User:SCLAX01/Marine biogenic calcification

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Marine biogenic calcification refers to the organic production of calcium carbonate by organisms in the global ocean.

Marine biogenic calcification is the biologically-mediated process by which marine organisms produce and deposit calcium carbonate minerals to form skeletal structures or hard tissues. This process is a fundamental aspect of the life cycle of some marine organisms, including corals, mollusks, foraminifera, certain types of plankton, and other calcifying marine invertebrates. The resulting structures, such as shells, skeletons, and coral reefs, functions as protection, support, and shelter and create some of the most biodiverse habitats in the world. Marine biogenic calcifiers also play a key role in the biological carbon pump and the biogeochemical cycling of nutrients, alkalinity, and organic matter.[1]

Overview of Marine Biogenic Calcifiers[edit]

Calcifying Organisms[edit]

Corals[edit]

Blue Linckia Starfish

Coral reefs, physical structures formed from calcium carbonate, are important on biological and ecological scales to the regions they are endemic to.[2] Most of the calcium carbonate zooxanthellae acquire to build their coral structures comes from storage reservoirs in older, permeable sediments. Their robust calcification abilities have resulted in extensive calcium carbonate deposits, some housing significant hydrocarbon reserves.[3] Today, these reefs exert essential controls on the global climate and marine environment, particularly in carbon recycling. However, this group only accounts for about 10% of the global production of calcium carbonate.  Corals undergo extracellular calcification and first develop an organic matrix and skeleton on top of which they will form their calcite structures.[4] Coral reefs uptake calcium and carbonate from the water to form calcium carbonate.

It is proposed that calcification via pH upregulation of the coral’s extracellular calcifying fluid occurs at least in part via Ca2+-ATPase. Ca2+-ATPase is an enzyme in the calicoblastic epithelium that pumps Ca2+ ions into the calcifying region and ejects protons (H+). This process circumvents the kinetic barriers to CaCO3 precipitation that exist naturally in seawater.[5]

Dissolved inorganic carbon (DIC) from the seawater is absorbed and transferred to the coral skeleton. An anion exchanger will then be used to secrete DIC at the site of calcification. This DIC pool is also used by algal symbionts (dinoflagellates) that live in the coral tissue.[6] These algae photosynthesize and produce nutrients, some of which are passed to the coral. The coral in turn will emit ammonium waste products which the algae uptake as nutrients. There has been an observed tenfold increase in calcium carbonate formation in corals containing algal symbionts than in corals that do not have this symbiotic relationship. The coral algal symbionts, Symbiodinium, show decreased populations with increased temperatures, often leaving the coral colorless and unable to photosynthesize and losing pigments (known as coral bleaching).[7]

Mollusks[edit]

Cuttlefish komodo large

This diverse group contains slugs, oysters, limpets, snails, scallops, mussels, clams, cephalopods and others. Mollusks employ a strategic approach to protect their soft tissues and deter predation by developing an external calcified shell. This process involves specialized cells following genetic instructions to synthesize minerals under non-equilibrium conditions.[8] The resulting minerals exhibit complex shapes and sizes along with being formed within a confined space, modulated by a mix of internal and external factors, some of which become part of the shell during calcification.[9] These organisms also pump hydrogen out so that it will not bond to the carbonate ions and make them unable to crystallize as calcium carbonate.


Echinoderms[edit]

Brittle star among other small crustaceans in the intertidal zone near the central coast of Oregon, US

Echinoderms, of the phylum Echinodermata, include organisms such as sea stars, sea urchins, sand dollars, crinoids, sea cucumbers and brittle stars. These organisms form extensive endoskeletons consisting of magnesian calcite.[10] Adult enchinoderm skeletons consist of teeth, spines, tests, tubule feet, and in some cases, spicules.[11] Echinoderms serve as excellent blueprints for biomineralization. Adult sea urchins are a particulary popular species studied to better understand the molecular and cellular processes that the calcification and biomineralization of their skeletal structures requires.[10] Unlike many other marine calcifiers, echinoderm tests are not formed purely from calcite; instead, their structures also heavily consist of organic matrices that increases the toughness and strength of their endoskeletons.[11]

Crustaceans[edit]

