calcium cycle
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The calcium cycle is a transfer of calcium between dissolved and
solid Solid is one of the State of matter#Four fundamental states, four fundamental states of matter (the others being liquid, gas, and Plasma (physics), plasma). The molecules in a solid are closely packed together and contain the least amount o ...
phases. There is a continuous supply of
calcium ions Calcium ions (Ca2+) contribute to the physiology and biochemistry of organisms' cells. They play an important role in signal transduction pathways, where they act as a second messenger, in neurotransmitter release from neurons, in contraction of ...
into waterways from
rocks In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's ...
,
organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
s, and
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
s. Calcium ions are consumed and removed from aqueous environments as they react to form insoluble structures such as calcium carbonate and calcium silicate, which can deposit to form sediments or the
exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...
s of organisms. Calcium ions can also be utilized
biologically Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
, as calcium is essential to biological functions such as the production of
bone A bone is a Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provid ...
s and
teeth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...
or cellular function. The calcium cycle is a common thread between terrestrial, marine, geological, and biological processes. Calcium moves through these different media as it cycles throughout the Earth. The marine calcium cycle is affected by changing
atmospheric carbon dioxide Carbon dioxide in Earth's atmosphere is a trace gas that plays an integral part in the greenhouse effect, carbon cycle, photosynthesis and oceanic carbon cycle. It is one of several greenhouse gases in Earth's atmosphere that are contributing t ...
due to
ocean acidification Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
.


Calcium weathering and inputs to seawater

Calcium is stored in geologic reservoirs, most commonly in the form of calcium carbonate or as calcium silicate. Calcium-containing rocks include
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
,
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
,
phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...
, and
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywall. ...
. Rocks slowly dissolve by physical and chemical processes, carrying
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
ions into rivers and oceans.
Calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
ions (Ca2+) and
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
ions (Mg2+) have the same charge (+2) and similar sizes, so they react similarly and are able to substitute for each other in some minerals, such as
carbonates A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
. Ca2+-containing minerals are often more easily weathered than Mg2+ minerals, so Ca2+ is often more enriched in waterways than Mg2+. Rivers containing more dissolved Ca2+ are generally considered more
alkaline In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a base (chemistry), basic, ionic compound, ionic salt (chemistry), salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as ...
.
Calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
is one of the most common elements found in seawater. Inputs of dissolved calcium (Ca2+) to the ocean include the weathering of
calcium sulfate Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Pari ...
, calcium silicate, and calcium carbonate, basalt-seawater reaction, and
dolomitization Dolomitization is a geological process by which the carbonate mineral dolomite is formed when magnesium ions replace calcium ions in another carbonate mineral, calcite. It is common for this mineral alteration into dolomite to take place due to ev ...
.


Biogenic calcium carbonate and the biological pump

Biogenic calcium carbonate is formed when marine organisms, such as
coccolithophore Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdo ...
s,
coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...
s,
pteropods Pteropoda ( common name pteropods, from the Greek meaning "wing-foot") are specialized free-swimming pelagic sea snails and sea slugs, marine opisthobranch gastropods. Most live in the top 10 m of the ocean and are less than 1 cm long. The mo ...
, and other
mollusks Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is esti ...
transform calcium ions and
bicarbonate In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula . Bicarbonate serves a crucial biochemic ...
into shells and
exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...
s of
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
or
aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including prec ...
, both forms of calcium carbonate. This is the dominant sink for dissolved calcium in the ocean. Dead organisms sink to the bottom of the ocean, depositing layers of shell which over time cement to form
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
. This is the origin of both marine and terrestrial limestone. Calcium precipitates into calcium carbonate according to the following equation: Ca2+ + 2HCO3 → CO2+ H2O + CaCO3 The relationship between dissolved calcium and calcium carbonate is affected greatly by the levels of carbon dioxide (CO2) in the atmosphere. Increased carbon dioxide leads to more
bicarbonate In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula . Bicarbonate serves a crucial biochemic ...
in the ocean according to the following equation: CO2 + CO32− + H2O → 2HCO3 With
ocean acidification Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
, inputs of
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
promote the dissolution of calcium carbonate and harm marine organisms dependent on their protective calcite or
aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including prec ...
shells.The
solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubil ...
of calcium carbonate increases with pressure and carbon dioxide and decreases with temperature. Thus, calcium carbonate is more soluble in deep waters than surface waters due to higher pressure and lower temperature. As a result,
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
of calcium carbonate is more common in shallower oceans. The depth at which the rate of calcite dissolution equals the rate of calcite precipitation is known as
calcite compensation depth Carbonate compensation depth (CCD) is the depth in the oceans below which the rate of supply of calcite (calcium carbonate) lags behind the rate of solvation, such that no calcite is preserved. Shells of animals therefore dissolve and carbonate p ...
.


