Reef Elements Articles

Replacement Chemistry: Supplementation Necessary To Maintain Calcium, Magnesium and Alkalinity Levels In Reef Aquaria.

By Ronald L. Shimek, Ph. D.

The Rule: Supplement Only What Is Used.

The soup that constitutes either natural sea water or any of the various artificial flavors used by aquarists is fairly complex.  Once organisms have been in it for any length of time, its chemistry becomes exceptionally complicated; determining what is in it, in any sort of entirety, is well beyond any kind of reasonable testing and analysis available to hobbyists. 

A well-constructed and maintained reef aquarium is a reasonable analogue of the dynamic natural ecosystem it mimics, and it stands to reason that that such sophisticated aquaria are also multifaceted, changeable and dynamic systems.  An established reef aquarium is a small “slice of life” that duplicates most, if not all, of the biological reactions occurring in the natural system.  Scientists refer to such systems as microcosms, “small universes,” and use them to study the behaviors and interactions of organisms.  Aquarists use them for many things, but the bottom line is that they are similar to the real systems in most essential regards.

An Open Door

Reef aquaria are “open” systems, energy and materials pass freely in and out of them.  The passage of materials is often facilitated by the aquarist in any number of ways, such as by feeding, skimming, water changes, and biomass export, as well as by the introduction and removal of decorative organisms.  Additionally, a large amount of material is passively exchanged with the aquarium’s environment, mostly through gas exchange or evaporation.  All of these mechanisms cause chemical changes in the water environment of the system.

Obviously, the question arises, “How can the aquarist maintain or adjust the system to facilitate the well being of the organisms in it?”       

A lot of excess verbiage may be dispensed with by realizing that the only things that need be replaced or supplemented in “coral reef aquaria” are those materials essential to the health of the organisms that are consumed and removed from the system.  Of course, THE most important replacement is pure water, to replace what is lost by evaporation.  Of all of the levels an aquarist must maintain, the most important is the overall concentration of dissolved materials, and the easiest way to measure and maintain that level is to measure and maintain salinity by ensuring that there is the proper amount of water for the amount of dissolved materials in the system. 

The only other levels that need to be supplemented or maintained are those materials that are consumed by the organisms in their daily metabolism.  At first glance, determining those materials might seem a daunting task.  If one is truly interested in maintaining the levels of everything utilized by all the organisms in one’s miniature universe, it would seem that one would have to monitor just about every substance in that aquarium.  The well-read aquarist will realize that many marine organisms have some ability to absorb nutrients and other materials directly out of the water.  Consequently, it would seem that to keep track of it all would involve continuous, complicated, and detailed chemical testing. 

Fortunately, things are not as bad as they might seem.  Many organisms have been shown in a laboratory to have the capability to absorb materials directly out of the surrounding water; however, in reality, except for a couple of cases, this capability is unimportant.  The organisms that actually utilize absorption as a primary, or even noticeable, source of many materials are primarily bacteria, fungi and algae.  These particular kinds of organisms can only obtain nutrients or basic materials of any sort by absorption.  On the other hand, the absorption of nutrients by animals is very limited, probably because in nature, most of the important things that animals use are not found in high abundances dissolved in water.

Most animals obtain their raw materials and nutrients by a combination of ingestion, maceration, and chemical decomposition, followed by mass transport and then phago- or pinocytosis.  Sounds complicated!  On one level, it is.  But another, equally precise, way of saying all this is: “Animals eat food, digest it, and use a circulatory system of some sort to transfer those dissolved foodstuffs to their constituent cells where those cells actively transport those materials into their interiors.  The vast majority of animals in any oceanic environment obtain almost all of their raw materials by eating.  With very few specific exceptions discussed below, they don’t absorb much of anything other than water.

The major exceptions to the above scenario are found in those animal groups whose constituents obtain the dissolved ions of a few necessary elements for the deposition of minerals for their skeletons.  These groups are the sponges, the scleractinian stony corals, the spicule-forming alcyonarian “soft” corals, and hydrocorals, a few types of tube-dwelling annelid worms (serpulids, and some cirratulids), most mollusks, some crustaceans, all echinoderms, and possibly some fishes.  Dissolved ionic silica in several forms is absorbed by some sponges, while other sponges may absorb calcium ions, or a combination of both silica and calcium ions.  Individuals in all of the other mentioned groups absorb ionic calcium and perhaps magnesium.  In addition to the sponges, some mollusks and crustaceans utilize silica, but they appear to get most of what they need from their diets, and not by absorption of dissolved oceanic ions.        

