6.1 Calcium-Based Biominerals in Aquatic Organisms
There are over 60 different biological minerals in nature. H, C, O, Mg, Si, P,
S, Ca, Mn, and Fe are common constituents of the 20–25 essential elements
required by living organisms. Over half of the elements essential for life are
incorporated in biomineral deposits. Of these, calcium is distinguished in being
widespread and the common constituent of bones, teeth, and shells. It is interesting
to note that bones are composed of calcium phosphate, while shells are built from
calcium carbonate. The reasons for this significant difference are still not known.
In both cases, however, the inorganic mineral is intimately associated with a
complex assemblage of organic macromolecules, the organic matrix, is of fundamental importance.
Calcium carbonate and calcium phosphate minerals have high lattice energies
and low solubilities, and are therefore thermodynamically stable within biological
environments. In contrast, hydrated phases, such as calcium oxalate and calcium
sulfate, are much more soluble and therefore less common. In general, the
precipitation of calcium salts provides an effective means to control the Ca
2+ ion
concentration in biological fluids. This helps to maintain a steady-state condition
corresponding to an intracellular calcium concentration of around 10
À7 M
(Mann 2001).
6.1.1 Calcium Carbonate
There are six calcium carbonate minerals with the same principal composition but
different structure: calcite, aragonite, vaterite, calcium carbonate monohydrate,
calcium carbonate hexahydrate, and amorphous calcium carbonate. But only the
two most thermodynamically stable structures (calcite and aragonite) are deposited
extensively as biominerals (Table 6.1). Magnesium ions are readily accommodated
in the calcite lattice; many biological calcites also contain Mg
2+ ions up to levels of
30 mol%.
6.1.1.1 Calcite and Aragonite
Mollusc Shell
Although molluscs build shells with all sorts of shapes and sizes that are generally
conservative when it comes to the choice of mineral used, shells are made pure and
simply of calcium carbonate usually in the form of calcite and aragonite. Interestingly, many types of seashell contain both calcite and aragonite but the minerals are
spatially separated in distinct parts of the shells.
6 Principles of Calcium-Based Biomineralization
143
There are over 60 different biological minerals in nature. H, C, O, Mg, Si, P,
S, Ca, Mn, and Fe are common constituents of the 20–25 essential elements
required by living organisms. Over half of the elements essential for life are
incorporated in biomineral deposits. Of these, calcium is distinguished in being
widespread and the common constituent of bones, teeth, and shells. It is interesting
to note that bones are composed of calcium phosphate, while shells are built from
calcium carbonate. The reasons for this significant difference are still not known.
In both cases, however, the inorganic mineral is intimately associated with a
complex assemblage of organic macromolecules, the organic matrix, is of fundamental importance.
Calcium carbonate and calcium phosphate minerals have high lattice energies
and low solubilities, and are therefore thermodynamically stable within biological
environments. In contrast, hydrated phases, such as calcium oxalate and calcium
sulfate, are much more soluble and therefore less common. In general, the
precipitation of calcium salts provides an effective means to control the Ca
2+ ion
concentration in biological fluids. This helps to maintain a steady-state condition
corresponding to an intracellular calcium concentration of around 10
À7 M
(Mann 2001).
6.1.1 Calcium Carbonate
There are six calcium carbonate minerals with the same principal composition but
different structure: calcite, aragonite, vaterite, calcium carbonate monohydrate,
calcium carbonate hexahydrate, and amorphous calcium carbonate. But only the
two most thermodynamically stable structures (calcite and aragonite) are deposited
extensively as biominerals (Table 6.1). Magnesium ions are readily accommodated
in the calcite lattice; many biological calcites also contain Mg
2+ ions up to levels of
30 mol%.
6.1.1.1 Calcite and Aragonite
Mollusc Shell
Although molluscs build shells with all sorts of shapes and sizes that are generally
conservative when it comes to the choice of mineral used, shells are made pure and
simply of calcium carbonate usually in the form of calcite and aragonite. Interestingly, many types of seashell contain both calcite and aragonite but the minerals are
spatially separated in distinct parts of the shells.
6 Principles of Calcium-Based Biomineralization
143