The Chemistry Behind the Coral Colony

Keywords: Biomineralization, aragonite, photosynthesis, symbiosis, Zooxanthellae, corals.

Abstract

Corals rely on a wide array of biochemical processes that govern their growth, development, and interactions with their environment. This paper aims to analyze these processes and the chemical drivers behind them, focusing on key mechanisms such as biomineralization, calcification, metabolism, and symbiosis. Biomineralization, driven by the deposition of calcium carbonate into complex structures, is essential for coral skeleton formation and the creation of coral reefs. This process is determined by the intricate interplay of chemical reactions that includes carbonization, ion transport, and proton transports. The development and structure of coral reefs are also heavily reliant on environmental factors such as ocean acidity and symbiotic relationships, specifically with zooxanthellae. Zooxanthellae and coral are mutually dependent, the former benefiting from protection and waste products for its development while the latter relies on zooxanthellae for energy production through photosynthesis. This symbiotic relationship is critical to the metabolic levels and overall health of coral reefs. Coral reefs survive thanks to the interplay of many bioprocesses and chemical reactions that determine every aspect of their life cycle and their function within their ecosystem.

Introduction

Coral reefs are one of the most biologically diverse ecosystems on Earth. These feats of nature can span thousands of miles and house thousands of species of marine life. Coral reefs are made up of colonies of invertebrate coral polyps which construct massive reef structures through a process known as biomineralization. This ability to build calcium carbonate skeletons not only builds the framework of coral reefs, but also provides an essential habitat for many marine species. However, the chemical processes governing such a massive structure are complex and intricately related to their environment.

The production of calcium carbonate for coral skeletons is regulated by a range of factors, including the availability of dissolved ions in the water and ocean acidity. First off, coral polyps must intake a massive amounts of calcium ions to form their skeletons, up to 149 moles of Ca²⁺ per square meter per year (Gattuso, 2015). This dependency on an abundance of calcium ions dictates the conditions required for corals to survive and thrive, which will be discussed in more detail later in this paper. The Ca²⁺ ions in the water must react with a molecule containing carbon, usually carbon dioxide, to form calcium carbonate. This carbon is usually found in the form of either carbon dioxide or carbonic acid. This reaction is illustrated at a base level in Fig. 1 below. The interplay between these factors affects how efficiently corals can secrete calcium carbonate which is directly tied to their growth rate (Cornell, 2016).

Fig. 1. A graphic describing the process through which calcium carbonate is formed (Cornell, 2016).

Another key aspect of coral chemistry is their symbiotic relationship with zooxanthellae, a type of algae that lives within the coral’s tissues. Zooxanthellae provide coral with energy through photosynthesis, converting light, along with carbon dioxide expelled by the coral, into chemical energy, providing nutrients such as glucose and oxygen that the coral need to survive. In exchange, corals supply the algae with protection, carbon dioxide, and nitrogen. The symbiotic relationship between these two organisms allows both to thrive in nutrient-poor environments. However, the balance between these organisms is delicate, and disruptions—often caused by rising water temperatures—can lead to coral bleaching, a harmful event in which corals expel their symbiotic algae.

Biomineralization

Physiology and Mechanism

Marine invertebrates, like molluscs, anemones, and coral, exhibit two different biomineralization processes. The first variant, termed “biologically induced mineralization,” does not depend on specialized cellular or molecular machinery from the organism and instead emphasizes inorganic processes that can be reproduced in a laboratory. The second variant, termed “biologically controlled mineralization,” is characterized by the utilization of an organic matrix framework designed by the organism to control the development and growth of the crystal. Current research is divided on the pathway coral utilizes. In fact, some research suggests coral may be able to control exoskeletal development via both physiochemical processes and an organic matrix (Allemand et al., 2010)!

As the external epidermal tissue, the ectoderm is responsible for the coral’s skeletogenic activity. The cells responsible for calcification are termed calicoblasts and form the thin calicodermis. (An image of the calicodermis and its associated skeletal structure is shown below in Fig 2). The shape of these cells varies considerably, with cup-like cells presenting the highest calcification rates. The region where calcification occurs is also heavily debated; Some suggest the presence of an empty layer between the calicodermis and the skeleton. Others imply a gel-like coating called the “extracellular calcifying medium” composed of proteins and glycosaminoglycans responsible for crystal nucleation. For this paper, we will assume the second hypothesis. Nevertheless, each polyp favours an intracellular mode of calcification, with crystal formation occurring in ectodermal cells before deposition (Allemand et al., 2010).