Alkali Earth Metals

With the exception of silicon, absorbed by a few animals as one or another of many forms of silica, the important absorbed elements are all alkali earth metals.  Without going into detailed chemistry, these elements all form ions with a +2 oxidative state, and because they are in the same column of the periodic table, they have the same number of electrons in their outermost shell of electrons.  This, in turn, gives them many similar chemical properties.  In a simplified sense, at the elemental level they may be considered to differ primarily in size from one another.

Table 1.  Alkali Earth Metals; Column 2 of the Periodic Table.  The Atomic Radii Are In Picometers (10-12 m).
Element	Atomic Number	Atomic Ionic Radius	Symbol
Beryllium	4	41, 59	Be
Magnesium	12	71, 86, 103	Mg
Calcium	20	114, 126	Ca
Strontium	38	132, 140	Sr
Barium	56	149, 156	Ba
Radium	88	162	Ra
Information from: WebElements™, the periodic table on the WWW, URL: http://www.webelements.com/ Copyright 1993-2007 Mark Winter [The University of Sheffield and WebElements Ltd, UK]. All rights reserved.

Of these elements, the important ones for aquarists are calcium and magnesium.  The relative sizes of the ions formed by each element are relatively similar, particularly for magnesium and calcium, and calcium and strontium (Table 1).  This allows an occasional “mistake” to occur, where one element will inadvertently be substituted for another in a biologically manufactured substance.  Barium and radium are also similar in size, but are so uncommon that their substitutions are generally considered to be immaterial.  Strontium is common enough that its inadvertent and incidental absorption by corals causes problems with the calcium metabolism, and as a result the corals have to remove the strontium from their systems.  That removal and the toxicity of strontium is discussed a bit below, but in more detail in another article in this series.

Calcium

One of the most abundant elements on Earth, calcium compounds form more than three percent of the planetary crust.  It is a silvery metal, but is never found in a pure state in nature (Weast, 1966).  In its various forms, calcium is part of the skeletal structure of most animals with a mineralized skeleton.  In those chordates that have bones, the basic mineral component is apatite, which is primarily calcium phosphate.  In most other animals, calcium combines with carbonate to form skeletal structures made of calcium carbonate, either as the mineral aragonite or calcite (Pilson, 1998).  Interestingly enough, there is a third naturally occurring form of calcium carbonate, the mineral, vaterite, but it is never found as a skeletal material.

In thinking about or discussing calcium, aquarists tend to focus on its use as a constituent of coral skeletons.  This is understandable, but it causes a very misleading emphasis; that being on calcium in the skeleton.  Perhaps the best way to put this use in perspective, regarding corals, is to realize that the scleractinian coral skeleton is non-living and wholly external to the living animal.  It is not “dead,” as it has never been alive.  In this regard it is important to realize that all the calcium used in such a coral’s skeleton has to pass completely through the coral before it can be used (Figure 1). 

1. The calcium ion must be absorbed by the coral, either through the outer epidermis or, more commonly, through the digestive gastrodermis. 

2. Then that ion has to pass completely through the coral:

1. That means it first must move through the epidermis or gastrodermis,

2. Then it must cross the largely acellular protein layer between the inner and outer tissues, the mesoglea,

3. Then, it must pass through the epidermis in the region of active skeletal secretion,

4. Then, it must be secreted by that epidermis in such a manner that it will bind with carbonate ions to form aragonite.

Figure 1.  This figure is a diagram showing the passage of Ca+2 ions (blue dots) from the gut cavity (orange), which although enclosed by the animal is really continuous with the environment outside the animal, across the gastrodermal layer (brown), through the mesogleal layer (violet) and through the epidermis (yellow) to be secreted into those very thin and tiny areas of skeletogenesis in the “hot pink” areas just outside the epidermis.  The pale green areas in the diagram represent previously secreted skeleton.  Note, each calcium ion deposited in the skeleton has to pass across 4 cell membranes.  Strontium interferes with this passage.  To block this interference, corals trap strontium and deposit it separately in the skeleton.  Never add or supplement strontium to a reef aquarium.  