Chthamalus stellatus

Crustaceans have a hard outer shell formed from calcium carbonate. These organisms form a network of chitin-protein fibers and then precipitate calcium carbonate within this matrix. The chitin-protein fibers are first hardened by sclerotization, or crosslinking of protein and polysaccharides, followed by the crosslinking of proteins with other proteins.[12] The presence of a hard, calcified exoskeleton means that the crustacean has to molt and shed the exoskeleton as its body size increases. This links molting cycles to calcification processes, making access to a regular source of calcium and carbonate ions crucial for the growth and survival of crustaceans.[12] Various body parts of the crustacean will have a different mineral content, varying the hardness at these locations with the harder areas being generally stronger. This calcite shell provides protection for the crustaceans, meaning between molting cycles the crustacean must avoid predators while it waits for its calcite shell to form and harden.

A fossil test from a planktonic globigerininan foraminifera.

Foraminifera[edit]

Foraminifera, or forams, are single-celled protists that form chambered shells (tests) from calcium carbonate.[13] Forams first appeared approximately 170 million years ago, and populate oceans globally. Forams are microscopic organisms, typically no larger than 1 mm in length.[13] Foraminiferal classification is dependent on the characteristics of the shell, such as chamber shape and arrangement, surface ornamentation, wall composition, and other features. The calcification and dissolution of their shells causes changes both in the surface seawater carbonate chemistry, and in deep-water chemistry.[13] These organisms are excellent paleo-proxies given their well-preserved fossil record. Planktonic foraminifera, found in large numbers in the ocean, contribute significantly to oceanic carbonate production.[14] Unlike their benthic counterparts, more of these species have algal symbionts. We have a better understanding of the factors influencing planktonic species, with key variables being temperature, salinity, and surface seawater productivity. They're distributed in specific latitudinal provinces, some with bipolar distributions, and can serve as tracers for ocean currents. Understanding their present-day temperature limits helps estimate past sea-surface temperatures in the Quaternary period.[14] Recent advancements in foraminiferal shell geochemistry focus on paleoceanographic questions about past water masses. Modern foraminiferal shells, both planktonic and benthic, are crucial for establishing the principles behind these inquiries. Culturing studies are especially important to understand the species-specific 'vital effect' in trace element incorporation and the acquisition of oxygen and carbon isotope signatures in the shell.[14]

Coccolithophores[edit]

Coccolithus pelagicus

Phytoplankton, especially haptophytes such as coccolithophores, are also well known for their calcium carbonate production. It is estimated that these phytoplankton may contribute up to 70% to the global calcium carbonate precipitation, and coccolithophores are the largest phytoplankton contributors. Contributing between 1 and 10% of total primary productivity, 200 species of coccolithophores live in the ocean, and under the right conditions they can form large blooms in subpolar regions. These large bloom formations are a driving force for the export of calcium carbonate from the surface to the deep ocean in what is sometimes called “Coccolith rain”. As the coccolithophores sink to the seafloor they contribute to the vertical carbon dioxide gradient in the water column.[15]

Great Calcite Belt of the Southern Ocean

Coccolithophores produce calcite plates termed coccoliths which together cover the entire cell surface forming the coccosphere.[16] The coccoliths are formed using the intracellular strategy where the plates are formed in a coccoliths vesicle, but the product forming within the vesicle varies between the haploid and diploid phases. A coccolithophore in the haploid phase will produce what is called a holococcolith, while one in the diploid phase will produce heterococcoliths. Holococcoliths are small calcite crystals held together in an organic matrix, while heterococcoliths are arrays are larger, more complex calcite crystals.[16] These are often formed over a pre-existing template, giving each plate its particular structure and forming complex designs. [17] Each coccolithophore is a cell surrounded by the exoskeleton coccosphere, but there exists a wide range of sizes, shapes and architectures between different cells. Advantages of these plates may include protection against infection by viruses and bacteria, as well as protection from grazing zooplankton. The calcium carbonate exoskeleton enhances the amount of light the coccolithophore can uptake, increasing the level of photosynthesis. Finally, the coccoliths protect the phytoplankton from photodamage by UV light from the sun. [17]