Fig. 2. The top left image was taken using a transmission electron microscope, which illustrates the interface between the calicoblasts and the aragonite exoskeleton. The bottom left image was captured with a scanning electron microscope, which depicts the external skeletal structure. The rightmost image shows a magnified view of the calicodermis (Allemand et al., 2010).

To form aragonite, the polyp must harbour the right concentrations of calcium and carbonate ions in this layer. The calicodermis, therefore, exhibits selective permeability, which it accomplishes using a combination of tight and leaky junctions that enable paracellular transport to the extracellular calcifying medium (ECM). The space between the skeleton and the tissue has been experimentally determined to be slightly alkaline, with a higher relative concentration of calcium to the surrounding sweater, suggesting metabolic energy is needed to counter the electrochemical gradient of the ion. However, calcium may be able to avoid this and move through a passive transport via an unknown mechanism. The method the calcium cation employs to reach the ECM is also disagreed upon, with four different mechanisms proposed. The first and second routes emphasize a paracellular strategy, with either the diffusion of individual ions or bulk seawater via polyp pulsation (as mentioned in Coral Physics). The third hypothesis focuses on a transcellular approach, with calcium ions spanning two phospholipid bilayers. The fourth method outlines a simultaneous application of the third and the first method (Allemand et al., 2010). A schematic of each hypothesis is shown below in Fig. 3.

Fig. 3. A schematic view of each hypothesis mentioned above outlining the transport of ions through the calicodermis. A) ions traverse through the tissue via a paracellular pathway B) bulk seawater, labelled SW, utilizes a similar pathway through a calicoblast’s leaky junction C) ions instead are supplied through the coral cell itself by a transcellular pathway D) a combination of the pathways outlined in A) and C) (Allemand et al., 2010).

The chemical equilibrium that defines coral calcification employs four molecules. The first, simplest, mechanism is shown below:

Ca²⁺ + CO₃^2- ⇒ CaCO₃ (1)

Unfortunately, the ratio between carbonate and bicarbonate in physiological pH (between 7.5- and 9.0) is extremely low. Hence, this reaction likely does not contribute to calcification in biological systems. The following mechanism is much more prevalent:

Ca²⁺ + HCO₃^- ⇒ CaCO₃ + H⁺ (2)

To maintain homeostasis, any protons produced must be transported elsewhere, usually the coelenteron. There is evidence of a calcium proton pump, in accordance with the third hypothesis, that may allow calicoblasts to remove H⁺ while overcoming the electrochemical gradient of calcium cations. Protons may also be titrated by hydroxyl ion byproducts from zooxanthellae photosynthetic processes and carbon capture. If the source of any dissolved organic carbon (DIC) is instead carbon dioxide, the following mechanism prevails:

Ca²⁺ + CO₂ + H₂O ⇒ CaCO₃ + H⁺ (3)

Due to the increased presence of mitochondria in calicoblasts, the rate of carbon dioxide released from respiration is four times calcification’s uptake. Hence, around 60-70% of the dissolved inorganic carbon (DIC) needed for calcification is provided by internal carbon dioxide, but the relationship between mechanisms determined by equations 2 and 3 is still unclear. There is evidence that calcium uptake and bicarbonate production obey Michaelis-Menten kinetics, which is a series of differential equations that describe the rate of product formation based on the presence of an enzyme. This signals the presence of a carrier-mediated step by an enzyme produced by the coral. Unfortunately, the complex dynamics of the ion transport needed for biomineralization in the coral colony are still poorly understood and are subject to further research (Allemand et al., 2010).

The exoskeleton of coral is made of the orthorhombic polymorph of calcium carbonate: aragonite. As mentioned in our Coral Physics, this crystal analog of calcite is less stable and tends to transform into calcite on the order of millions of years. The initial crystal nucleation is kinetically unfavourable and heavily depends on high concentrations of both calcium and carbonate, as shown in the solubility equilibrium in Eqn. 4 (Allemand et al., 2010). W_calcite is termed the internal calcite saturation state and is analogous to the aragonite saturation state, while K_sp is the solubility product constant.