The causes of this movement are not wholly understood.  Movement across the mesoglea must be due to diffusion, but the movement through the cells appears to be actively mediated by the coral’s physiology.

While it must come from sea water originally, the calcium for the skeleton has to both enter and then be actively secreted by the coral.

In fact, the use of calcium in the skeleton may only occur after all other physiological “needs” for calcium are met.  There are many physiological uses for calcium ions in the corals metabolic reactions, but probably the two primary ones are 1) in the manufacture and maintenance of nematocysts, and 2) in muscle contraction and relaxation.  Nematocysts are the small “explosive” capsules that corals and other cnidarians use to capture their food.  As the animal will die without sufficient food, and as corals devote more of their body to food capture than do any other animals, the production of nematocysts is tremendously important and it consumes a lot of calcium.  In the interior of an undischarged nematocyst calcium-protein complexes stabilize and maintain the structure of the capsule.  Even small corals produce millions of nematocysts per day, and this accounts for much of the utilization of calcium in the corals’ tissues (See, for example: Tardent and Holstein, 1982; Weber, 1989).

Additionally, calcium is necessary for all coral movement.  In corals, as well as in all other animals, calcium ions must be used to “reset” the proteins in muscles so that they may contract.  Muscles contract by moving long protein strands past one another.  Once the contraction is over, the muscle will remain contracted unless internal cellular environment changes allowing the proteins to be pulled back to their “relaxed” state.  High concentrations of calcium ions must be present in the muscles to allow this to occur (Prosser, 1991).

These two uses of calcium, in nematocyst production and in muscular physiology, are interfered with by excessive amounts of strontium.  This is why 1) evolution has given corals a way to remove strontium from their cells, and 2) strontium should never be added to a marine aquarium.

Once calcium carbonate precipitates in the coral’s skeleton, the coral cannot redissolve it to use it again.  It is therefore lost to that animal.  Consequently, the regulatory biochemistry of coral tissues will not allow calcium to be utilized in skeletal production unless there is an internal excess of it.

Magnesium

Magnesium is one of those good news, bad news things.  The good news is that magnesium is pretty easy test for, maintain, and add to reef aquaria.  The bad news is that you REALLY don’t want to go overboard with it.  Magnesium is very similar to calcium, not surprisingly as they are adjacent to one another in the column of the periodic table (Table 1).  It is also abundant in the Earth’s crust, and like calcium, it is never found as pure uncombined metal.  When manufactured as a metal, it is a silvery white metal that burns with a dazzlingly bright flame (Weast, 1966).  Chemically similar to calcium, and being found in relatively high concentrations dissolved in sea water, magnesium has many metabolic uses.  However, the cautionary note is important:  Although important and necessary in its own right, magnesium in excessive amounts interferes with calcium metabolism, and may cause severe problems.  EXCESSIVE, in this context, means concentrations significantly above the normal sea water concentration of magnesium.  Magnesium concentrations of up to about 1500 ppm probably won’t cause any long term damage.  Concentrations above that level, roughly 120% of normal, should be avoided.

As an example of how magnesium’s interference with calcium is particularly evident and impressive, consider that one of the ways to immobilize invertebrates in research, when it is necessary to examine them without the disturbances caused by muscular activity, is to flood the animals’ bodies with magnesium ions.  This flushes away the calcium and causes paralysis.  As an example, in teaching and research, a common and widely used “anesthetic” or “narcotizing agent” for invertebrates is not an anesthetic or narcotic at all.  It is simply a solution of MgCl2 in distilled water that is isosmotic with sea water.  (NOTE: SUCH A HIGH CONCENTRATION OF MAGNESIUM SHOULD NEVER, AND LIKELY NEVER WOULD, BE USED IN AN AQUARIUM SETTING).  Such a solution has the same total ionic level as sea water, but ALL of those ions are magnesium.  If an animal that doesn’t have a waterproof integument is placed in it, the magnesium rapidly infiltrates the organism’s cells and displaces the calcium.  As a result of this the animal is not able to utilize any of its muscles.  The organism becomes flaccid and may be handled without it responding to that handling (See, for example: Strathmann, 1987; Ruppert, et al., 2003).  If, in some situations, after this treatment, the animal is relatively rapidly returned to normal sea water, the magnesium ions may be flushed out and the poisoning reversed.  If the magnesium and calcium levels do not return to normal, however, the animals will die.  This relaxation unto death may well occur with some delicate animals if the magnesium level in a reef tank becomes excessive.  The critical level will vary with the animal and its condition, so it is better to err on the side of caution and maintain magnesium levels near those that are normally found in sea water.  