Changes in global climate and the calcium cycle

Ocean acidity due to carbon dioxide has already increased by 25% since the industrial revolution. As carbon dioxide emissions continually increase and accumulate, this will negatively affect the lives of many marine ecosystems. The calcium carbonate used to form many marine organisms' exoskeletons will begin to break down, leaving these animals vulnerable and unable to live in their habitats. This ultimately has a flow on effect to predators, further affecting the function of many food webs globally.


Changes in calcium concentrations over geologic time

Calcium stable isotopes have been used to study inputs and outputs of dissolved calcium in marine environments. For example, one study found that calcium levels have decreased between 25 and 50 percent over a 40 million year timespan, suggesting that dissolved Ca2+outputs have exceeded its inputs. The isotope Calcium-44 can help to indicate variations in calcium carbonate over long timespans and help explain variants in global temperature. Declines in the isotope Calcium-44 usually correlate with periods of cooling, as dissolution of calcium carbonate typically signifies a decrease in temperature. Thus, Calcium isotopes correlate with Earth's climate over long periods of time.


Human/animal use of calcium

Being an essential element, calcium is obtained through dietary sources, the majority of which comes from dairy products. The three most significant mechanisms controlling calcium use within the body are intestinal absorption, renal absorption and bone turnover, which is controlled predominantly by hormones and their corresponding receptors in the gut, kidneys and bones respectively. This allows for calcium use throughout the body, namely in
bone A bone is a Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provid ...
growth,
cellular signalling In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property of all cellula ...
, blood clotting, muscle contraction and
neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
function. Calcium is one of the essential components of bone, contributing to its strength and structure in addition to being the main site at which it is stored within the body. Within the muscles, its primary use is to enable contractions. Muscle cells draw calcium from the blood, allowing it to bind with troponin, a component of the muscle fibre that signals for a contraction by moving actin and myosin. After a contraction, calcium dissipates and the filaments move back to a resting state before the release of more calcium for the next contraction. Furthermore, calcium plays a significant role in allowing nerve impulses to be transmitted between neurons. The release of calcium ions from voltage gated ion channels signals for the release of neurotransmitters into the synapse. This allows for the depolarisation of a neuron, thus transmitting the signal to the next neuron where this process is once again repeated. Without the presence of calcium ions, the release of neurotransmitters would not occur, preventing signals from being sent and hindering body processes. Negative feedback mechanisms are implemented in order to control calcium levels. When low calcium levels are detected in the body, the parathyroid releases
parathyroid hormone Parathyroid hormone (PTH), also called parathormone or parathyrin, is a peptide hormone secreted by the parathyroid glands that regulates the serum calcium concentration through its effects on bone, kidney, and intestine. PTH influences bone re ...
(PTH) which travels through the bloodstream to the bones and kidneys. In the bones, the presence of PTH stimulates osteoclasts. These cells break down bone to release calcium into the bloodstream where it can be used by the rest of the body in the above processes. In the kidneys, PTH stimulates re-absorption of calcium so it in not lost from the body through urine and returned to the bloodstream instead. Lastly, PTH acts on the intestines by indirectly promoting enzymes that activate vitamin D, a signal for the intestines to absorb more calcium, further increasing blood calcium levels. This will continue until the body releases too much calcium into the bloodstream. Excess calcium then promotes the release of calcitonin from the thyroid gland, effectively reversing the process of PTH. Osteoclast activity is stopped and osteoblasts take over, utilising the excess calcium in the bloodstream to form new bone. Calcium re-absorption in the kidney is prevented, allowing the excretion of excess calcium through the urine. Through these hormonal mechanisms, calcium homeostasis is maintained within the body.