[Ca²⁺][CO₃^2-] = W_calcite > K_sp = 20 ~ 25 (4)

To tackle this, coral produces a transient thermodynamically unstable amorphous variant to aid the subsequent formation of the more stable aragonite exoskeleton. With Raman spectroscopy, amorphous calcium carbonate (ACC) was found in the nucleation centers of Scleractinian coral; This leads some to claim that ACC is necessary for sustained aragonite nucleation. Nevertheless, further research on the role of the intermediate is needed. Carbonic anhydrase, an enzyme responsible for the conversion between carbon dioxide and bicarbonate shown in the mechanism below, was also found to be instrumental to the regulation of both forms of DIC (Allemand et al., 2010).

CO₂ + H₂O ⇄ HCO₃^- + H⁺ (5)

If carbon dioxide is the primary form of DIC present, carbonic anhydrase can produce additional bicarbonate ions to improve calcification rates. If carbonate is the dominant form of DIC, carbonic anhydrase can accelerate the dehydration of carbonic acid into carbon dioxide and help buffer the excess acidity produced by reactions 2 and 3 described above (Allemand et al., 2010).

Aragonite Crystal Structure

Most corals exclusively use the aragonite polymorph of calcium carbonate to manufacture their skeletons, excluding a few species that include insets of calcite. The crystal can be divided into unit cells with three axes a, b, and c, as shown in Figure 4 below.

Fig. 4. A diagram of the unit cell of aragonite. This structure is repeated throughout the entire crystal. Notice the equal number of calcium and carbonate ions due to their equal charges (Farfan et al., 2021).

Statistically, the molecular basis does not change much between different species of coral, even if they live in very different environments. Volume and unit-cell axis lengths may change minimally, which can have far-reaching implications on the solubility, toughness, and other material properties that define the skeleton. In general, these changes are relatively insignificant between species at a microscopic level outside of a few outliers. On the contrary, coral aragonite is fundamentally different from other biomineralizing marine invertebrates, like molluscs. The difference in axis lengths between these groups is substantial, with coral presenting much larger unit cell volumes. This difference is rooted in each crystal’s direction of growth, with coral extending the c-axis through spherulite formation. The c-axis was found to have a strong correlation with increasing crystal volume compared to the other two axes, allowing coral to be more metabolically efficient in skeletal development (Farfan et al., 2021). This is the basis of a variety of considerations that study coral reefs as a readily available analog of synthetic aragonite and spherulite formation from a material science perspective.

Other alkali earth metals, such as magnesium and strontium, can interstitially replace calcium and cause distortions to the unit cell. Incorporations of beryllium and strontium both increase the volume of the unit cell along the x-axis and distort the trigonal planar structure of the carbonate ion. This relationship was analyzed using the linear regressions shown in Figure 5 below. On the other hand, magnesium can lead to shorter a-axis and c-axis lengths. Key differences between seawater and coral aragonite can somewhat be attributed to differences in these trace metal concentrations. For example, the concentration of beryllium in aragonite is strongly correlated with pH, temperature, and the calcification rate of the polyp (Farfan et al., 2021). In a sense, these inclusions and the dimensions of the unit cell allow marine biologists to examine what environmental and chemical conditions were like during reef formation.

Fig. 5. Each of these graphs depicts a linear regression between a trace metal’s ratio with calcium in the aragonite crystal and the volume of the unit cell. Note that there is no relationship between the atomic mass or volume of each interstitial replacement and the volume of the unit cell (Farfan et al., 2021).

At the micrometer level, the aragonite exoskeleton is composed of spherulites. Spherulites are polycrystalline structures where acicular crystals radiate outwards from a single nucleation site and grow simultaneously outwards in all directions to form a sphere. If the nucleation site falls along a line instead of a point, a “plumose spherulite” may form where aragonite fibres radiate outwards from the line at a specific angle. These tend to look like feathers. A depiction of both a spherical and plumose spherulite is shown below in Figure 6. Each spherulite begins as a group of small acicular, or asymmetric, crystals at the nucleation site termed fibres that eventually grow to form a “sheaf of wheat” structure. Eventually, these fibres will become spheres. Aragonite is described as acicular due to its increased growth rate along the c-axis. In fact, the c-axis demonstrates increased 10 x more growth than the a/b axes via particle attachment and ion-by-ion attachment (Sun et al., 2017).