The coccolithophores are also important in the geological history of Earth. The oldest coccolithophore fossil records are more than 209 million years old, placing their earliest presence in the Late Triassic period. Their calcium carbonate formation may have been the first deposition of carbonate on the seafloor. [17]

Species in the intertidal zone near the central coast of Oregon
Species in the intertidal zone near the central coast of Oregon

Corallinales (red algae)[edit]

Calcifying rhodophytes stock their filamentous cell walls with calcium carbonate and magnesium.[18] Corallinales is the one genus of red algae exists but their distribution ranges across the world's oceans.[19] Examples include Corallina, Neogoniolithon, and Harveylithon.[19] The magnesium-rich calcium carbonate of Corallinales cell wall provides shelter from predators and structural integrity in the intertidal zone.[20][21][22] The CaCO3 production in Coralline also plays a role in habitat formation and provides resources for benthic invertebrates.[21]

Calcifying bacteria[edit]

Evidence shows that some calcifying cyanobacteria strains have existed for millions of years and contributed to large land formations.[23] About 70 strains of cyanobacteria can precipitate calcium carbonate, including some strains of Synechococcus, Bacillus sphaericus, Bactilus subtilus,and Sporosarcina psychrophile.[24][25]

Morphological variations in calcium carbonate skeletons[edit]

Structural adaptations in different marine organisms[edit]

Diverse algae exhibit distinct mechanisms of CaCO3 formation, with calcification occurring internally or externally. The partial reaction of calcification suggests a potential physiological role in providing H+ for CO2 production or other H+ requiring processes. Phytoplankton species relying on CO2 diffusion for photosynthesis may face limitations due to CO2 concentration and diffusion to the chloroplast's Rubisco site. Some algae have evolved mechanisms for utilizing external HCO3- as a carbon substrate, facilitated by carbonic anhydrase. The giant-celled brackish-water alga Chara deposits bands of external CaCO3 in alkaline regions, suggesting spatial separation of H+ regions involved in calcification. Calcifying macroalgae like Halimeda and Corallina also produce CaCO3 in alkaline extracellular spaces.[26]

Coccolithophorid phytoplankton form CaCO3 in crystalline structures known as coccoliths, with holococcoliths formed externally and heterococcoliths produced intracellularly. Various coccolithophores produce two coccolith types: Heterococcoliths, from diploid cells, are complex, while holococcoliths, from haploid stages, are less studied. Factors influencing life cycle phase transitions and the role of specific proteins like GPA in coccolith morphology are explored. Polysaccharides, particularly coccolith-associated polysaccharides (CAPs), emerge as key regulators of calcite growth and morphology. CAPs' diverse roles, including nucleation promotion and inhibition, vary between species. External polysaccharides also influence coccolith adhesion and organization. Recent findings link cellular transport processes, carbonate saturation conditions, and regulatory processes determining calcite precipitation rate and morphology. Unexpectedly, silicon's role in coccolith morphology regulation is species-dependent, highlighting physiological distinctions among coccolithophore groups. These revelations raise questions about ecological implications, evolutionary adaptations, and the impact of changing ocean silicate levels on coccolithogenesis.[27]

Calcification rates in coccolithophores often correlate with photosynthesis, implying a potential metabolic role. Heterococcoliths develop inside intracellular vesicles, with coccolith formation showing a unity ratio with photosynthetic carbon fixation under high calcification rates. The variability in isotope fractionation and calcification mechanisms underscores these organisms' adaptability and complexity in responding to environmental factors.[28]

Evolution of biogenic calcification[edit]

The evolution of biogenic calcification and carbonate structures within the eukaryotic domain is a complex narrative, highlighted by the distribution of mineralized skeletons across major clades. Five out of the eight major clades feature species with mineralized skeletons, and all five clades involve organisms that precipitate calcite or aragonite. Skeletal evolution occurred independently in foraminiferans and echinoderms, suggesting two separate origins of CaCO3 skeletons. The common ancestry for echinoderm and ascidian skeletons is less clear, but a conservative estimate indicates that carbonate skeletons evolved at least twenty-eight times within Euckarya.[29]

