Biomineralization
Biomineralization, also written biomineralisation, is the process by which living organisms produce minerals, often resulting in hardened or stiffened mineralized tissues. It is an extremely widespread phenomenon: all six taxonomic kingdoms contain members that can form minerals, and over 60 different minerals have been identified in organisms. Examples include silicates in algae and diatoms, carbonates in invertebrates, and calcium phosphates and carbonates in vertebrates. These minerals often form structural features such as sea shells and the bone in mammals and birds.
Organisms have been producing mineralized skeletons for the past 550 million years. Calcium carbonates and calcium phosphates are usually crystalline, but silica organisms are always non-crystalline minerals. Other examples include copper, iron, and gold deposits involving bacteria. Biologically formed minerals often have special uses such as magnetic sensors in magnetotactic bacteria, gravity-sensing devices and iron storage and mobilization.
In terms of taxonomic distribution, the most common biominerals are the phosphate and carbonate salts of calcium that are used in conjunction with organic polymers such as collagen and chitin to give structural support to bones and shells. The structures of these biocomposite materials are highly controlled from the nanometer to the macroscopic level, resulting in complex architectures that provide multifunctional properties. Because this range of control over mineral growth is desirable for materials engineering applications, there is interest in understanding and elucidating the mechanisms of biologically-controlled biomineralization.
Types
Mineralization can be subdivided into different categories depending on the following: the organisms or processes that create chemical conditions necessary for mineral formation, the origin of the substrate at the site of mineral precipitation, and the degree of control that the substrate has on crystal morphology, composition, and growth. These subcategories include biomineralization, organomineralization, and inorganic mineralization, which can be subdivided further. However, the usage of these terms varies widely in the scientific literature because there are no standardized definitions. The following definitions are based largely on a paper written by Dupraz et al., which provided a framework for differentiating these terms.Biomineralization
Biomineralization, biologically controlled mineralization, occurs when crystal morphology, growth, composition, and location are completely controlled by the cellular processes of a specific organism. Examples include the shells of invertebrates, such as molluscs and brachiopods. Additionally, the mineralization of collagen provides crucial compressive strength for the bones, cartilage, and teeth of vertebrates.Organomineralization
This type of mineralization includes both biologically induced mineralization and biologically influenced mineralization.- Biologically induced mineralization occurs when the metabolic activity of microbes produces chemical conditions favorable for mineral formation. The substrate for mineral growth is the organic matrix, secreted by the microbial community, and affects crystal morphology and composition. Examples of this type of mineralization include calcareous or siliceous stromatolites and other microbial mats. A more specific type of biologically induced mineralization, remote calcification or remote mineralization, takes place when calcifying microbes occupy a shell-secreting organism and alter the chemical environment surrounding the area of shell formation. The result is mineral formation not strongly controlled by the cellular processes of the animal host ; this may lead to unusual crystal morphologies.
- Biologically influenced mineralization takes place when chemical conditions surrounding the site of mineral formation are influenced by abiotic processes. However, the organic matrix is responsible for crystal morphology and composition. Examples include micro- to nanometer-scale crystals of various morphologies.
Biological roles
Among animals, biominerals composed of calcium carbonate, calcium phosphate, or silica perform a variety of roles such as support, defense, and feeding.If present on a supracellular scale, biominerals are usually deposited by a dedicated organ, which is often defined very early in embryological development. This organ will contain an organic matrix that facilitates and directs the deposition of crystals. The matrix may be collagen, as in deuterostomes, or based on chitin or other polysaccharides, as in molluscs.
In molluscs
The mollusc shell is a biogenic composite material that has been the subject of much interest in materials science because of its unusual properties and its model character for biomineralization. Molluscan shells consist of 95–99% calcium carbonate by weight, while an organic component makes up the remaining 1–5%. The resulting composite has a fracture toughness ≈3000 times greater than that of the crystals themselves. In the biomineralization of the mollusc shell, specialized proteins are responsible for directing crystal nucleation, phase, morphology, and growths dynamics and ultimately give the shell its remarkable mechanical strength. The application of biomimetic principles elucidated from mollusc shell assembly and structure may help in fabricating new composite materials with enhanced optical, electronic, or structural properties.The most described arrangement in mollusc shells is the nacre, known in large shells such as Pinna or the pearl oyster. Not only does the structure of the layers differ, but so do their mineralogy and chemical composition. Both contain organic components, and the organic components are characteristic of the layer and of the species. The structures and arrangements of mollusc shells are diverse, but they share some features: the main part of the shell is crystalline calcium carbonate, though some amorphous calcium carbonate occurs as well; and although they react as crystals, they never show angles and facets.