In both coral and synthetic aragonite, crystal formation is often not perfect, with small but consistent branching under 30° from the crystal lattice’s orientation. This variant of branching is not the same as crystallographic branching, which is responsible for the characteristic >60° bend of snowflakes. This special form of branching is one of the key characteristics of spherulite formation.

Fig. 6. Aragonite can either form a (A) spherical or (B) plumose spherulite. Each line depicts both the direction where the crystal grows and the orientation of the c-axis, which is the primary direction of growth in the unit cell. Each fibre, however, is much wider than the diagram shows and will completely fill up the volume provided. The little coloured insets in both diagrams indicate the different c-axis orientations of that region of the spherulite. Data was gathered using photoemission electron microscopy (PEEM), specifically at the oxygen K-edge π* peak energy, 534 eV. (Sun et al., 2017).

Another key characteristic is the ability of spherulites to fill up large volumes of space. Each aragonite crystal can orient its c-axis in all possible directions to support multidimensional expansion. Non-crystallographic branching is also essential, as some slight branching is necessary to fill up all possible regions of space. This effect can be visualized in Figure 7. The only way for each fiber to fill up more space is to branch out the farther the fiber is from the center.

Fig. 7. Colour-coded diagrams depicting the polarization-dependent imaging contrast maps of two aragonite samples. (Three “bundles” of coral aragonite that demonstrate non-crystallographic branching. This can be seen by the colour range of each bundle (blue-green, purple-pink, orange-yellow) confining themselves to within 30 angles of variation in the c’ angle. (B) Ability of spherulites to grow in all directions from the nucleation site, as shown by the entire range of c’ angles in the Polarization-Induced Color map of a synthetic aragonite spherulite (Sun et al., 2017).

The coral skeleton, specifically, is divided into the centers of calcification (CoCs), acicular aragonite fibres, their consequent sheaths, and eventually a structure often compared to a “feather duster” called the “trabecular.” Each CoC lies inside the septa of each polyp and is composed of various organic compounds and microscopic mineral granules (Sun et al., 2017). Recent studies have suggested the presence of these organic compounds may evidence “biologically controlled crystallization” as the primary pathway to skeletal formation; This organic matrix will be further examined in the next section.

Feedback and Control

The coral colony does have a role in the rate and direction of skeletal formation. The role the colony fulfills, however, is still unknown. Currently, three control systems have been discovered and coral likely practice a combination of the three. The bio-inorganic, “biologically induced” approach elevates the saturation state and pH, facilitates diffusion of Ca²⁺, removes inhibitory compounds, introduces enzymes like carbonic anhydrase, and provides carrier molecules or vesicles for rapid transport of aragonite to and from the ECM. The additive-mediated model, which is like the bio-inorganic model, emphasizes additional growth control other than the saturation state by promoting nucleation, positioning growth faces, or inhibiting growth entirely via some organic additives. The organic matrix template, or the “biologically controlled” model, directly includes organic compounds into the lattice of the biomineral. The arrangement of the functional groups of the inclusions acts as a framework for the skeleton to grow into, in contrast to the physiochemical method of indirectly adjusting ion concentrations. So far, the organic matrix theory seems to be the most appropriate, as most crystallographic studies of coral aragonite have detected various organic inclusions. The identity of these molecules, outside of a few proteins like chitin and galaxin, is still unclear (Tambutté et al., 2011).

Even with such uncertainty over the composition of the matrix, the synthesis is a relatively understood process. Each skeletogenic calicoblast releases key compounds, like acidic polysaccharides, through the cell membrane into the ECM via intracellular vesicles. The function of the matrix is akin to a sheath which restricts the crystal to a certain direction and space. Also, secreted glycoproteins have been found to directly interact with ACC and eventually form a separate layer outside the crystal network. Individual molecules, like calcium-binding proteins, can directly regulate Ca²⁺ availability (Tambutté et al., 2011). A histology of the organic matrix is shown below in Figure 8.

Fig. 8. A) Confocal microscopy of the immunolabelled organic matrix with orange corresponding to the matrix and blue corresponding with calicoblast nuclei. The coelenteron (COEL), symbiotic zooxanthellae (ZOOX), external seawater (ESW), and skeleton (SK) are labelled as follows. B) The same tissue was observed with bright-field microscopy (Tambutté et al., 2011).