Magnesium carbonate is found in trace amounts in many animals’ skeletons, particularly if the primary skeletal material is calcium carbonate (Ruppert, et al., 2003).  Magnesium-calcite, a mineral consisting primarily of the calcitic form of calcium carbonate, but incorporating sizeable amounts of magnesium, is found in many echinoderm skeletal ossicles.  It is not known if the presence of magnesium in these ossicles is necessary or is simply the result of an artifact of the similar chemistry of calcium and magnesium in the internal echinoderm tissue environment.  Unlike scleractinian corals, the skeletal components of echinoderms are all secreted internally in tissues and except in a few special cases, such as the large spines of pencil urchins, can be redissolved and remodeled.

Magnesium appears to be necessary in normal amounts in aquaria for one simple reason: regulation (See any good physiology text, for example, Prosser, 1991, for a detailed discussion of this process).  In brief, in all marine biological systems where dissolved calcium is used, the chemically-similar dissolved magnesium is always present in relatively large concentrations.  Because of this, all biological systems that utilize calcium are “adjusted” and proceed as if the magnesium is always present in “normal” concentrations.  If the magnesium concentration is abnormally low, these reactions respond as if a brake is released, and tend to accelerate abnormally.  Conversely, if the magnesium concentration is abnormally high, the reactions will slow down.  For this reason, it is critically important to maintain magnesium at normal levels.

Alkalinity

Alkalinity is a troublesome term in that it doesn’t really measure a “thing.”  Because, in the early days of chemistry, “alkali” was considered to be the opposite of “acid,” the term “alkalinity” was considered the opposite of the term “acidity.”  In this case the term alkalinity was applied to, and measures, a “process.”  In oceanographic situations, the term “alkalinity” was coined to describe the ability of a sea water solution to “neutralize” a strong acid by turning all of the various ionic forms of carbonate into fully neutral materials (Pilson, 1998).  This means it was measured by using a strong acid, which produces a lot of  H+, to change all of the bicarbonate ion, HCO3-, and carbonate ion, CO3=, to water and carbon dioxide.  There are many ways of conceptualizing alkalinity, but perhaps the easiest is to consider it as the sum of all reactions that can react with, or produce, CO2.  So, in the simplest conception, one could consider alkalinity a measure of all of the types of carbonate ions in solution.  Unfortunately for this conception, however, during the acidification process a number of other negatively-charged ions also get oxidized, primarily borate, B(OH)4-, and hydroxyl, OH-, ions.  In natural sea water the amounts of these ions are relatively constant.  That is not the case in artificial sea water where the amounts of these ions will vary with the formulation.  Additionally, in some natural situations, other ions may contribute to alkalinity.  Nonetheless, we normally consider alkalinity as a measure of the amount of dissolved carbon dioxide, or carbonate, in the water, and we adjust it by using materials that either contain soluble carbonate in various forms, or increase carbonate solubility.

Alkalinity measures an exceptionally important series of ions in sea water media containing living things.  There are two reasons for this importance.  Relatively high alkalinity means relatively high concentrations of those ions that help maintain the pH at levels that are necessary for good animal health; additionally, those same high alkalinity levels in many ways drive the solubility of calcium, and also determine the mineral form of calcium carbonate that is precipitated.  At high alkalinity, dissolved carbonate ions are supersatured in oceanic water by a factor of three and a half to four times what they would be if the system was at “thermodynamic equilibrium.  Under those conditions, the mineral aragonite is the form of calcium carbonate formed.  At lower alkalinity, where the supersaturation factor is less than threefold, calcite will the mineral form of calcium carbonate that is formed.  As most marine animals utilize aragonite rather than calcite, it is necessary to maintain the alkalinity at a reasonably high level otherwise the animals will not be able to form skeletal materials.  There is a fine line in this maintenance; when the alkalinity is excessively high, inorganic calcium carbonate precipitation can proceed at a correspondingly high rate.  In nature, this may result in precipitation of calcareous oolitic sands in shallow waters.  This is not a big deal.  In aquaria, however, such inorganic precipitation can mean that there is mineral precipitation inside of the plumbing and pumps, and that may well become a serious problem.

