Modern ReefKeeping

Aquarium Calcification

The water chemistry of closed system aquaria tends to be significantly different than that of seawater; a result of the metabolism of highly unnatural groupings of organisms contained in such displays, the myriad additives and devices employed by aquarists, and the almost limitless potential chemical and biochemical reactions which may occur in such systems.

Yet, little investigation of calcification has been done at all in private aquaria. Some public aquaria have published a few studies; those resulting from the aquaria at Waikiki, the Smithsonian, Monaco, and Biosphere 2 are notable exceptions (Bingman 1998, 1999, Small and Adey 1998 and others).

Without question, the subject of calcium supplementation and their benefits to the growth of coral and coralline algae growth is a well discussed subject in aquarist circles. The best study is shown shortly below.

I want to cover some recent papers of interest to aquarists now

Paper #1

The addition of 2 mM bicarbonate to aquaria containing tropical ocean water and branches of Porites porites caused a doubling of the skeletal growth rate of the coral.

Nitrate or ammonium addition to oligotrophic sea-water caused a significant reduction in coral growth, but when seawater containing the extra bicarbonate was supplemented with combined nitrogen, no depression of the higher growth rate was evident.

We infer that (1) the present dissolved inorganic carbon (DIC) content of the ocean limits coral growth, (2) this limitation is exacerbated by nitrate and ammonium, and (3) adding DIC increases coral calcification rates and confers protection against nutrient enrichment

By adding 2 mM bicarbonate, we changed not only the total concentration of DIC but also the relative proportion of each carbon species as pH increased from 8.10 to 8.27. While CO2 increased only by a third, HCO23 doubled and CO23 became three times as large

Competition for carbon between calicoblastic cells and a larger zooxanthellae population could explain this paradox. After a 32-d exposure to elevated N, the photosynthetic pigment concentration was twice as high as controls.

Growth rate was reduced, but only in the established growth phase and only in seawater with no DIC addition. No difference was found between NO3-N– and NH3-N–enriched corals.

Nitrogen does not affect calcification immediately; only after 2 weeks of elevated N is the symbiont biomass high enough to cause a reduction in calcification.

The reduced growth rate is compensated for by DIC enrichment. These results fit the hypothesis that N affects calcification indirectly by enhancing the zooxanthellae biomass, which in turn limits the supply of DIC available for calcification (Stambler et al. 1991; Marubini and Davies 1996).

Fragments of Porites compressa and Montipora capitata were collected near Coconut Is. in Kaneohe Bay, HI and placed in a flume of flowing seawater. Corals received full natural sunlight

Each day the calcification rate of the assembled coral was measured for 1.5 hours with normal seawater or acidified seawater. On alternate days the treatments were reversed and the corals were exposed to acidified seawater first. There was an immediate drop in calcification

Effect is observed over a wide range of irradiance. Marubini et al. 2001

Changes in CO32- clearly effect the calcification of corals but changes in Ca2+ within the range of natural seawater may not.

Aurelie Moya and colleagues have now characterized the first coral gene that responds to the light cycle; this gene, called STPCA, makes an enzyme that converts carbon dioxide to bicarbonate (baking soda) and is twice as active at night compared to daytime.

The researchers found that the enzyme concentrates in the watery layer right under the calcified skeleton, which combined with studies showing that STPCA inhibitors lower calcification rates, confirms a direct role for STPCA in this process.

Moya and colleagues propose that STPCA becomes more active at night to cope with acid buildup.

The calcification process requires many hydrogen atoms, which during the day can be removed by photosynthesis; at night, however, hydrogen accumulates which increases the acidity of the coral, and therefore STPCA creates extra bicarbonate as a buffer to prevent acid damage.

Paper #2

The rate of calcification in the scleractinian coral Galaxea fascicularis was followed during the day-time using 45Ca tracer. The coral began the day with a low calcification rate, which increased over time to a maximum in the afternoon. Since the experiments were carried out under a fixed light intensity, these results suggest that an intrinsic rhythm exists in the coral such that the calcification rate is regulated during the daytime.

When corals were incubated for an extended period in the dark, the calcification rate was constant for the first 4 h of incubation and then declined, until after one day of dark incubation, calcification ceased, possibly as a result of the depletion of coral energy reserves.