Metabolism and Symbiosis

Coral Metabolism

As discussed in Coral Physics of this series, corals have a symbiotic relationship with single-celled dinoflagellates called zooxanthellae that reside in coral polyp tissues. Zooxanthellae perform photosynthesis by using sun energy and water in light-dependant reactions (Fig. 8) to produce ATP and NAPDPH, followed by the Calvin Cyle (Fig. 9) which produces glucose using the products of the latter reaction. The net products of photosynthesis (glucose and O₂) are used for respiration by corals, which in turn produce the reactants (CO₂ and water) needed for zooxanthellae to continue performing photosynthesis (National Oceanic and Atmospheric Administration).

Fig. 8. Figure depicting the light dependant reaction of photosynthesis performed by zooxanthellae (Khan Academy)

Fig. 9. Figure depicting the reactants and products of the Calvin cycle performed by zooxanthellae (Khan Academy)

Carbonate Chemistry

DIC is essential for both photosynthesis and calcification in corals and can be present in the form of dissolved CO₂, HCO₃^-, or carbonate ions (CO₃^2-). The reaction representing the distribution of these species is shown in Eqn. 6 and has the equilibrium constants K₁ and K₂ (Eqn.s 7 and 8) resulting from the acid-base chemistry reactions of inorganic carbon in seawater. Changes in temperature and pH have strong effects on the distribution of these species.

CO₂ + H₂O ⇄ HCO₃^- + H⁺ ⇄ CO₃^2- + 2H⁺ (6)

K₁ = [HCO₃^-][ H⁺] / [CO₂] (7)

K₁ = [CO₃^2-][2H⁺] / [HCO₃^-] (8)

Standard seawater usually has a distribution of 90% HCO₃^-, 10% CO₃^2-, and <1% CO₂. Other biological or chemical processes that produce or consume these molecules also contribute to the molecular distribution. These concentrations of DIC contribute to the supply of CO₂ needed for zooxanthellae to perform photosynthesis and reaches the coral tissue through the gastrovascular cavity which is open to surrounding seawater through the mouth. The gastrovascular cavities of neighboring polyps are also open to each other. This gastrovascular fluid is also exposed to the epithelial layers of the polyps which have two components: the ectoderm and the endoderm. The oral ectoderm is in contact with external seawater, while the aboral ectoderm is in contact with the CaCO₃ skeleton. The ectodermal cells of this layer are primarily responsible for calcification, while the endodermal cells of the oral layers contain zooxanthellae through which photosynthesis occurs. The pathway for carbon through the coral to these sites is hypothesized to be either transcellular, which involves membrane proteins, an energy dependant step, and uptake or efflux from the cell, or paracellular, involving molecular diffusion through lateral cell junctions (Allemand, Frankignoulle, Gatuso, 1999). For more details on the exact mechanism of ion transport, read the previous section “Mechanism and Physiology.”

Carbon Pathways

The DIC supply in corals is carbon-saturated, suggesting that there is a carbon-concentrating mechanism (CCM) to actively absorb HCO₃^- for photosynthesis, as it is the preferred form of DIC for this process. A CCM would ensure that there are high concentrations of surrounding Rubisco, allowing for more efficient fixation in the light independent reactions. The uptake of HCO₃^- involves two anion carriers, one of which is a Na⁺ dependant Ca^- / HCO₃^- exchanger or Na⁺ / HCO₃^- cotransporter. In anemone, DIC is supplied by transepithelial active mechanisms in endodermal and ectodermal cells. This process produces a net efflux of OH^- under light conditions into the gastrovascular cavity leading to a pH gradient of about 0.8 units across the tentacles. It is theorized that if this also happens in corals it could contribute to the buffering mechanism for H⁺ produced by CaCO₃ precipitation (Allemand et al., 1999).

Carbon-Concentrating Mechanism

As mentioned in the previous section, it is thought that corals have a CCM mechanism in order to increase photosynthesis efficiency. Some studies have shown that the symbiosome is acidic, and that when the pH of the symbiosomal lumen is below 6.1, the formation of CO₂ is favored, followed by its diffusion across the algal membrane and thus the accumulation of CO₂ in the algal cell. One study sought to quantify the pH of the symbiosomal lumen and identify the cellular mechanisms of this acidification, specifically, the role played by V-Type proton ATPase (VHA) as a mechanism to acidify intracellular compartments by transporting H⁺ against the concentration gradient. ATP in the coral cytoplasm is used by VHA at the symbiosome membrane and is used to transport protons from the cytoplasm to the lumen where the pH is about 4. This low pH in the lumen allows HCO₃^- to be converted to CO₂ through enzymes called carbonic anhydrases. As a result, CO₂ concentrations increase, and diffuse across the cell wall and plasma membrane, followed by transportation to the site of photosynthesis (Barott et al.,2014).

