5.3.2.1 Role of Calcium Ions in the Cell and Bacterial Calcium Metabolism
Life evolved in an environment containing many different cations, including Ca
2+ .
Cations present during early evolution would also have participated in the
random selection processes that eventually yielded the structural and functional
components of cells. According to Smith (1995), the role that Ca
2+ came to play in
the early cells may have been conserved during subsequent evolution. The
attributes of specific cations suited them to specific roles. Cations of related
elements often are associated in pairs, with one needed for intracellular nutrition
and the other not used intracellularly (Silver 1997). Unlike Mg
2+ , which functions
in many intracellular roles and must be transported inward and carefully regulated,
Ca
2+ frequently functions extracellularly, rather than intracellularly, in biological
processes and is maintained at low intracellular levels by efflux transport pathways
(Silver 1997).
Many of the functions that Ca
2+ performs in eukaryotes may therefore be
expected to be present in prokaryotes. The common evolutionary origin of the
prokaryotes and eukaryotes and the many examples of evolutionary conservation of
structure and function that have been shown to exist between them, support the
concept that such evolutionary conservation should extend to the role of Ca
2+
(Smith 1995).
Extensive investigations on Ca
2+ in eukaryotic cells have shown its important
role in signal transduction, as a signal transmitter in membrane depolarization
events, as an intracellular second messenger and as an effector of actin–myosin
contraction (for references, see Smith 1995). Relatively few systematic studies on
the role of Ca
2+ have been done in bacteria and its role is less well established.
Recently, however, there has been an increased interest in the role of Ca
2+ in
prokaryotes and a number of investigations have reported data demonstrating a
clear Ca
2+ contribution to structure and regulatory functions of the prokaryotic cell
(Norris et al. 1996; Smith 1995). The evidence in favor of Ca
2+ as a mediator of
regulatory phenomena in prokaryotes continues to accumulate. There is evidence
that calcium is involved in a number of bacterial processes such as maintenance of
cell structure, motility, cell division, gene expression, and cell differentiation
processes such as sporulation, heterocyst formation, and fruiting body development
(Dominguez 2004).
Since Ca
2+ plays a pivotal role in numerous biological processes in both
prokaryotes and eukaryotes, its intracellular concentration must be strictly
regulated and maintained to constant values. The free intracellular Ca
2+ concentration in bacteria is tightly regulated ranging from 100 to 300 nM, with similar values
to those found in eukaryotic cells (Dominguez 2004), against large changes in
external Ca
2+
concentrations. This is also necessary to avoid toxic effects of free Ca
2+
excess and irreversible damage to the cells, such as formation of calcium salts
relatively insoluble. Thus the maintenance of low intracellular concentrations of
calcium is essential both for the survival of an organism and for calcium to function
as a secondary messenger. Since calcium was present during the early stages of
5 Molecular Basis of Bacterial Calcium Carbonate Precipitation
127
Life evolved in an environment containing many different cations, including Ca
2+ .
Cations present during early evolution would also have participated in the
random selection processes that eventually yielded the structural and functional
components of cells. According to Smith (1995), the role that Ca
2+ came to play in
the early cells may have been conserved during subsequent evolution. The
attributes of specific cations suited them to specific roles. Cations of related
elements often are associated in pairs, with one needed for intracellular nutrition
and the other not used intracellularly (Silver 1997). Unlike Mg
2+ , which functions
in many intracellular roles and must be transported inward and carefully regulated,
Ca
2+ frequently functions extracellularly, rather than intracellularly, in biological
processes and is maintained at low intracellular levels by efflux transport pathways
(Silver 1997).
Many of the functions that Ca
2+ performs in eukaryotes may therefore be
expected to be present in prokaryotes. The common evolutionary origin of the
prokaryotes and eukaryotes and the many examples of evolutionary conservation of
structure and function that have been shown to exist between them, support the
concept that such evolutionary conservation should extend to the role of Ca
2+
(Smith 1995).
Extensive investigations on Ca
2+ in eukaryotic cells have shown its important
role in signal transduction, as a signal transmitter in membrane depolarization
events, as an intracellular second messenger and as an effector of actin–myosin
contraction (for references, see Smith 1995). Relatively few systematic studies on
the role of Ca
2+ have been done in bacteria and its role is less well established.
Recently, however, there has been an increased interest in the role of Ca
2+ in
prokaryotes and a number of investigations have reported data demonstrating a
clear Ca
2+ contribution to structure and regulatory functions of the prokaryotic cell
(Norris et al. 1996; Smith 1995). The evidence in favor of Ca
2+ as a mediator of
regulatory phenomena in prokaryotes continues to accumulate. There is evidence
that calcium is involved in a number of bacterial processes such as maintenance of
cell structure, motility, cell division, gene expression, and cell differentiation
processes such as sporulation, heterocyst formation, and fruiting body development
(Dominguez 2004).
Since Ca
2+ plays a pivotal role in numerous biological processes in both
prokaryotes and eukaryotes, its intracellular concentration must be strictly
regulated and maintained to constant values. The free intracellular Ca
2+ concentration in bacteria is tightly regulated ranging from 100 to 300 nM, with similar values
to those found in eukaryotic cells (Dominguez 2004), against large changes in
external Ca
2+
concentrations. This is also necessary to avoid toxic effects of free Ca
2+
excess and irreversible damage to the cells, such as formation of calcium salts
relatively insoluble. Thus the maintenance of low intracellular concentrations of
calcium is essential both for the survival of an organism and for calcium to function
as a secondary messenger. Since calcium was present during the early stages of
5 Molecular Basis of Bacterial Calcium Carbonate Precipitation
127