The addition of glucose and Artemia reduced the dark calcification rate for the short duration of the experiment, indicating an expenditure of oxygen in respiration

Artificial hypoxia reduced the rate of dark calcification to about 25% compared to aerated coral samples. It is suggested that G. fascicularis obtains its oxygen needs from the surrounding seawater during the nighttime, whereas during the day time the coral exports oxygen to the seawater. From the results of this study it is concluded that in G. fascicularis the daily rate of calcification is regulated by an intrinsic rhythm whose mechanism is yet unknown

Paper #3

Laboratory experiments were designed to estimate the ingestion rates of the scleractinian coral Stylophora pistillata under varying prey concentrations and feeding regimes and to assess the effect of feeding on the tissue and skeletal growth

Six sets of corals were incubated under two light (80 and 300 umol photons m2s-1) and three feeding levels (none, fed twice, and fed six times per week) using freshly collected zooplankton.

Results showed that the number of prey ingested was proportional to prey density, and no saturation of feeding capability was reached.

Capture rates varied between 0.5 and 8 prey items 200 polyps/hr. Corals starved for several days ingested more plankton than did fed corals. Fed colonies exhibited significantly higher levels of protein, chlorophyll a, and chlorophyll c2 per unit surface area than starved colonies.

Feeding had a strong effect on tissue growth, increasing it by two to eight times. Calcification rates were also 30% higher in fed than in starved corals. Even moderate levels of feeding enhanced both tissue and skeletal growth.

Paper #4
The rate of calcification exhibited the trend previously reported; it decreased as a function of increasing pCO2 and decreasing aragonite saturation state. The dissolution process of calcareous sand does not seem to be affected by open seawater carbonate chemistry; rather, it seems to be controlled by the biogeochemistry of sediment pore water interesting in terms of sand beds and calcium reactors

Paper #5
This study demonstrates that the growth of fragments of the scleractinian coral Madracis mirabilis depends on their origin from within a branch, and suggests that an approximately two-fold difference in physiological age is responsible for the variation in calcification rates

To our knowledge, only two other experimental studies have suggested that polyp position, as a proxy for module age, has biological consequences for scleractinian corals, specifically with respect to reproduction (Kojis and Quinn, 1985) and tissue regeneration (Meesters and Bak, 1995).

In addition, the calcification rates of Stylophora pistillata decrease prior to the appearance of tissue mortality, thus indirectly supporting senescence, or the deterioration of physiological function with age (Rinkevich and Loya, 1986).

Notably, in Acropora palmata, the ability to regenerate tissue after a lesion decreased exponentially during the first 2–3 years of polyp life (Meesters and Bak, 1995), an age range equivalent to the relative difference in tissue age observed to affect calcification in the present study. Despite the cnidarian potential for cell renewal and exchange through gastrovascular connections between polyps, as demonstrated for hydrozoans (Crowell, 1953, Martínez, 1998, Müller et al., 2004), young polyps of branching scleractinian corals appear to be distinct physiologically from older polyps, exhibiting faster calcification and regeneration rates.

In summary, the results of our study contrast markedly with the traditional assumption that age does not matter for scleractinian corals. In Madracis mirabilis, the age of coral tissue appears to have considerable biological importance, and the same is probably true for other coral species that have developmental gradients associated with a branching morphology (Soong and Lang, 1992.)

Together, the senescence of proximal modules reported in many other clonal organisms and the physiological patterns presented here may be a general feature of taxa sharing this body plan.

Paper #6

Low concentrations of calcium ions (Ca2+) are maintained in living cells by the Ca-ATPase calcium pump of the plasma membrane. The presence of a calcium pump in corals has been confirmed and its temperature related breakdown underlies coral bleaching

There is also mounting evidence from stable calcium isotope ratios in the skeleton for involvement of a calcium pump in coral calcification.

Hydrogen peroxide (H2O2) is known to be generated by zooxanthellae during photosynthesis and passes easily through cell membranes. H2O2 is known to cause lipid peroxidation of the plasma membrane and make it leaky to Ca2+ is suggested that the lipid peroxidation makes the plasma membrane leaky to Ca2+ and thus provides a route by which Ca2+ enters cells of the calicoblastic layer.

A model of calcification, following Adkins et al. (2003), is presented in which calcification takes place from the extracellular calcifying fluid (ECF) between the calicoblastic layer and the skeleton.

The Ca-ATPase calcium pump drives calcification by actively transporting Ca2+ into the ECF and setting up pH and carbon dioxide (CO2) gradients that enhance the passive diffusion of CO2 and the formation of calcium carbonate.

