How Coral Reefs Grow

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How coral reefs grow 2009 coralscience.org

How coral reefs growBy Tim Wijgerde

Coral reefs are widely known for their majestic beauty; these porousunderwater mountains belong to the most species-rich biotopes on the planet.They are home to countless vertebrate and invertebrate animals, and have been

documented and visited by many. These unique ecosystems have been createdby the combined forces of billions of tiny invertebrate animals; the coral polyps.After many scientific studies, it has become clear how these animals have beenable to create a Garden of Eden from virtually nothing.

Coral polyps; the bringers of life

Corals are mostly sessile, colonial

polyps, although solitary species exist.

Coral polyps have tentacles, a mouth, agastrovascular cavity and are

connected to one another by commontissue called coenosarc. The outer,cellular layer of a polyp's tentacles ishighly loaded with nematocysts; cellswhich can fire stinging barbs filled withneurotoxins. This allows corals andanemones to paralyze prey, rangingfrom small plankton to even small fish,depending on the size of the polyp.

Figure 1: Coral polyps provide life to a staggering amount of species, such as this yellowfeather duster and deep blue tunicates (photograph: Hans Leijnse).

The polyp gut is a simple sac, and many coral species actually have guts which areconnected together, allowing them to share nutrients. The gut is also the location whereits gonads are located. Along the gut mesenteries, ovaries and testes will produce

oocytes and spermatocytes, which are released during specific times of the year (formore info on coral anatomy read the articlecoral reefs, an introduction').

Many coral species build skeletons, which provide a refuge against predators. They alsohelp newly settled coral polyps to attach themselves onto rocky substrate. These

skeletons, which are secreted by the underside of the polyp skin, eventually created the

colorful reefs which we know today. The basal plate of a polyp is where its skeleton starts(fig.1), which is made from calcium carbonate (or aragonite). The molecules which makeup the skeleton are secreted by the calicoblastic layer or epithelium; the ectoderm or

skin lining the lower part of the polyp. This process consumes significant amounts ofenergy, and the rate at which it occurs is quite slow. Some stony corals may grow about5 mm (0.2 inches) each month, while others such as deep water corals may grow muchslower. This so-called process of calcification is very energy-demanding; this energy is

provided by algae residing in coral tissue. A group of algae from the genus Symbiodiniumhas formed a partnership with corals; these are called the zooxanthellae. They producesugars by using the suns energy, just like higher plants do. We call this processphotosynthesis, and it provides up to 95% of the energy corals need.

When coral reefs die, such as after bleaching due to warm summers, most reefinhabitants die as well, or simply leave the grounds. This is because the corals do not
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A part of the ions splits into CO2 and hydroxide ions (OH-). A major portion of the CO2-

molecules is taken up by the zooxanthellae. The hydroxide ions help stabilize the pH ofthe coelenteron by reacting with protons (H+-ions) into water (H2

Step 2: Transporting the building blocks to the growing skeleton

O).

The next step is to transport the bicarbonate ions to the so-called calicoblastic fluid; thisis the stagnant layer of water located directly beneath a coral polyp, where aragonitedeposition takes place. This process is energy-demanding, which requires an energy

carrier. This is provided by very commonly used molecule by all life on our planet, and itis called ATP (adenosine triphosphate). ATP itself is produced in the power reactors of aliving cell; the mitochondria (fig.3). the calicoblastic cells in the outer skin layer are

highly enriched with these cell organelles, and work hard on a daily basis to allow coralsto build their skeleton. ATP is produced by oxidizing carbohydrates and fatty acids, andthe yielded energy is used to transport mainly calcium ions over the calicoblastic layer(fig.3). The needed carbohydrates are produced by the zooxanthellae, and provide up to95% of the energy budget4,5,6. The transport of bicarbonate ions takes place byexchanging of negatively charged molecules from the external environment (represented

as A-). This principle is called antiport. The pumping of calcium ions over the cellularmembrane is also carried out by an antiport system; the only difference is that the

calcium ions as well as the protons (H+) have to be transported across a gradient. This issimilar to a salmon trying to swim up a river having a strong downward current; this ofcourse uses up a lot of energy. ATP, in the end, is the energy carrier for this processallowing the calcium/H+

-pump to perform its duty.

