Biomineralization is the formation of biominerals, which are ‘elements or compounds, amorphous or crystalline, formed through biogeochemical processes’ (Skinner, 2005) by a life form. These life forms range from the Archae, bacteria, eukaryotes, up to and including, vertebrates and plants (Skinner, 2005). Biomineralization involves the selective extraction and uptake of elements from the local environment, which are then incorporated into a structure under strict biological control (Mann, 2001). Biomineralization may offer an organism structural support and mechanical strength, but may also be involved in biological functions such as protection, motion, cutting and grinding, buoyancy, optical, magnetic and gravity sensing and storage (Mann, 2001).
Carbonate biomineralized creatures are multi-cellular with several organs each of which may employ a separate mechanism to deposit the carbonate biomineral (Skinner, 2005). Calcium carbonate, CaCO3, exists as six polymorphs: calcite, aragonite, vaterite, calcium carbonate monohydrate, calcium carbonate hexahydrate and amorphous calcium carbonate. Of these polymorphs, only calcite and aragonite are deposited extensively as biominerals. These two polymorphs have the most thermodynamically stable structure (Mann, 2001). However, calcite is thermodynamically more stable than aragonite at ambient temperatures and pressures (Falini et al., 1996). Aragonite has an orthorhombic crystal structure, while calcite has a rhombohedral crystal structure (Leeuw and Parker, 1998). The inorganic mineral calcium carbonate is associated with an organic matrix, which consists of organic macromolecules (Mann, 2005). This organic matrix may influence the species of biomineral (Skinner, 2005).
Besides carbonate biomineralized creatures, the cyanobacteria, which are uni-cellular bacteria and algae, can also precipitate calcium carbonate. Cyanobacteria foster local mineral extracellular precipitation, while other carbonate biomineralizers do so intra- or inter-cellularly (Skinner, 2005).
In nature, calcium carbonate precipitation occurs both abiotic and biotic. Abiotic chemical precipitation occurs from solutions that are saturated in Ca2+ and CO32-, e.g. due to evaporation, by a reduction in CO2 pressure and by a temperature increase. Calcium carbonate precipitation in solution occurs via the overall equilibrium reaction: Ca2+ + CO32- ‘ CaCO3. Calcium carbonate precipitation readily occurs in alkaline environments (Ghashghaei and Emtiazi, 2013). Biotic calcium carbonate precipitation can occur either active or passive precipitation. Several biotic processes that lead to calcium carbonate precipitation will be discussed in this paper.
Carbonate biominerals often exhibit crystal chemical specificity (Skinner, 2005). For example the ratio aragonite/calcite within species depends on several conditions. These different conditions will be reviewed here. The kinetics and specificity of carbonate biomineral precipitation should reflect the integrated chemical, biochemical and physical environments in which they form, which will also be discussed here.
Biotic calcium carbonate precipitation
Calcium carbonate can be precipitated by animals and plants, bacteria, fungal carbonates and precipitates influenced by photosynthesis (Riding and Awramik, 2000). According to Ghashghaei and Emtiazi (2013) there are a number of conditions that can lead to calcium carbonate precipitation by microorganisms: 1) extracellular polymeric substances, that carry a net negative charge and have the high binding ability to Ca2+ ions, can result in CaCO3 precipitation, 2) an environment with higher content of dissolved inorganic carbon, 3) autotrophic pathways that reduce local carbon dioxide in the bacterial environment and thus increases the pH of the medium, and 4) heterotrophic pathways that can lead to active or passive precipitation. When carbonate particles are produced by ionic exchange through the cell membrane by activation of calcium and/or magnesium ionic pumps or channels, active precipitation or active carbonatogenesis occurs (Riding and Awramik, 2000; Ghashghaei and Emtiazi, 2013). This is probably coupled with ion production. In contrast, passive precipitation or passive carbonatogenesis operates by producing carbonate and bicarbonate ions and inducing various chemical modifications in the medium, e.g. an increase in pH, that lead to the precipitation of calcium carbonate (Riding and Awramik, 2000).
Chemical specificity of aragonite versus calcite
Carbonate biominerals often prefer either aragonite or calcite, depending on e.g. seawater composition, temperature and organic matrix. According to Stanley et al. (2002), the selection of aragonite versus calcite depends on the ambient (in water at the ionic strength of modern seawater (0.7) and temperatures typical of tropical seas (25-30??C)) Mg2+/Ca2+ mole ratio. According to Falini et al. (1996), the presence of other doubly charged ions in CaCO3 solutions, in particular Mg2+ as well as a variety of small organic molecules, favours the formation of aragonite. Stanley et al. (2002) found that when the ambient Mg2+/Ca2+ mole ratio is below ~1, then low-Mg calcite precipitates. When the ambient Mg2+/Ca2+ mole ratio is above ~1, high-Mg calcite precipitates. The percentage of magnesium carbonate incorporated into a crystal increases with this ratio, reaching ~6-8 at a ratio of 2, depending on the temperature. At ambient Mg2+/Ca2+ mole ratios between ~2 and slightly above 5, aragonite precipitates along with high-Mg calcite, but at ratios above 5 only aragonite precipitates (Stanley et al., 2002) (figure 1). They conclude that the percentage of magnesium in skeletal calcite is positively correlated with the Mg2+/Ca2+ mole ratio of seawater over geological time.