It is suggested that, in light, H2O2 produced by zooxanthellae makes the membranes leaky to Ca2+. The calcium pump has to work harder to maintain low internal levels of calcium and thus more Ca2+ would be actively transported into the ECF to be deposited as calcium carbonate

Paper #7

This suggests that organic matrix biosynthesis and its migration towards the site of calcification may be a prerequisite step in the calcification process. ….Taken together, the results of the present study suggest that organic matrix biosynthesis, rather than calcium deposition, may be the limiting factor controlling skeletogenesis, as suggested previously by Wainwright (1963) in hexacorallians and by Kingsley and Watabe (1984) in octocorallians

Paper #8

Variation in skeletal microstructure of the coral Galaxea fascicularis: effects of an aquarium environment and preparatory techniques

The use of aquaria to grow and maintain corals for scientific experimentation has become increasingly popular (see Carlson, 1999) and, with the degradation of coral reefs across the globe, it may soon be necessary for experimental corals to be constantly maintained in this way.

In the past, keeping scleractinian corals alive under artificial conditions for long periods of time was exceedingly difficult, but advances in filtration, lighting, and water systems have made the propagation of these corals in aquaria much easier. However, little is known about the impact of artificial environments upon scleractinian coral calcification, behavior, growth, and reproduction, with direct comparisons between field and aquarium-maintained corals rare

A comparison of the crystalline structure of exsert septa between colonies growing under aquarium conditions and natural, field conditions has revealed that drastic changes to crystal deposition and skeletal formation may occur in corals maintained for long periods in closed-circuit aquaria.

It is evident that calcification in scleractinian corals can be severely affected by changes in the surrounding environment. Much recent attention has focused upon how major changes in environmental conditions such as atmospheric CO2 levels and water temperature affect coral calcification (Done, 1999; Kleypas et al., 1999; Pittock, 1999).

Our results indicate that even relatively small modifications to the surrounding environment, particularly those that are artificially generated, may significantly alter the pattern of crystal deposition and the rate of calcification in captive corals.

Variability in the growth rate of aquarium-kept corals may be influenced by, and dependent upon, what are often considered minor details, such as water quality and movement, food availability (Mortensen, 2001), and subtle changes in light intensity and wavelength (see Carlson, 1999).

As a result of the discontinuous, unordered growth, skeletal porosity was likely to increase. When compared with those growing in their natural environment, a variety of scleractinian corals show a significant reduction in skeletal porosity following containment within an aquarium

The fact that the region of disoriented growth in G. fascicularis extends to an approximate depth of only 1 um in septa sampled from corals kept in aquaria for 26 weeks suggests that calcification and growth rates of these corals may be significantly reduced, with drastic changes in calcification patterns also expected.

In many branching scleractinian corals, gross colony morphology changes dramatically following maintenance in aquaria (see Carlson, 1999), with colonies developing unnatural morphological traits.

In addition to directly affecting the physical processes of coral calcification, suboptimal conditions may also invoke changes indirectly through stress responses. Colonies maintained under aquarium conditions appeared to lose a large proportion of their symbiotic algae.

Loss of zooxanthellae from gastrodermal cells in times of stress is common to many zooxanthellate scleractinian corals (Hoegh-Guldberg and Smith, 1989; Brown et al., 1995), and calcification rates can be greatly reduced through the induced expulsion ofhese symbionts (Goreau, 1959).

These results raise concerns about the validity of experiments, particularly those investigating calcification rates, etc., with corals kept in closed-circuit aquaria, where artificially modified or unnatural environments may have induced atypical patterns of calcification.

Marked differences in skeletal organization between corals sampled from natural and artificial environments highlight the importance of accurately imitating natural conditions in closed-circuit aquaria where experimental corals are to be maintained.

Summary for aquarists to increase calcification

pH – keep above 8.0 up to 8.4-8.6
Alkalinity – want bicarbonate as source in water (via pH or direct addition of carbonate/bicarbonate) and limits calcification – keep high
Calcium – keep within normal range – rarely limits calcification
Oxygen – keep high especially at night
Feeding – feeding (up to 4800 nauplii/l continuous availability) increases tissue and calcification
Use growing tips (younger polyps) for best calcification in fragmentation
CO2 – keep environmental and water levels low (pH, buffering, fresh air, remote algae, reverse daylight, watch calcium reactors)
Water motion – increases calcification by increasing food availability, skeleton density, metabolism, gas exchange, DIC exchange increasing photosynthesis, decreasing hypoxia

No matter what, our aquariums are still not going to calcify normally like reef corals.