Marine biogenic calcification
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 corals can incorporate both calcite and aragonite polymorphs into their skeletons. Some species, like corals and byrozoans, can incorporate other minerals to form complex protein matrices that perform specific functions.
A range of biochemical calcification (biocalcification) mechanisms exist, indicated by the fact that marine calcifiers use 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. In particular, 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 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.
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.
Some of the calcite polymorphs are further subdivided by relative magnesium content (Mg/Ca ratio), with calcite solubility increasing with increasing Mg. The solubility of various forms of CaCO3 differs in seawater; specifically, aragonite exhibits greater solubility compared to pure calcite. This difference in solubility has profound implications for how marine organisms construct their shells and skeletons under varying ocean conditions.
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, bicarbonate, and hydrogen ions. Carbonate and bicarbonate are also deposited into the global ocean by rivers through the weathering
of rock formations.
Approximately 90% of dissolved organic carbon is bicarbonate ions, 10% is carbonate ions, and less than 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. 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 carbonate ions.
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.
Coral reefs, physical structures formed from calcium carbonate, are important on biological and ecological scales to the regions they are endemic to. Their robust calcification abilities have resulted in extensive calcium carbonate deposits, some housing significant hydrocarbon reserves. However, this group only accounts for about 10% of the global production of calcium carbonate.
Mollusks are a diverse group including 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. The resulting minerals exhibit complex shapes and sizes along with being formed within a confined space.
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 magnesium-rich calcite. Magnesium-rich calcite maintains the chemical composition of CaCO3, yet features substitutions of Mg for Ca as calcite and aragonite are mineral forms or polymorphs of CaCO3. Adult echinoderm skeletons consist of teeth, spines, tests, tubule feet, and in some cases, spicules.
The evolution of biogenic calcification and carbonate structures within the eukaryotic domain is complex, 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 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. 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.
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 calcite 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.
Upon reaching the seafloor, CaCO3 undergoes a diagenetic process that ends in either dissolution or burial. 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. 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 formation of biogenic calcium carbonate by marine calcifiers is one way to add ballast to sinking particles and enhance transport of carbon to the deep ocean and seafloor. The calcium carbonate counter pump refers to the biological process of precipitation of carbonate and the sinking of particulate inorganic carbon. This process releases CO2 into the surface ocean and atmosphere across timescales spanning 100 to 1,000 years.
Ocean acidification 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, which has an emissions trajectory that results in atmospheric CO2 reaching approximately 740 ppm in
2100.
Global aquaculture production for shellfish contributed US$29.2 billion to the world economy. 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 can lead to reductions ranging from 35% to 70%. 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.
Tourism is integral to the Caribbean region with the sector contributing to over 15 percent of GDP and sustaining 13 percent of jobs in the region as a whole. In the face of these challenges, the worldwide combined economic value of coral reefs is an estimated average of US$490 per hectare annually. Specific regions showcase the economic significance of coral reefs, with Hawai'i's contributing US$360 million annually to its economy, and the Philippine economy receiving at least US$1.06 billion each year from coral reefs.
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Common questions
What minerals make up marine calcifier skeletons?
The skeletal structures of marine calcifiers are predominantly composed of calcium carbonate minerals, specifically aragonite and calcite. Some species like corals can incorporate both calcite and aragonite polymorphs into their skeletons.
How do mollusks form extracellular mineralization shells?
Mollusks 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. Specialized cells follow genetic instructions to synthesize minerals under non-equilibrium conditions within a confined space.
When did mineralized skeletons first appear during the Cambrian Period?
Skeletal diversity increased during the Cambrian Period with a gradual increase in abundance and diversity over 25 million years. The appearance of mineralized skeletons in various groups marked a significant watershed in skeletal evolution driven by predation pressure favoring protective armor.
Why does ocean acidification threaten shellfish production?
Ocean acidification presents a formidable threat to global shellfish production by reducing calcification rates due to lower saturation states of seawater. Projections indicate that by the end of the century mussel and oyster calcification could witness substantial reductions of 25% and 10% respectively under the IPCC IS92a scenario.
What is the economic value of coral reefs for the Philippine economy?
The Philippine economy receives at least US$1.06 billion each year from coral reefs. This figure represents specific regional economic significance alongside the worldwide combined economic value of coral reefs which is an estimated average of US$490 per hectare annually.