Phylogenetic insights highlight repeated innovations in carbonate skeleton evolution, raising questions about homology in underlying molecular processes. Skeleton formation involves controlled mineral precipitation in specific biological environments, requiring directed calcium and carbonate transport, molecular templates, and growth inhibitors. Biochemical similarities, including the synthesis of acidic proteins and glycoproteins guiding mineralization, suggest an ancient capacity for carbonate formation in eukaryotes. While skeletons may not share structural homology, underlying physiological pathways are common, reflecting multiple cooptations of molecular and physiological processes across eukaryotic organisms.[30]

Silica skeletons, found in five to six eukaryotic clades, differ from CaCO3 skeletons in their distribution and intracellular precipitation. Phosphate skeletons, primarily limited to animals, contrast with amorphous granules used in phosphate storage. The Cambrian Period marks a significant watershed in skeletal evolution, with the appearance of mineralized skeletons in various groups. Skeletal diversity increased during this period, driven by predation pressure favoring protective armor evolution. The Cambrian radiation of mineralized skeletons was likely part of a broader animal diversity expansion.[31]

The evolution of mineralized skeletons during the Cambrian did not occur instantly, with a gradual increase in abundance and diversity over 25 million years. Environmental changes and predation pressure played key roles in shaping skeletal evolution. The diversity of minerals and skeletal architectures during this period challenges explanations solely based on changing ocean chemistry. The interplay between genetic possibility and environmental opportunity, influenced by factors like increased oxygen tensions, likely contributed to Cambrian diversification. Later Cambrian oceans witnessed a decline in mineralized skeletons, potentially influenced by high temperatures and pCO2 associated with a super greenhouse. Skeletal physiological responses to environmental conditions remain an area of study. Large-scale variations in carbonate chemistry suggest a connection between ocean chemistry and the mineralogy of carbonate precipitation. Skeletal organisms that precipitate massive skeletons under limited physiological control show stratigraphic patterns corresponding to shifts in seawater chemistry. This interplay between physiology, evolution, and environment underscores the complexity of mineralized skeleton evolution across geological time.[32]

Processes of Marine Biogenic Calcification[edit]

Inorganic sources of calcium carbonate[edit]

Of all the metals important to biogeochemical cycles in the ocean, calcium is one of the most significant in both its mobility and the role it plays in regulating climate over millions of years through its presence in calcium carbonate. Calcium has the ability to migrate relatively easily between the hydrosphere, the biosphere, and the crust of the Earth.[33]

Calcium and bicarbonate ions are largely deposited into the ocean from the weathering of rock formations and are transported via riverine input. This process occurs on very long timescales.[34] Weathering accounts for approximately 60-90% of solute calcium within the global calcium cycle.[35] Limestone rock, which consists mostly of calcite, is a prime example of a rich source of calcium to the ocean. The source of the majority of inorganic calcium present in the ocean is due to riverine deposition, though volcanic activity interacting with seawater does provide some calcium as well. The distribution of calcium sources described above is the case for both the present day oceanic calcium budget, and the historical budget over the last 25 million years.[36] The formation of biogenic calcium carbonate is the primary mechanism of removal of calcium in the ocean water column.[37]

Chemical processes and saturation state[edit]

The surface ocean engages in air-sea interactions and absorbs carbon dioxide (CO2) from the atmosphere, making the ocean the Earth’s largest sink for atmospheric CO2. Carbon dioxide dissolves in and reacts with seawater to form carbonic acid. Subsequent reactions then produce carbonate (CO32−), bicarbonate (HCO3−), and hydrogen (H+) ions. Carbonate and bicarbonate are also deposited into the global ocean by rivers through the weathering of rock formations. The three species of carbon in seawater, carbon dioxide, bicarbonate, and carbonate, make up the total concentration of dissolved organic carbon (DIC) in the ocean. Approximately 90% of DIC is bicarbonate ions, 10% is carbonate ions, and <1% is dissolved carbon dioxide, with some spatial variation. The equilibria reactions between these species result in the buffering of seawater in terms of the concentrations of hydrogen ions present.[38]

The following chemical reactions exhibit the dissolution of carbon dioxide in seawater and its subsequent reaction with water:

CO2(g) + H2O(l) ⥨ H2CO3(aq)