Relation between Photosynthesis and Calcification

Photosynthesis and calcification both consume inorganic carbon and can take place simultaneously; however, it has been shown to complement and support rather than inhibit each other as would be expected. A study by Kawaguti and Sakumoto (Kawaguti et al., 1948) showed that calcification is higher in the light than in darkness, indicating that calcification might be light enhanced. One hypothesis for this relationship is that photosynthetic uptake of CO₂ reduces the extracellular CO₂ partial pressure in the coral tissue, increasing carbonate saturation for calcium carbonate precipitation. The second hypothesis is called transcalcification. This is where Ca²⁺-ATPase supplies Ca²⁺ to and H⁺ removes from the site of calcification. By removing H⁺ from the coelenteron, the amount of dehydrated HCO₃^- to CO₂ is decreased, promoting calcium carbonate precipitation due to increased pH in the extra cytoplasmic calcifying fluid while also maintaining a high concentration of CO₂ in the gastrovascular cavity for photosynthesis (Allemand et al., 1999).

Effect of Water Motion on Metabolism

Turbulence and wave exposure are known to affect growth forms of corals; exposed regions tend to have more growth than protected areas due to enhanced exchange of CO₂ and O₂ between corals and their environment. The boundary layer is a stagnant layer of water around the organism. If this layer is thick enough, the rate of carbon that diffuses across may be limited. However, increased water flow around the organism causes this boundary layer to be diminished, which may result in changes to photosynthetic and calcification rates. To test this, a study collected samples of A. formosa collected from shallow water and the stir rates were experimentally manipulated to study the impacts on metabolism and growth. The results found that stirred rates were higher than unstirred, with photosynthetic and respiratory rates being about 25% lower in unstirred conditions. The results of calcification and photosynthesis at various stir rates are shown below in Table 1. These results indicate that water motion may impact the rate of diffusion of substances needed for photosynthesis and other metabolic processes by altering the thickness of the boundary layer (Barnes, Dennison, 1988).

Table 1. Rates of calcification and photosynthesis at various stir rates and light conditions (Dennison and Barnes, 1988)

Case Study: Precious Coral

Precious corals, known for their rich and vibrant color, are deep-water branching corals. They differ from other reef-forming corals due to differences in skeletal formation: precious corals’ skeleton consists of calcite while reef corals use aragonite for their skeleton (Macchia et al., 2016) . Falling under the Corallidae family, precious coals have been used to make jewelry and beads due to their beauty. Researchers deduced that corals of the Corallidae family derives its red color carotenoids in their spicules and skeleton. Carotenoids, used for light absorption and photoreception (France, 2017), are pigments used in photosynthesis. These pigments are produced by zooxanthellate, which are the organisms responsible for photosynthesis (Maoka, 2020). In this study, researchers separated spicules and skeleton from soft tissues with a treatment of 1 molar sodium hydroxide (NaOH) at 100℃ for ten minutes. After, with the help of a centrifuge, researchers found that the organic matrix (in the exoskeleton) was orange, illustrating how precious corals’ color is attributed to its exoskeleton and spicules (Fig. 10) (Cvejic et al., 2007).

Fig. 10. Illustrates the red color in precious coral in a) corals, b) dried skeleton after tissue removal, c) dried spicules, d) magnification of dried spicules after NaOH treatment (Cvejic et al., 2007).

The chemical structure of the carotenoid, itself, affects the color precious corals reflect. Through extraction with demineralization (using a mixture of dichloromethane and methanol), researchers determined that the major carotenoid in all samples were in a geometrically trans-position, named trans-canthaxanthin (4, 4’ – diketo-β carotene) rather than the cis-position (Cvejic et al., 2007). They theorized that the cis-position is present, however, it is considered the minor carotenoid in tissues. Additionally, researchers measured the quantity of canthaxanthin (carotenoids) to be, on average, four times greater than in the skeleton due to the comparatively higher percentage of organic matrix in spicules (Cvejic et al., 2007). Physical and chemical properties of carotenoids (i.e. cis vs trans geometric positions) affect which part of the light wavelengths absorbed, thereby affecting its color (France, 2017). Interestingly, carotenoids are produced through feeding, meaning that metabolism plays a huge factor in giving precious corals its red color (Cvejic et al., 2007).