Calcium in plants and soil

Calcium is an essential component of soil. When deposited in the form of lime, it cannot be used by plants. To combat this, carbon dioxide produced by plants reacts with water in the environment to produce carbonic acid. Carbonic acid is then able to dissolve limestone, enabling the release of calcium ions. This reaction is more readily available with smaller particles of limestone than it is with large pieces of rock due to the increased surface area. When lime is leached into soil, calcium levels inevitably increase, both stabilising pH and enabling calcium to mix with water to form Ca2+ ions, thus making it soluble and accessible to plants to be absorbed and utilised by the root system. The calcium ions travel up the xylem of the plant alongside water to reach the leaves. The plant can utilise this calcium in the form of calcium pectate to stabilise cell walls and provide rigidity. Calcium is also used by plant enzymes to signal growth and coordinate life-promoting processes. Additionally, the release of calcium ions enables microorganisms to access phosphorus and other micro nutrients with greater ease, improving the soil ecosystem drastically thus indirectly promoting plant growth and nutrition. Inevitable plant and animal death results in the return of calcium contained within the organism back into the soil to be utilised by other plants. Decomposing organisms break them down, returning the calcium back into the soil and enabling the cycling of calcium to continue. Additionally, these animals and plants are eaten by other animals, similarly continuing the cycle. It is however important to note the modern introduction of calcium into the soil by humans (through fertilisers and other horticultural products) has resulted in a higher concentration of calcium contained within soil.


Industrial uses of calcium and its impact on the calcium cycle

The naturally occurring calcium cycle has been altered by human intervention. Calcium is predominantly extracted from limestone deposits to be utilised by many industrial processes. Purification of iron ore and aluminium, replacing asbestos brake linings and some coatings for electric cables. Furthermore, calcium is used within the household to maintain alkaline pH of swimming pools, counteracting acidic disinfectants and in the food production industry to produce bicarbonate soda, some wines and dough. With its widespread uses, a large volume of calcium must be obtained from mines and quarries to supple the high demand. As more limestone and water is removed from mines, underground stores of rock are often weakened making the ground more susceptible to sink holes. Sinkholes and mining both affect the presence of groundwater, potentially leading to a lower water table or altered pathways of flowing water. This may affect local ecosystems or farmland as the water supply is restricted. Additionally, the water that is released from mining areas will have higher concentrations of dissolved calcium. This can either be released into oceans or absorbed by the soil. Whilst not always detrimental, it alters the natural calcium cycle which may have flow-on effects for ecosystems. Furthermore, water being pumped from mines increases the danger of downstream flooding whilst simultaneously decreasing the volume on water in upstream reservoirs such as marshes, ponds of wetlands It is however important to note than limestone mining is comparatively less damaging than other mining process, with potential to restore the environment after the mine is no longer in use


The importance of the calcium cycle and future predictions

The calcium cycle links ionic and non ionic calcium together in both marine and terrestrial environments and is essential for the functioning of all living organisms. In animals, calcium enables neurons to transmit signals by opening voltage gated channels that allow neurotransmitters to reach the next cell, bone formation and development and kidney function, whilst being maintained by hormones that ensure calcium homeostasis is reached. In plants, calcium promotes enzyme activity and ensures cell wall function, providing stability to plants. It also enables crustaceans to form shells and corals to exist, as calcium provides structure, rigidity and strength to structures when complexed (combined) to other atoms. Without its presence in the environment, many life-preserving processes would not exist. In the modern context, calcium also enables many industrial processes to occur, promoting further technological developments. With its close relation to the
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...
and the effects of greenhouse gasses, both calcium and carbon cycles are predicted to change in the coming years. Tracking calcium isotopes enables the prediction of environmental changes, with many sources suggesting increasing temperatures in both the atmosphere and marine environment. As a result, this will drastically alter the breakdown of rock, the pH of oceans and waterways and thus calcium sedimentation, hosting an array of implications on the calcium cycle. Due to the complex interactions of calcium with many facets of life, the effects of altered environmental conditions are unlikely to be known until they occur. Predictions can however be tentatively made, based upon evidence-based research. Increasing carbon dioxide levels and decreasing ocean pH will alter calcium solubility, preventing corals and shelled organisms from developing their calcium-based exoskeletons, thus making them vulnerable or unable to survive.


References

{{Reflist Biogeochemical cycle Biochemistry