H2CO3(aq) ⥨ HCO3-(aq) + H+(aq)

HCO3-(aq) ⥨ CO32−(aq) + H+(aq)

This series of reactions governs the pH levels in the ocean and also dictates the saturation state of seawater, indicating how saturated or unsaturated the seawater is with ions. Consequently, the saturation state significantly influences the balance between the dissolution and calcification processes in marine biogenic calcifiers. When seawater is oversaturated with calcium carbonate, the concentration of calcium ions and carbonate ions exceed the saturation point for a particular mineral, such as aragonite or calcite, which make up the skeletons of many marine organisms. Such conditions are favorable to marine calcifiers for the formation of calcium carbonate skeletons or shells. When seawater is undersaturated, meaning the concentration of calcium and carbonate ions is below the saturation point, it becomes challenging for marine calcifiers to build and maintain their skeletal structures, as the equilibrium conditions favor dissolution of calcium carbonate.[39] The saturation state of seawater with respect to CaCO3 is a measure of its capacity to erode the shells and skeletons of marine calcifiers. As a general rule, seawater that is undersaturated (Ω < 1) can dissolve the structures of calcifying organisms.[40] However, many organisms see negative effects on growth at saturation states above Ω = 1. For example, a saturation state of Ω = 3 is optimal for coral growth, so a saturation state Ω < 3 can potentially have negative effects on coral growth and survival.[41]

For marine calcifiers to build and maintain calcium carbonate structures, CaCO3 production must be greater than CaCO3 loss through physical, chemical, and biological processes. This net production can be thought of as follows:

CaCO3 accretion = CaCO3 production – CaCO3 dissolution – physical loss of CaCO3

The decreasing saturation of seawater with respect to calcium carbonate, often attributed to ocean acidification resulting from increased carbon dioxide (CO2) absorption by the oceans, poses a significant threat to marine calcifiers. As CO2 concentrations in seawater rise, a decrease in pH and a reduction in carbonate ion concentrations in seawater follows. This can make it difficult for marine organisms to precipitate and maintain their calcium carbonate structures, affecting growth, development, and overall health.[42][43]

The widespread use of calcification by marine organisms has relied on the ability of calcium carbonate to readily form in seawater, where the saturation states (Ω) of aragonite and calcite minerals have consistently surpassed Ω = 1 (indicating oversaturation) in surface waters for hundreds of millions of years.[44] The impacts of reduced calcium carbonate saturation on marine calcifiers have broader ecological implications, as these organisms play vital roles in marine ecosystems. For example, coral reefs, which are built by coral polyps secreting calcium carbonate skeletons, are particularly vulnerable to changes in calcium carbonate saturation.[45]

Calcium carbonate saturation can be determined using the following equation:

Ω = ([Ca2+][CO32−])/Ksp

where the numerator ([Ca2+][CO32−]) denotes the concentration of calcium and carbonate ions and the denominator (Ksp) refers to the mineral (solid) phase stoichiometric solubility product of calcium carbonate.[46]

Another way to calculate saturation is:

Ω = ([Ca2+][CO32-])/[CaCO3]

where the numerator is again the concentration of calcium and carbonate ions, and the denominator is the concentration of calcium carbonate at saturation.

When Ω > 1, seawater is supersaturated with respect to CaCO3. When Ω < 1, seawater is undersaturated with respect to CaCO3 (dissolution).[47]

When saturation is high, organisms can extract calcium and carbonate ions from seawater, forming solid crystals of calcium carbonate:

Ca2+(aq) + 2HCO3−(aq) → CaCO3(s) + CO2 + H2O

Calcification rates and the success of calcifying organisms in the oceans often correlate with carbonate system parameters. Marine organisms precipitate 0.5–2.0 Gt of carbon annually as calcium carbonate (CaCO3), significantly influencing global biogeochemical cycles. Biotic calcification relies on calcium ions (Ca2+) and typically bicarbonate ions (HCO3-) as CaCO3 substrates, with high proton (H+) concentrations capable of inhibiting this process. Despite calcification rates correlating well with carbonate ions (CO32-) and the CO32- dependent CaCO3 saturation state, seawater concentrations of these parameters appear irrelevant in the calcification process. A rearrangement of carbonate system equations reveals a proportionality between CO32- or CaCO3 and the ratio of HCO3- to H+. Calcification rates correlate equally well with HCO3- = H+ as with CO32- or CaCO3 when temperature, salinity, and pressure are constant. CO32- and CaCO3 may function as good proxies for the control exerted by HCO3- = H+, where HCO3- serves as the inorganic carbon substrate and H+ functions as a calcification inhibitor. If the "substrate–inhibitor ratio" (HCO3- = H+) rather than CO32- or CaCO3 controls biotic CaCO3 formation, common paradigms in ocean acidification research may need reassessment. The absence of a latitudinal gradient in HCO3- = H+ in contrast to CO32- and CaCO3 could challenge the assumption that high latitudes are most severely affected by ocean acidification.[48]

Forms of calcium carbonate[edit]

The three most common calcium carbonate minerals are aragonite, calcite, and vaterite. Although these minerals have the same chemical formula (CaCO3), they are considered polymorphs because the atoms that make up the molecule are stacked in different arrangements. For example, aragonite minerals have an orthorhombic crystal lattice structure, while calcite crystals have a trigonal structure.[49] Some of the calcite polymorphs are further subdivided by relative magnesium content (Mg/Ca ratio), with calcite solubility increasing with increasing Mg.[50] The solubility of various forms of CaCO3 differs in seawater; specifically, aragonite exhibits greater solubility compared to pure calcite, and the solubility of calcite rises with its magnesium content.[51]

The ratio of magnesium to calcium (Mg/Ca) in the composition of seawater has been found to have a significant impact on marine calcifiers. There is evidence suggesting that throughout Earth's history, variations in the Mg/Ca ratio of seawater have influenced which calcifying marine organisms were the major reef-builders and sediment-producers throughout different periods of Earth’s history. When the molar Mg/Ca ratio has been greater than 2 (termed "aragonite seas"), aragonite and high-Mg calcite secreting organisms dominate, while ratios less than 2 (termed "calcite seas"), favor calcite-secreting organisms.[52]

Additionally, laboratory experiments have shown that aragonite-secreting bryopsidalean algae and scleractinian corals, as well as calcite-secreting coccolithophores, demonstrate higher rates of calcification and growth when exposed to experimental seawaters with favorable Mg/Ca ratios for their particular skeletal mineral. These findings support the idea that seawater Mg/Ca played a crucial role in determining which calcifying marine organisms were the primary builders of reefs and producers of sediment throughout Earth's history.

Biochemical mechanisms[edit]

Cellular and molecular processes of biogenic calcification[edit]

Calcium carbonate plays a fundamental role in the skeletal formation of marine calcifiers. The skeletal structures of these organisms are predominantly composed of calcium carbonate minerals, specifically aragonite and calcite. These structures provide support, protection, and housing for marine calcifiers and are formed through the biochemical processes of biomineralization to precipitate the crystal structures that form the hard tissues of these organisms.

The biogenic formation of calcium carbonate structures is the result of a combination of biological and physical processes such as genetics, cellular activity, crystal competition, growth in confined spaces, and self-organization processes. The composition of these structures, and the mechanisms involved in building them, are highly diverse. For example, some organisms can incorporate both calcite and aragonite polymorphs into their skeletons. Some species, often found near hydrothermal vents, can incorporate other minerals to form complex protein matrices that perform specific functions.[53][54]

The key steps involved in marine biogenic calcification include the uptake of dissolved calcium ions (Ca2+) and carbonate ions (CO32-) from seawater, the precipitation of calcium carbonate crystals, and the controlled formation of skeletal structures through biomineralization processes. These organisms often regulate the calcification process through the secretion of organic molecules and proteins that influence the nucleation and growth of crystalline structures.