Synchronized Spawning

Synchronized spawning, as the name suggests, refers to the simultaneous release of gametes to reproduce (Fig. 11). Corals can reproduce both asexually and sexually. Asexual reproduction starts with daughter clonal polyps budding off from parent polyps for the purposes of expansion or to start new colonies (NOAA, 2024a). This is primarily caused when the parent polyp hits a size limitation (NOAA, 2024a).

Fig. 11. Illustrating synchronized spawning where corals release gametes simultaneously (NOAA, 2024a).

Interestingly, about three-quarters of all stony corals produce both sperm and/or eggs. When synchronize spawning occurs, all corals release their gametes into the water simultaneously. In the water, the egg and the sperm meet to form larvae known as planulae. Corals produce a over 15 million (Humphrey) larvae due to the hazards of the ocean, such as predators and other dangers in the water (NOAA, 2024a). This causes a fraction of offspring to survive between the formation of larvae and time the larvae settle to the bottom. Corals evolved to spawn in synchrony to compensate for their sessile lifestyle and the large distance between each other (i.e. they are unable to move to meet). Timing the release of gametes must be precise due to the limited viability (of a few hours) (NOAA, 2024b), which is often dictated by changes in temperature or lunar cues (NOAA, 2024a). In a study, researchers illustrate that rising sea temperature causes the maturation of eggs and sperm while moon phases coordinate the timing of mass spawning (Kaniewska et al., 2015). This study found that the lunar cycle, specifically the light intensity from moon phases, increased RNA transcription in genes linked to coral’s circadian clock and light signaling. This helps signal to corals when to release their gametes (Kaniewska et al., 2015). Unlike other organisms who utilize specialized visual structures to see, corals are “likely mediated through photosensitive molecules such as opsins or cryptochromes” (Kaniewska et al., 2015) to receive their cues. One light is detected by opsins, specific G-proteins are activated, affecting the genes that dictate life phase transitions (i.e. the release of gametes). Further evidence suggests that gamete release during synchronized spawning is mechanically caused by this G-protein-coupled signaling pathways, illustrated in Figure 12 (Kaniewska et al., 2015).

Fig. 12. Signaling pathway that results in the release of gametes (due to the presence of moonlight or another signal) (Kaniewska et al., 2015).

Ultimately, once the coral larva forms, they “float in the ocean, some for days, some for weeks, before dropping to the ocean floor” until they find suitable seafloor conditions to attach their substrate to and to start a new colony (NOAA, 2024b).

Ocean Acidification

Chemical Basis of Ocean Acidification

Ocean acidification “refers to a reduction in the pH of the ocean over an exerted period of time, caused primarily by uptake of CO₂ (Ocean Acidification). When the ocean absorbs carbon dioxide, the water molecules react with the carbon dioxide, forming carbonic acid (H₂CO₃), illustrated in Eqn. 9 (Barker, 2012). Carbonic acid then reacts with water (since it is in an aqueous environment), causing carbonic acid to lose a hydrogen atom to ultimately produce hydronium ions and bicarbonate ions (HCO₃) (Barker, 2012) (Eqn. 10). Additionally, since HCO₃^- still has a hydrogen atom, it is also an acid (Barker, 2012) (Eqn. 11). Thus, in water, HCO₃^- donates a proton to water, making hydronium ions and carbonate ions (CO₃^2-)(Barker, 2012). From these Eqn.s, hydronium ions are produced, lowering the overall pH of the ocean (since pH is the measure of how many hydronium ions there are) (Barker, 2012).

