Modern ReefKeeping

Introduction

The process of calcification, or the accretion of calcium carbonate to form reefs, is one of the distinguishing characteristics of coral reefs and reef aquaria. Both plants and animals calcify, although perhaps it is the corals and the coralline algae that are most noted to aquarists as the builders of limestone skeletons and frameworks.

Octocorals (soft corals, gorgonians, etc.) also calcify by producing internal calcium plates and inclusions (spicules, sclerites); these are occasionally external in the case of the blue and pipe-organ corals, Heliopora sp. and Tubipora sp., primnoid gorgonians, and others.

Other significant calcifying organisms are Halimeda sp. algae, foraminiferans, and mollusks. Even polychaete worms and echinoderms can contain calcium inclusions to strengthen their invertebrate bodies.

Many other marine organisms also frequently and advantageously make use of the vast stores of oceanic calcium. True coral reefs – and the “live rock” used in aquaria – are built on the skeletons and deposits of these calcifying organisms.

Despite some seventy years of research on calcification, the mechanisms by which calcification occur are still not entirely known. Growth in corals and other calcifiers is a much-sought goal of those maintaining reef aquaria

Calcification – General Principles

For the purposes of this chat, I will mainly be referring to the calcification of scleractinian (stony) corals. The body of literature from which to draw is more complete for this group, and it is probably the most appropriate group of interest to aquarists.

Calcification in stony corals is closely coupled to the process of photosynthesis. Photosynthesis in many corals can occur from their symbiotic union with various types, strains and species of single celled algae called zooxanthellae.

Such corals are termed zooxathellate or symbiotic, while those without symbionts such as Tubastraea spp.- the sun corals – are termed aposymbiotic or azooxanthellate (Schumacker, Zibrowius 1985).

Corals with zooxanthellae are found to calcify 3 times faster (on average) than those without (Gattuso et al. 1999). Calcification also takes place in the dark, although it occurs at a much-reduced rate in zooxanthellate corals.

Calcification can be defined as the ability of plants and animals to form calcium carbonate from calcium and inorganic carbon. It is different from the process of skeletogenesis; the process by which corals lay down calcium carbonate to form new skeletal growth.

The Building Blocks of the Reef

For simple purposes, there are only two chemical compounds that form coral skeleton: calcium and inorganic carbon. Calcium (Ca) is predominantly found in one form in seawater. Inorganic carbon can be found in seawater in a number of different forms; carbon dioxide (CO2), carbonate (CO32-) and bicarbonate (HCO3-).

The end product of calcification is calcium carbonate (CaCO3), principally in the forms of aragonite and calcite. Aragonite is by far the commonest form of which stony corals build their skeletons today (although seas have geologically varied between calcitic and aragonitic).

In seawater, the amounts of various inorganic carbon sources of the three forms listed above depend on temperature, pH, and various biochemical processes that can effect concentrations.

CO2 + H2O > H2CO3 > > HCO3- + H+ >CO32- + 2 H+

Although more complex relationships and coefficients exist, they are beyond the scope of this talk. At normal tropical seawater temperature and pH, approximately 90% of inorganic carbon is in the form of bicarbonate, with carbonate (approx. 10%) and carbon dioxide (<1%) occurring in much lower amounts in solution.

It was long assumed that carbon dioxide (from coral respiration) was the preferred carbon substrate for calcification, although it is now known that both coral and zooxanthellae (Gattuso et al. 1999) preferentially utilize bicarbonate although carbon dioxide concentration inside corals is the proximate source of cola skeletons.

In aquaria, the amount of CO2 present may tend to be higher, or subject to greater variations, because of the high animal bioload relative to water volume and surface area and how tight houses are to fresh air as well as the use of calcium reactors.

The Reactions of Calcification

Calcification in zooxanthellate corals requires three basic biological processes:

1) Photosynthesis CO2 + H2O —-> CH2O + H2O

2) Respiration CH2O + O2 —-> CO2 + H2O

3) Calcification Ca2+ + 2 HCO3- —-> CaCO3 + CO2 + H2O

Calcification and photosynthesis have been defined above. Respiration is the process by which organic carbon molecules are broken down to release chemical energy needed for metabolism. The production of CO2 in the last two reactions can conveniently be used to fuel photosynthesis.

Thus, corals tend to make their symbiotic algae very happy by producing a double dose of CO2 in their metabolic activities, even though photosynthesis rates typically outpace respiration and calcification during the day in shallow water corals (P:R ratio > 1), and may therefore depend on additional carbon to meet its requirements

The Biological Processes and Cellular Dynamics of Calcification

Calcification takes place by coral polyps via their lower (aboral) epidermal tissue layer, called the calicoblastic epithelium. This is the area where the outer polyp surface that is in contact with the water joins the skeleton and continues to form the external underside of the polyp.

This layer lifts up from the skeleton in small sections called lappets during skeletogenesis. Lappets form a small pocket of fluid filled space that resides between the skeleton and the calicoblastic epithelium.