A range of biochemical calcification (biocalcification) mechanisms exist, indicated by the fact that marine calcifiers use a number of different forms of calcium carbonate minerals. Within this range of mechanisms, there are two broad categories of biogenic calcification in marine organisms: extracellular mineralization and intracellular mineralization. Mollusks and corals use the extracellular strategy, in which ion exchange pumps actively pump ions out of a cell into the extracellular space, where environmental conditions, such as pH, can be tightly controlled. In contrast, during intracellular mineralization the calcium carbonate is formed within the organism and can either be kept within the organism as an internal structure or is later moved to the outside of the organism, while retaining the cell membrane covering. Broadly, the intracellular mechanism pumps ions into a vesicle within the cell. This vesicle can then be secreted to the outside of the organism. Often, cells will fuse their membranes and combine these vesicles in order to build very large calcium carbonate structures that would not be possible within a single cell.[55]

Calcium carbonate cycling and the biological carbon pump[edit]

The calcium carbonate cycle in the global ocean is of great significance to the biological, chemical, and physical state of the ocean. Mineral calcium carbonate most commonly presents as calcite in the ocean, and the majority of it is produced biologically in the upper layer of the ocean. CaCO3 material is exported from the upper ocean to sediments on the ocean floor where it either dissolves or is buried.[56] Alternatively, CaCO3 can dissolve or be remineralized within the water column prior to reaching the seafloor.

Upon reaching the seafloor, CaCO3 undergoes a diagenetic process that ends in either dissolution or burial. [57] The distribution of sediments consisting of calcium carbonate is fairly even across the global oceans, but specific locations are determined by the solubility and saturation level of calcium carbonate.[58] Mineral CaCO3 is more commonly found in shallow, benthic regions of the ocean where the environment is supersaturated with respect to calcite. CaCO3 is less commonly found, if at all, in sediments in the pelagic zone, or deep open ocean. The deep ocean sediments are typically undersaturated with respect to CaCO3.[59]

The “biological carbon pump” is a colloquial term coined by scientists to summarize the global carbon cycle in the ocean and its relationship to the biological processes that occur throughout the ocean. The calcium carbonate cycle is inherently linked to the biological pump. The formation of biogenic calcium carbonate by marine calcifiers is one of the mechanisms for transport of carbon to the deep ocean and seafloor.[60]

Impact of environmental factors on calcification[edit]

Rising temperature and light exposure[edit]

Marine biogenic calcifiers, such as corals, are facing challenges due to increasing ocean temperatures, leading to prolonged warming events. When sea surface temperatures exceed the local summer maximum monthly mean, coral bleaching and mortality occur as a result of the breakdown in symbiosis with Symbiodiniaceae. Predicted increases in summer-time temperatures, coupled with ocean warming, are expected to impact coral health and overall rates of calcification, particularly in tropical regions where many corals already live close to their upper thermal limits.[61]

Corals are highly adapted to their local seasonal temperature and light conditions, influencing their physiology and calcification rates. While increased temperature or light levels typically stimulate calcification up to a certain optimum, beyond which rates decline, the effects of temperature and light on the calcifying fluid chemistry are less clear. Coral calcification is a biologically-mediated process influenced by the regulation of internal calcifying fluid chemistry, including pH and dissolved inorganic carbon. The impacts of temperature and light on these factors remain a knowledge gap, with laboratory studies yielding contrasting results. Decoupling the effects of temperature and light on calcification processes is challenging due to their seasonal co-variation, highlighting the need for further research to address this gap and enhance our understanding of how marine biogenic calcifiers respond to future climate change.[62]

Ocean Acidification[edit]

Calcifying organisms are particularly at risk due to changes in the chemical composition of ocean water associated with ocean acidification. As pH decreases due to ocean acidification, the availability of carbonate ions (CO32-) in seawater also decreases. Therefore, calcifying organisms experience difficulty building and maintaining their skeletons or shells in an acidic environment. There has been considerable debate in the literature regarding whether organisms are responding to reduced pH or reduced mineral saturation state[63] [64][65][66]as both variables decline with ocean acidification. However, recent studies that have isolated the effects of saturation state independent of pH changes point toward saturation state as the most important factor impacting shell formation development[67][68]. However, we still need to fully constrain the carbonate chemistry to better interpret the ecological responses around ocean acidification.

Responses of marine calcifiers to reduced carbonate ion availability are seen in different ways. For example, coral reefs experience inhibited growth at decreased pH[69], and live calcium carbonate structures experience weakening of existing structures[70]. Other organisms are particularly vulnerable in the early stages of their life cycle. Bivalves for instance are particularly susceptible during early larval stages during initial shell formation since these early stages have a high energetic cost to the individual’s development. In contrast, adult bivalves are considerably more resilient to reduced pH.