In fungi
are a diverse group of organisms that belong to the eukaryotic domain. Studies of their significant roles in geological processes, "geomycology", have shown that fungi are involved with biomineralization, biodegradation, and metal-fungal interactions.In studying fungi's roles in biomineralization, it has been found that fungi deposit minerals with the help of an organic matrix, such as a protein, that provides a nucleation site for the growth of biominerals. Fungal growth may produce a copper-containing mineral precipitate, such as copper carbonate produced from a mixture of 2CO3 and CuCl2. The production of the copper carbonate is produced in the presence of proteins made and secreted by the fungi. These fungal proteins that are found extracellularly aid in the size and morphology of the carbonate minerals precipitated by the fungi.
In addition to precipitating carbonate minerals, fungi can also precipitate uranium-containing phosphate biominerals in the presence of organic phosphorus that acts as a substrate for the process. The fungi produce a hyphal matrix, also known as mycelium, that localizes and accumulates the uranium minerals that have been precipitated. Although uranium is often deemed toxic to living organisms, certain fungi such as Aspergillus niger and Paecilomyces javanicus can tolerate it.
Though minerals can be produced by fungi, they can also be degraded, mainly by oxalic acid–producing strains of fungi. Oxalic acid production is increased in the presence of glucose for three organic acid producing fungi: Aspergillus niger, Serpula himantioides, and Trametes versicolor. These fungi have been found to corrode apatite and galena minerals. Degradation of minerals by fungi is carried out through a process known as neogenesis. The order of most to least oxalic acid secreted by the fungi studied are Aspergillus niger, followed by Serpula himantioides, and finally Trametes versicolor.
In bacteria
It is less clear what purpose biominerals serve in bacteria. One hypothesis is that cells create them to avoid entombment by their own metabolic byproducts. Iron oxide particles may also enhance their metabolism.Other roles
Biomineralization plays significant global roles terraforming the planet, as well as in biogeochemical cycles and as a carbon sink.Composition
Most biominerals can be grouped by chemical composition into one of three distinct mineral classes: silicates, carbonates, or phosphates.Silicates
Silicates are common in marine biominerals, where diatoms form frustules and radiolaria form capsules from hydrated amorphous silica.Carbonates
The major carbonate in biominerals is CaCO3. The most common polymorphs in biomineralization are calcite and aragonite, although metastable vaterite and amorphous calcium carbonate can also be important, either structurally or as intermediate phases in biomineralization. Some biominerals include a mixture of these phases in distinct, organised structural components. Carbonates are particularly prevalent in marine environments, but also present in freshwater and terrestrial organisms.Phosphates
The most common biogenic phosphate is hydroxyapatite, a calcium phosphate 6 and a naturally occurring form of apatite. It is a primary constituent of bone, teeth, and fish scales. Bone is made primarily of HA crystals interspersed in a collagen matrix—65 to 70% of the mass of bone is HA. Similarly, HA is 70 to 80% of the mass of dentin and enamel in teeth. In enamel, the matrix for HA is formed by amelogenins and enamelins instead of collagen. Remineralisation of tooth enamel involves the reintroduction of mineral ions into demineralised enamel. Hydroxyapatite is the main mineral component of enamel in teeth. During demineralisation, calcium and phosphorus ions are drawn out from the hydroxyapatite. The mineral ions introduced during remineralisation restore the structure of the hydroxyapatite crystals.The clubbing appendages of the peacock mantis shrimp are made of an extremely dense form of the mineral which has a higher specific strength; this has led to its investigation for potential synthesis and engineering use. Their dactyl appendages have excellent impact resistance due to the impact region being composed of mainly crystalline hydroxyapatite, which offers significant hardness. A periodic layer underneath the impact layer composed of hydroxyapatite with lower calcium and phosphorus content inhibits crack growth by forcing new cracks to change directions. This periodic layer also reduces the energy transferred across both layers due to the large difference in modulus, even reflecting some of the incident energy.
File:Glomerula piloseta tube microstructure.jpg|thumb|Glomerula piloseta, longitudinal section of the tube showing aragonitic spherulitic prismatic structure