In contrast, photosynthesis takes place mainly in endodermal cells lining the polyp body cavity (coelenteron) and the upper exposed surface between polyps (coenosarc). Polyps of some corals may also have zooxanthellae in their tentacle tissue or other areas, but this is less common.

In order to calcify, the calcium and carbon sources must be available to these epidermal cells. Seawater enters the body cavity through the mouth where the needed components are then available for uptake.

The exact pathways for calcium and carbon are not entirely known, but it is likely that the majority of calcium crosses the two major tissue layers by squeezing between the adjacent cells of the polyp’s tissue layers. This is called paracellular transport, and it is driven by diffusion and does not require energy expenditure

Because cells are anchored to each other by tight junctions formed by attachments called desmosomes (Muscatine et al. 1997), only small molecules can pass through (Benazet-Tambutte 1996). The only place where energy is required to transport calcium is when it is passed through a cell instead of around it.

This is called transcellular transport, and it may also occur in all tissue layers, although it does not seem to be required or of a significant amount.

In such cases, calcium channels open into the cell, allowing its entrance. Calcium is then pumped out or exits via exocytosis from the cells into the space adjacent to the skeleton; a type of active transport that requires energy.

However, transcellular transport is required when calcium ions reach the calcicoblastic epithelial cells. There, they must be taken into the cell for calcification to occur.

For photosynthesis, calcium is not required, but carbon sources are typically metabolically respired CO2 from the coral polyp, CO2 dissolved in seawater, and bicarbonate. The coral polyp may also concentrate carbon from seawater derived bicarbonate, buffering itself intracellularly

The carbon source most available and used in calcification is bicarbonate from seawater. For calcification, bicarbonate itself is probably mostly carried to the site of calcification by active transport across cells (transcellular) while CO2, a smaller molecule, may diffuse passively.
Carbonic anhydrase is a well-known enzyme that can catalyze the transformation of this CO2 into bicarbonate within the calicoblastic epithelium.

How Calcification Might Proceed

(McConnaughey 1991, 1996). The process of calcification generates protons (H+ ions) which are swapped in a 2:1 ratio for calcium using the enzyme, Calcium ATPase. The protons are pumped into the body cavity where they lower pH and cause the formation of CO2 from bicarbonate.

In the process of pumping out protons, the area where calcification occurs maintains a high pH that favors the precipitation of skeletal material (similar to the first model). Furthermore, the CO2 released into the body cavity would then be available to fuel more photosynthesis by zooxanthellae.

This process was named trans calcification. While it has been shown that photosynthesis does not actually depend on calcification derived carbon dioxide, it is still apparently complementary and can potentially provide up to 78% of zooxanthellae CO2 needs (Gattuso et al. 1999).

One aspect of pumping protons through a cell is that intracellular pH would be significantly lowered. However, the buffering of carbon concentration mechanisms resulting from the uptake of bicarbonate from seawater releases -OH- ions in the light.

These anions help buffer the constant flow of protons arising from calcification. The result is the formation of water, which could also help maintain turgidity of expanded coral tissue without active or passive removal of materials to establish inward flowing concentration gradients.

Effectors of Calcification

What are the influences over calcification? In other words, what will make corals grow slower and faster? Unfortunately, there is no one good answer.

The complexity and synergism of various factors involved in the metabolic processes of corals are complex and not entirely understood. With many details yet to be worked out, it does seem that the mechanisms of carbon delivery are the crucial factor affecting calcification. These are dependent on temperature, pH and the relative availability of various carbon sources.

It has been shown that fairly significant changes in pH do not dramatically affect the rates of calcification or photosynthesis, presumably because of biological control by the coral

While even slight reductions in seawater pH can cause many enzymatic and cellular processes to be altered, the internal fluids of coral tissue can vary significantly over the course of a day (especially at sunrise and sunset where pH levels within coral tissue can change from 7.5 to 8.5 within minutes of the appearance or disappearance of sunlight (Kuhl et al. 1995).

The availability of various carbon sources and the presence of light and water flow delivery systems can also affect the growth rates of corals. Furthermore, stony corals and their zooxanthellae are bicarbonate users, rather than carbon dioxide users (like true plants, seagrasses, diatoms, microalgae, macroalgae, coralline algae, etc.).

As such, they depend on the increased levels of bicarbonate relative to carbon dioxide in seawater (Al-Moghrabi et al. 1996, Goiran et al. 196, Allemande et al. 1998). Such availability seems to indicate that increased bicarbonate results in increased calcification; less cellular energy is required to convert carbon dioxide to bicarbonate by carbonic anhydrase, and corals (rather than algae) thrive

The effects of other variables, such as energy status from feeding and/or absorption, phosphorous and nitrogen enrichment, growth form, breakage, and other water chemistry attributes are complex and beyond the scope of this article.

I will cover nitrogen more below, but as a general rule phosphate poisons calcification…this is just a given

Included are the substitution of various other divalent cations (such as strontium, radium, etc.), and the incorporation of various organic material (such as chitin) into skeletal material.