mid-ocean-ridge brine production, and the composition of seawater. This increase
in the seawater [Ca
2+ ] during the Early Cambrian may have created a chemical
environment favorable for the initial development of CC and calcium phosphate
hard parts, which have dominated marine biota ever since. Supporting this hypothesis, biocalcification in modern microorganisms can be affected by changes in
seawater [Ca
2+ ] as well as the Mg
2+ /Ca
2+ ratio as observed in cyanobacteria
(Sect. 5.2.3), coccolithophorids, and coralline algae. Calcifying cyanobacteria are
abundant in the Phanerozoic rock record during times of high oceanic [Ca
2+ ]
(Arp et al. 2001), and the first appearance of widespread calcifying cyanobacteria
coincides with the proposed spike in the Early Cambrian.
During the Early Cambrian, the ambient oceanic [Ca
2+ ] may have risen to such a
degree that cells could no longer effectively exclude or expel the Ca
2+ ions, causing
intracellular [Ca
2+ ] in marine organisms to reach toxic levels and triggering the
metabolic changes that led to pervasive biocalcification, so spurring evolutionary
changes and diversification in marine biota.
Even if hard shells and exoskeletons are considered defense mechanisms against
predators, they would have occurred first as defense mechanisms against high
levels of free Ca
2+ (detoxification hypothesis according to Simkiss 1977).
5.4 Cell Surface Structures and Precipitation
Bacterial surfaces play an important role in CCP since they make exceptional
interfaces for the precipitation of metal ions and the development of fine-grained
minerals (Fortin et al. 1997). Bacterial surfaces, and consequently bacterial cells,
can act as important sites for the absorption of cations and constitute particularly
favorable templates for heterogeneous nucleation and crystal growth. Due to the
presence of several negatively charged groups, at neutral pH, positively charged
metal ions could be bound on bacterial surfaces, favoring heterogeneous nucleation
(Fortin et al. 1997; Douglas and Beveridge 1998; B€ auerlein 2003).
Among surface structures, bacterial cell walls have been studied for their ability
to complex metals, a process not well understood yet (Jiang et al. 2004). Negative
charges result predominantly from deprotonation of carboxyl, phosphate, and
hydroxyl functional groups exposed on the outer surface of the cell wall (Fein
et al. 1997). The B. subtilis cell wall is one of the most studied for interactions with
metals. Ca
2+ ions are among the metals strongly bound to isolated walls of
B. subtilis. Beveridge and Murray (1980) have predicted at least a two-step mechanism for the development of metal precipitation on the cell wall of B. subtilis. The
first step is a stoichiometric interaction of metal with reactive chemical groups,
which reside primarily in the peptidoglycan. After complexation, these sites nucleate the deposition of more metal as a chemical precipitate.
Surface reactivity of bacterial cells depends on the metabolic state of the cells
(Jiang et al. 2004). In the absence of metabolic activity, passive interactions may
5 Molecular Basis of Bacterial Calcium Carbonate Precipitation
133
in the seawater [Ca
2+ ] during the Early Cambrian may have created a chemical
environment favorable for the initial development of CC and calcium phosphate
hard parts, which have dominated marine biota ever since. Supporting this hypothesis, biocalcification in modern microorganisms can be affected by changes in
seawater [Ca
2+ ] as well as the Mg
2+ /Ca
2+ ratio as observed in cyanobacteria
(Sect. 5.2.3), coccolithophorids, and coralline algae. Calcifying cyanobacteria are
abundant in the Phanerozoic rock record during times of high oceanic [Ca
2+ ]
(Arp et al. 2001), and the first appearance of widespread calcifying cyanobacteria
coincides with the proposed spike in the Early Cambrian.
During the Early Cambrian, the ambient oceanic [Ca
2+ ] may have risen to such a
degree that cells could no longer effectively exclude or expel the Ca
2+ ions, causing
intracellular [Ca
2+ ] in marine organisms to reach toxic levels and triggering the
metabolic changes that led to pervasive biocalcification, so spurring evolutionary
changes and diversification in marine biota.
Even if hard shells and exoskeletons are considered defense mechanisms against
predators, they would have occurred first as defense mechanisms against high
levels of free Ca
2+ (detoxification hypothesis according to Simkiss 1977).
5.4 Cell Surface Structures and Precipitation
Bacterial surfaces play an important role in CCP since they make exceptional
interfaces for the precipitation of metal ions and the development of fine-grained
minerals (Fortin et al. 1997). Bacterial surfaces, and consequently bacterial cells,
can act as important sites for the absorption of cations and constitute particularly
favorable templates for heterogeneous nucleation and crystal growth. Due to the
presence of several negatively charged groups, at neutral pH, positively charged
metal ions could be bound on bacterial surfaces, favoring heterogeneous nucleation
(Fortin et al. 1997; Douglas and Beveridge 1998; B€ auerlein 2003).
Among surface structures, bacterial cell walls have been studied for their ability
to complex metals, a process not well understood yet (Jiang et al. 2004). Negative
charges result predominantly from deprotonation of carboxyl, phosphate, and
hydroxyl functional groups exposed on the outer surface of the cell wall (Fein
et al. 1997). The B. subtilis cell wall is one of the most studied for interactions with
metals. Ca
2+ ions are among the metals strongly bound to isolated walls of
B. subtilis. Beveridge and Murray (1980) have predicted at least a two-step mechanism for the development of metal precipitation on the cell wall of B. subtilis. The
first step is a stoichiometric interaction of metal with reactive chemical groups,
which reside primarily in the peptidoglycan. After complexation, these sites nucleate the deposition of more metal as a chemical precipitate.
Surface reactivity of bacterial cells depends on the metabolic state of the cells
(Jiang et al. 2004). In the absence of metabolic activity, passive interactions may
5 Molecular Basis of Bacterial Calcium Carbonate Precipitation
133