CO₂ + H₂O ⇄ H₂CO₃ (9)

H₂CO₃ + H₂O ⇄ HCO₃^- + H₃O⁺ (10)

HCO₃^- ⇄ CO₃^2- + H₃O⁺ (11)

Carbonic acid, however, is a weak acid, meaning that it does not fully dissociate in water (i.e. it is not a fully forward reaction – the reverse reaction occurs as well). Eqn. 12 occurs, allowing the ocean to act as a natural buffer against dramatic pH changes (due to CO₃^2- ‘s ability to neutralize CO₂). Unfortunately, as more CO₂ is produced, CO₃^2- will be consumed more, limiting the ocean’s ability to buffer pH changes. This creates a positive feedback loop until CO₂ production decreases or is limited (Barker, 2012).

CO₂ + CO₃^2- + H₂O ⇄ 2HCO₃^- (12)

This ultimately reduced the amount of carbonate ions available for the biological systems, causing the rate of calcification of marine organism to decrease.

Impact on Biomineralization

Coral reefs are the impressive products of CaCO₃ biomineralization. Ocean acidification not only decreases the net calcification of coral reefs but threatens to dissolve pre-existing reef structures (Erez et al., 2010).  Biomineralization occurs in the calicoblastic site, which consists of the calicoblastic epithelium that makes up the coral ectoderm and covers the coral skeleton, illustrated in Figure 13.

Fig. 13. The process of calcium ion transport and calcification in coral, with the calicoblastic layer positioned above the coral skeleton, with the plasma membrane acting as a barrier between them. The Ca²⁺-ATPase enzyme, embedded in the plasma membrane, actively pumps calcium ions (Ca²⁺) into the calcifying fluid while simultaneously expelling hydrogen ions (H⁺) into the surrounding seawater. This ion exchange maintains a higher pH in the calcifying fluid, which is crucial for the formation of calcium carbonate (CaCO₃). As Ca²⁺ ions accumulate, they combine with CO₃²⁻ to precipitate as calcium carbonate, which is deposited onto the organic matrix.

Ca²⁺-ATPase is the enzyme responsible for ensuring corals have ideal pH conditions and Ca²⁺ concentrations for biomineralization. It does this by removing protons from and transporting Ca²⁺ to the calicoblastic site (Holcomb et al., 2014). At a lower pH, and therefore increased proton concentration, the Ca²⁺- ATPase enzyme must work harder to maintain the proton gradient. The increased demand for removal of protons reduces the efficiency of Ca²⁺ pumping across the membrane of the calicoblastic cells. When the amount of Ca²⁺ ions is not satisfactory for biomineralization, then the coral skeleton is unable to grow.

The regulation of pH by Ca²⁺-ATPase is necessary as the site of coral skeleton formation, the calcifying fluid, requires a specific pH for the conversion of HCO₃^- to CO₃^2-. CO₃²are essential for the coral precipitation of calcium carbonate, the primary mineral in coral skeletons. The increase in protons caused by ocean acidification discussed above causes a shift in the equilibrium shown in Eqn. 13 toward bicarbonate ions, leading to a lower production of CO₃^2-.

HCO₃^- ⇄ CO₃^2- + H⁺ (13)

Conclusion

Coral reefs, the delicate and complex foundation of marine ecosystems, rely on a careful balance of chemical reactions to form and maintain their calcium carbonate skeletons. Yet, this fragile equilibrium is at a constant risk from environmental changes. Corals must be resilient in the face of fluctuating oceanic conditions and increasing threats such as ocean acidification. Corals’ ability to maintain their delicate chemical balances and precise processes represents unique design solutions in supporting one of the most complex, interconnected ecosystems on Earth. Their symbiotic relationship with zooxanthellae not only provides essential energy to fuel calcification but also enables efficient light absorption, maximizing photosynthetic productivity in their vibrant, shallow-water habitats. The process of biomineralization, where corals selectively deposit aragonite, serves a dual purpose of providing structural integrity as well as improved photosynthetic efficiency.

However, this delicately balanced system is very sensitive to environmental changes. Rising ocean temperatures can cause coral bleaching events, where corals expel the symbiotic algae living within their tissues. In addition, ocean acidification disrupts the availability of ions in the water that coral polyps use for their biomineralization. Corals regulate their Ca²⁺ levels to ensure proper biomineralization and growth rates through selective permeability via both passive and active ion transport. Without these abilities, coral structure may be compromised, thus endangering the numerous marine species that are reliant on reef structures and the ecosystem.

Understanding coral biochemical processes reveals the delicate balance they maintain under stress and emphasizes the need to protect coral reefs—not only for their biodiversity but also for the health of broader marine ecosystems amid climate change and human impact.