Figure 3: The deposition of calcium carbonate by the outer skin layer or ectoderm at thelow end of a coral polyp. The bicarbonate ions again diffuse through the mesoglea,although this process is not yet completely understood. The next step is however not

passive, but active; bicarbonate ions are pumped into the calicoblastic fluid by anantiporter system which uses up negatively charged ions (A -). Calcium ions (Ca2+) are

also translocated to the calcifying layer, and H+-ions are pumped into the calicoblasticcells at the same time. As this process works against a chemical gradient it uses upenergy, which is provided by the hydrolysis of ATP to ADP and inorganic orthophosphate(Pi). Note that most bicarbonates originate from the coral cells own metabolism; the

mitochondria exhale CO2, after which the enzyme carbonic anhydrase (CA) catalyzes thereaction to generate bicarbonate ions (HCO3-). Up to 75% of the available bicarbonateoriginates from the coral itself, and not from the water column! Eventually, the


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bicarbonate en calcium together precipitate as calcium carbonate (CaCO3). The releasedprotons (H+

Eventually, the bicarbonate and calcium together precipitate as calcium carbonate(CaCO

) are constantly pumped back into the coral cells to ensure a continuous highpH value in the calicoblastic layer. This remains about 9.3 during the day, and drops toabout 8 at night. This means that stony corals grow mostly during the day (modifiedfrom Furla et al, Journal of Exp. Biol., 2000).

3). The released protons (H+

) are constantly pumped back into the coral cells toensure a continuous high pH value in the calicoblastic layer. This has to do with a veryimportant chemical equilibrium in seawater:

Figure 4: The CO2

Without this high pH level, coral skeleton would dissolve quickly. As pH levels drop, morecarbonate ions are converted into bicarbonate ions. This provides more room for new

carbonate ions, such as those from the coral skeleton. For this reason, corals maintainhigh calicoblastic fluid pH levels to prevent newly produced skeleton from redissolving.The amount of free carbonate ions is called the aragonite saturation state. During theday, the pH level of the calicoblastic fluid lies around 9.3, and around 8 at night. This

means that corals mostly grow during the day! The current rise in atmospheric CO

-equilibrium. As pH levels drop, more carbonate ions are converted intobicarbonate ions. This provides more room for new carbonate ions, such as those fromthe coral skeleton. For this reason, corals maintain high calicoblastic fluid pH levels to

prevent newly produced skeleton from redissolving. The amount of free carbonate ions is

called the aragonite saturation state. During the day calicoblastic fluid pH levels liearound 9.3, and around 8 at night. At these pH levels, calcium carbonate cannot dissolve

properly and precipitates as aragonite (image: Tim Wijgerde).

2-concentration causes oceanic pH levels to drop slowly, as they absorb about 20% of thisgreenhouse gas. When CO2 dissolves in water, it lowers the pH value by releasing H

+-ions. If current CO2-emissions persist, this level will drop to about 7.4 in the year 2150,dissolving entire coral reefs4

Alkalinity, the main source of bicarbonates?

. Long before that, bleaching will have devastated virtuallyall reefs leaving behind barren patches of rubble. Even now, a decline in coralcalcification is noticeable, and calcifying organisms in temperate seas are especially

affected. This is has to with the physicochemical aspects of the ocean. Calcium dissolvesmore easily in cooler waters; think of the common calcium carbonate precipitation onaquarium heaters. This is simply the exact opposite phenomenon.

Although the dissolving of CO2 in the oceans forms a major future threat, this process isvery useful within coral tissue. Next to taking up bicarbonates, coral polyps produce a lot

2 -TDdtt2-8(e)1( 7-9(e)-14(1(d( 7- f)-2()-8(6(a)71(b)197oh)11(i9(s)-14(1(o)-10(y)-ba)71(b)197)-8(6a,


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sufficient bicarbonate ions are still requiredfor normal coral growth. Furthermore,(bi)carbonates help stabilize aquarium pH,especially during night time.

Figure 5: Corals acquire the bulk of their

bicarbonate ions from their ownmetabolism; up to 75%. The deposition ofcalcium carbonate by coral polyps hascreated a habitat for countless species(photo: Hans Leijnse).

Why do corals grow faster during theday?

Laboratory experiments have shown that corals grow a lot faster during the day; why isthis exactly? There are several possible explanations for this phenomenon. The firstprocess which may increase aragonite precipitation is the high production of ATP by the

calicoblastic cells, as they receive significant amounts of carbohydrates from thezooxanthellae; the measured ATP content in Galaxea fascicularis tissue was about 35%higher in light-incubated colonies compared to dark-incubated ones. A lot of ATP meansthat ample energy is available for transporting calcium and bicarbonate ions to the

calicoblastic fluid. The second process which mediates light-enhanced calcification is anincrease in tissue pH levels during the day, as zooxanthellae take up more CO 2. Coralshave less difficulty with depositing aragonite at higher pH-levels, as the aragonite

saturation state increases. The third possible reason for elevated calcification during theday may be the activation of the Ca2+/H+ pump, which is light-sensitive, thereby pumpingmore building blocks to the skeleton (table 1)3

Ca

. In overall, the process of calcification canbe summarized with the following equation:

2+ + 2HCO3- --> CaCO3 + CO2 + H2

Table 1: An overview of the calcium/bicarbonate concentrations and pH in different compartments of a coralpolyp during the day and night. The red figures indicate the primary factors stimulating calcification; the highproduction of bicarbonate ions and high pH in the calicoblastic fluid. The calcium concentration is essential aswell; this is even higher during the day, despite the large flux towards the calcium carbonate matrix. X: no dataavailable (compiled from Furl et al, Journal of Exp. Biol., 2000 and Al-Horani, Marine Biology, 2003).

