Biomineralization

• 1. Defining The Biological Process

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  Living organisms produce minerals through a process called biomineralization. This phenomenon results in hardened or stiffened tissues found across all six taxonomic kingdoms of life. Over 60 different minerals have been identified within these biological systems. Examples include silicates found in algae and diatoms, carbonates in invertebrates, and calcium phosphates 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 like sponges and diatoms 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 mechanisms. In terms of taxonomic distribution, the most common biominerals are phosphate and carbonate salts of calcium used with organic polymers like 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. Because this range of control over mineral growth is desirable for materials engineering applications, there is interest in understanding the mechanisms of biologically-controlled biomineralization.

• 2. Taxonomic Distribution And Types

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  Mineralization can be subdivided into different categories depending on the organisms or processes that create chemical conditions necessary for mineral formation. These subcategories include biomineralization, organomineralization, and inorganic mineralization. However, the usage of these terms varies widely in the scientific literature because there are no standardized definitions. Biomineralization 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 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. A more specific type of biologically induced mineralization takes place when calcifying microbes occupy a shell-secreting organism and alter the chemical environment surrounding the area of shell formation. This may lead to unusual crystal morphologies not strongly controlled by the cellular processes of the animal host.

• 3. Evolutionary History Of Skeletons

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  The first evidence of biomineralization dates back some 550 million years during the Cambrian explosion. Sponge-grade organisms may have formed calcite skeletons at that time. In most lineages, biomineralization first occurred in the Cambrian or Ordovician periods. Organisms used whichever form of calcium carbonate was more stable in the water column at the point in time when they became biomineralized. They stuck with that form for the remainder of their biological history. The stability is dependent on the Ca/Mg ratio of seawater which is thought to be controlled primarily by the rate of sea floor spreading. Biomineralization evolved multiple times independently and most animal lineages first expressed biomineralized components in the Cambrian period. Many of the same processes are used in unrelated lineages suggesting that biomineralization machinery was assembled from pre-existing off-the-shelf components already used for other purposes in the organism. Although the biomachinery facilitating biomineralization is complex involving signalling transmitters inhibitors and transcription factors many elements of this toolkit are shared between phyla as diverse as corals molluscs and vertebrates. The shared components tend to perform quite fundamental tasks such as designating that cells will be used to create the minerals.

• 4. Molluscan Shell Mechanics

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  The mollusc shell is a biogenic composite material that has been the subject of much interest in materials science because of its unusual properties. 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 approximately 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 growth dynamics. These proteins 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 Pinctada. Both contain organic components including proteins sugars and lipids which 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.

• 5. Fungal Mineral Interactions

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  Studies of fungi's roles in geological processes geomycology have shown that fungi are involved with biomineralization biodegradation and metal-fungal interactions. 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 ammonium carbonate and copper chloride. The production of the copper carbonate is produced in the presence of proteins made and secreted by the fungi. These fungal proteins 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.

• 6. Bacterial Magnetite And Sensing

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  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. Magnetotactic bacteria employ magnetic iron minerals magnetite and greigite to produce magnetosomes to aid orientation and distribution in the sediments. The most ancient example of biomineralization dating back 2 billion years is the deposition of magnetite observed in some bacteria as well as the teeth of chitons and the brains of vertebrates. It is possible that this pathway which performed a magnetosensory role in the common ancestor of all bilaterians was duplicated and modified in the Cambrian to form the basis for calcium-based biomineralization pathways. Iron is stored in close proximity to magnetite-coated chiton teeth so that the teeth can be renewed as they wear. Not only is there a marked similarity between the magnetite deposition process and enamel deposition in vertebrates but some vertebrates even have comparable iron storage facilities near their teeth.

• 7. Biomimetic Engineering Applications

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  Most traditional approaches to the synthesis of nanoscale materials are energy inefficient requiring stringent conditions such as high temperature pressure or pH often producing toxic byproducts. In contrast materials produced by organisms have properties that usually surpass those of analogous synthetically manufactured materials with similar phase composition. Biological materials are assembled in aqueous environments under mild conditions by using macromolecules. Organic macromolecules collect and transport raw materials and assemble these substrates into short- and long-range ordered composites with consistency and uniformity. Bacterially induced calcium carbonate precipitation can be used to produce self-healing concrete. Bacillus megaterium spores and suitable dried nutrients are mixed and applied to steel-reinforced concrete. When the concrete cracks water ingress dissolves the nutrients and the bacteria germinate triggering calcium carbonate precipitation resealing the crack and protecting the steel reinforcement from corrosion. This process can also be used to manufacture new hard materials such as bio-cement. Biomineralization may be used to remediate groundwater contaminated with uranium through the precipitation of uranium phosphate minerals associated with the release of phosphate by microorganisms.