oligosaccharide and upper
regions of lipid A are rich in phosphate groups that have an affinity for Mg
2+
and Ca
2+
(Ferris and Beveridge 1986b). The core has several keto-deoxyoctonate residues that
provide available carboxylate groups while many
O
-sidechains also contain residues rich
in carboxylate groups (Ferris and Beveridge 1986a). Phospholipid is mainly present in
the inner face of the outer membrane. In gram-negative cells, it is the LPS that is the
major factor in catalyzing mineral formation because of its high concentration of
phosphate and carboxyl groups (Ferris and Beveridge 1984, 1986a).
Members of the Archaea also show gram-positive and gram-negative staining
characteristics. However, the cell walls of the Archaea are very different chemically from
the Bacteria and from each other (König 1988). Some gram-positive Archaea have cell
walls composed of a layer of a peptidoglycan-like polymer, consisting of N-
acetyltalosaminuronic acid and N-acetylglucosamine, called pseudomurein that overlies
the plasma membrane. Others lack pseudomurein and have cell walls consisting of
polysaccharide, glycoproteins, or protein. Some gram-negative Archaea lack a cell wall
entirely but retain the plasma membrane. Thus, electrochemical charges present on the
cell surfaces of the Archaea vary.
Other layers external to the bacterial cell wall that may be involved in mineral
nucleation include S layers, capsules, slimes, and sheaths. S layers, very common in
Archaea, are paracrystalline cell surface assemblages composed of protein or
glycoprotein that self assemble and associate with the underlying wall through non-
covalent interaction (Koval 1988). When S layers are present, they are the outermost
layer of the cell facing the surrounding environment. S layers are acidic and possess a net
negative charge thereby having an affinity for metal cations (Southam 2000). Capsules
are dense, highly hydrated amorphous assemblages of polysaccharides or proteins that are
chemically attached to the cell surface. They can be quite thick and extend up to 1 µm
from the cell. Capsules are rich in carboxylate groups and may also contain a significant
number of phosphate groups, both giving the structure a net negative charge. Because
capsules are highly hydrated and cover the cell surface, there can be extensive interaction
between the capsule and metal cations. In some cases, capsules are known to form in
response to the presence of metal ions (Appanna and Preston 1987). Slime layers, a much
more loosely packed version of the capsule, are similar to capsules chemically but are not
attached to the cell so they can leave the cell entirely (Southam 2000). Sheaths are rigid
hollow cylinders generally surrounding chains of cells or filamentous bacteria produced
by a few species of prokaryotes (e.g,
Leptothrix
). In the Domain Bacteria, the sheath is a
rigid homo- or heteropolymer of carbohydrate or carbohydrate and protein. In some
organisms, the sheath is important in the nucleation of oxidized mineral precipitates and
active biomineralization since it sometimes contains proteins that oxidize metals, e.g., as
in the oxidation of manganese by
L. discophora
(Adams and Ghiorse 1986, 1987). In the
Archaea, sheaths are composed of protein and are covalently linked to the cell wall.
Minerals known to be formed via BIM through passive surface-mediated
mineralization include Fe, Mn, and other metal oxides, e.g., ferrihydrite (5Fe
2
O
3
•9H
2
O),
hematite (
-FeOOH); metal sulfates, phosphates, and carbonates;
phosphorite; Fe and Fe-Al silicates; and metal sulfides. Mineral formation results initially
from the neutralization of chemically reactive sites on the cell, and proceeds via nucleation
of additional metal ions with the initially sorbed metals (Southam 2000). Mineralization is
α
-Fe
2
O
3
), and goethite (
α