Human interactions and applications[edit]

Economic importance[edit]

Shellfish industry[edit]

Ocean acidification (OA) presents a formidable threat to global shellfish production, particularly exerting its impact on calcification processes. Projections indicate that by the end of the century, mussel and oyster calcification could witness substantial reductions of 25% and 10%, respectively, as outlined in the IPCC IS92a scenario, reaching approximately 740 ppm in 2100.[71]These species, integral to coastal ecosystems and representing a significant portion of global aquaculture, play crucial roles as ecosystem engineers. The anticipated decline in calcification due to OA not only jeopardizes coastal biodiversity and ecosystem functioning but also carries the potential for considerable economic losses. For example, global aquaculture production for shellfish contributed US$ 29.2 billion to the world economy. [72]

Shellfish harvesting at St. Benoit de Ondes

Beyond the direct effects on calcification, it underscores the collateral consequences of OA on shell surfaces. Damaged shell surfaces, primarily resulting from reduced calcification rates, contribute to a significant decrease in sale prices, marking a critical economic concern. Economic assessments reveal that such damages, particularly impacting culture quasi-profits, can lead to reductions ranging from 35% to 70%.[73] Furthermore, when accounting for assumed pH-driven changes occurring concurrently, quasi-profits diminish even more substantially, reaching levels of 49% to 84% across diverse OA scenarios. Consequently, the economic fallout is substantial, with the UK facing potential direct losses of £3 to £6 billion in GDP by 2100, and globally, costs exceeding $100 billion USD.[74][75] These findings emphasize the urgent need for proactive measures to mitigate OA's impact on bivalve farming and underscore the importance of comprehensive climate policies to address these multifaceted challenges.

Coral reef tourism[edit]

For organisms relying on calcification processes, OA can potentially disrupt entire ecosystems. As these calcifiers play crucial roles in maintaining marine biodiversity, the repercussions of coral reef decline extend beyond economic considerations, emphasizing the urgency of comprehensive conservation efforts. Extensive degradation is occurring in the Caribbean and Western Atlantic region's coral reefs, stemming from issues like disease, overfishing, and a range of human activities. Adding to the challenges, rapid climate-induced ocean warming and acidification exacerbate the threats to these vital ecosystems.[76] Tourism is integral to the Caribbean region, surpassing global dependence, with the sector contributing to over 15 percent of GDP and sustaining 13 percent of jobs in the area.[77] Amidst these challenges, the global economic value of coral reefs becomes evident, estimated at $490 USD/year for an average hectare. Specific regions showcase the economic significance of coral reefs, with Hawai'i's contributing $360 million USD annually to its economy, and the Philippine economy receiving at least $1.06 billion USD each year from coral reefs. In the St. Martin region, coral reefs contribute significantly, emphasizing the need for prioritized conservation and protection efforts.[78] Proposed solutions encompass ecological measures such as water quality management, sustainable fishing practices, ecological engineering, and marine spatial planning. Additionally, socio-economic strategies involve establishing a regional reef secretariat, integrating reef health into blue economy plans, and initiating a reef labeling program to foster corporate partnerships.[79]

Future Research[edit]

The future of research on marine biogenic calcification is largely focused on marine calcifiers’ responses to ocean acidification and warming ocean temperatures. Many marine calcifiers seem to be resilient to acute changes in the pH and carbonate chemistry of their surroundings.[80] There is a school of thought that many calcifying organisms’ success is dependent on a threshold, or tolerance, of pH levels. This will vary between various species. The genetic and biological mechanisms that allow calcifiers to develop this resiliency to ocean acidification is not yet fully understood. Understanding the response of marine calcifiers to ocean acidification and warming is important for a number of reasons. Firstly, many calcifiers are economically important to humans and provide us with food sources, or support large tourism industries. Secondly, marine calcifiers represent a huge portion of the global fossil record.[80] Studying their responses to climate change induced phenomena can help scientists make predictions for how the oceans will respond in the future to these changes.

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