O

time

point

compartment calcium

(mM)

bicarbonate

(nmol/mgprotein)

pH

constant seawater 10 x 8.2

daytime coelenteron 9.8 12 8.2tissue x 230 8.5

calicoblastic fluid 10.6 x 9.3

nighttime coelenteron 9.9 20 7.6

tissue x 5.9 7.6

calicoblastic fluid 10.2 x 8.1

The fusion between coral and skeleton

Now that it is clear how corals are able to create something beautiful from seemingly

nothing, the next question is how their tissue is connected to the aragonite matrix. Thishas to do with the calicoblastic epithelium, which contains other specialized cells next to


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calicoblastic cells. These other cells are specialized in adhesion, and are calleddesmocytes7

Smart chemistry

. These cells uniquely connect the coral tissue to the skeletal matrix, bymeans of numerous protrusions running into the mesoglea. Figure 6 basically shows onegiant desmocyte, which extends profoundly into the mesogleal connective tissue of the

coral. From the protrusions, countless protein bundles called filaments extend evenfurther into the tissue, effectively creating a very large surface contact area between

coral and skeleton. A desmocyte is comparable to the outer calicoblastic cells as depictedin figure 3, however these anchoring cells are much more erratic in morphology.

desmocytes are connected to coral skeleton with countless fibers which branch out intothe aragonite; these proteins together form the organic matrix. Adding amino acids toaquaria has reportedly increased coral growth, which may be explained in part bystimulation of the organic matrix buildup (see the coral science archive for moreinformation).

The models from this article have beencompiled by scientists, by interpreting

countless complex experiments which involvedusing radioactive isotopes of calcium and

carbon. This allowed the biologists to conductso-called pulse-chase experiments, whichmeans that the uptake and translocation of

chemicals by animals is carefully measured.Coral growth is a unique process, and it showsus how these remarkable creatures haveadapted to the harsh oceanic environment. By

utilizing biochemical processes in a smart way,many corals are able to build a skeleton whichallows them to attach to substrates and hide

from predators.

Figure 6, right: An overview of a desmocyte,which connects coral tissue (mesoglea, m) tothe coral skeleton (lower right) via numerous

protrusions. The protrusions again branch out into smaller fibers (sf). The desmocyte

nucleus (n) is located on the top right, and a mitochondrion (mt) can be seen on far left.The desmocytes are connected to the skeleton through countless fibers which branch outinto the skeleton (plaques, pq). It has become clear that sufficient amino acids arerequired for coral growth, which are ingested through particulate feeding and are taken

up from the water column. The amino acids are used for tissue buildup, and forsynthesizing the organic matrix which branches into the skeleton

(image: Toby Wright).

Figure 7, left: Electronmicroscopic photo of a desmocyte (D)which stretches out into the skeleton. Th


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Chave, KE, Smith SV and Roy KJ, Carbonate production by coral reefs, Mar. Geol., 1975,pp 123140(12)

Furla P, Galgani I, Durand I and Allemand D, Sources and mechanisms of inorganiccarbon transport for coral calcification and photosynthesis, Journal of ExperimentalBiology, 2000, pp 3445-3457(203)

Al-Horani FA, Al-Moghrabi SM, de Beer D, The mechanism of calcification and its relationto photosynthesis and respiration in the scleractinian coral Galaxea fascicularis, MarineBiology, 2003, pp 419-426(142)

Falkowski, PG, Dubinsky, Z, Muscatine, L, Porter, JW, Light and bioenergetics of asymbiotic coral. Bioscience, 1984, pp 705709(34)

Muscatine, L. Porter, JW, Reef corals: mutualistic symbioses adapted to nutrient-poor

environments. Bioscience, 1977, pp 454 460(27)

Edmunds, PJ, Davies, SP, An energy budget for Porites porites (Scleractinia). Mar. Biol,1986, pp 339 347(92)

Muscatine L, Tambutte E, Allemand D, Morphology of coral desmocytes, cells that anchorthe calicoblastic epithelium to the skeleton, Coral Reefs, 1997, pp 205-213(16)

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