Figure 2.  This image shows a retreating glacier.  The human-caused addition of carbon dioxide to the atmosphere is doing more than just warming the earth.  Increases in atmospheric concentration of CO2 result in wholesale changes in some marine oceanic chemical parameters.  One of these is alkalinity, which is dropping far faster than organisms can accommodate or adjust to.  If that drop continues at the present rate, corals will loose the ability to form their skeletons by the middle of this century.  At that point, stony corals will become extinct.

Recently, there has been a tendency for some ill-informed aquarists to tend to maintain the pH of their systems at an abnormally low level.  At such levels, the amount of calcium that may dissolve in sea water is quite a bit higher than normal.  And, “Gee, that sounds good.”  But, those aquarists don’t follow through with the thought.  “Why is that calcium level higher?”  It is because the system is more acidic.  And as the system becomes more acidic, the corals and other animals that need to precipitate calcium carbonate for their skeletons, find it physiologically and chemically impossible to do so.  In effect, the maintenance of pH at abnormally low levels is an experiment that mimics the change in the oceans with the advent of increasing carbon dioxide concentrations.  This progressive change in the world’s oceanic chemistry will result in the death of all corals, probably by 2100, and quite possibly much sooner (See Kleypas, et al., 1999; Orr et al., 2005 for starters, and follow the literature trail to find more information).  It seems foolish to do much the same thing on a smaller scale in one’s aquarium.  Maintain the pH at normal sea water levels, 8.4 or higher, and maintain the calcium and alkalinity properly!!!

Levels – A Moving Target

One can find a number of suggested levels for the various factors mentioned in this article.  Natural levels of calcium on reefs vary from about 300 ppm to about 450 ppm, with about 420 ppm as a reasonable target (Pilson, 1998).  Likewise magnesium varies, but not as much as calcium, probably because it is not as biologically active as is calcium.  A good target level for magnesium is about 1280 ppm.  Finding an appropriate level for alkalinity presents a problem.  Alkalinity varies drastically from place to place in the oceans; it also varies dramatically from time-to-time at the same place due to apparently “minor” changes in temperature, organism respiration, and other perturbations.  Additionally, and most importantly, the increase in atmospheric carbon dioxide since the start of the industrial revolution has drastically altered oceanic carbonate chemistry including alkalinity.  At the present time, a reasonable value for overall oceanic shallow-water alkalinity would be about 2.5 meq/L (7 dKH) to about 3 meq/L (8.4 dKH).  Physiologically, this value is probably far too low!  There is documentary evidence that the alkalinity of the world’s oceans has dropped by about 50% in the last 250 years (Kleypas, et al., 1999; Orr, et al., 2005).  In other words, the alkalinity of the oceans prior to mankind’s mucking with the atmospheric CO2 levels was 5 to 6 me/L (14 to 16.5 dKH).  Levels in that range would probably be best for the animals in the system.  However, in many aquaria, such levels may result in undue precipitation of calcium carbonate.  Aquarists should maintain alkalinity as high as they can without damaging their systems.

References:

Kleypas, J. A.,  R. W. Buddemeier, D. Archer, J.-P. Gattuso, C. Langdon,  and B. N. Opdyke. 1999.  Geochemical Consequences of Increased Atmospheric Carbon Dioxide on Coral Reefs.  Science. 284: 118 – 120.

Orr, J. C., V. J. Fabry, O. Aumont, L. Bopp, S. C. Doney, R. A. Feely, A. Gnanadesikan, N. Gruber, A. Ishida, F. Joos, R. M. Key, K. Lindsay, E. Maier-Reimer, R. Matear, P. Monfray, A. Mouchet, R. G. Najjar, G-K. Plattner, K. B. Rodgers, C. L. Sabine, J. L. Sarmiento, R. Schlitzer, R. D. Slater, I. J. Totterdell, M-F. Weirig, Y. Yamanaka and A. Yool. 2005. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature. 437: 681-686.

Pilson, M. E. Q. 1998.  An Introduction to the Chemistry of the Sea. Prentice Hall, Inc. Upper Saddle River, NJ. 431 pp.

Prosser, C. L. 1991. Ed. Environmental and Metabolic Animal Physiology. Wiley-Liss, Inc. New York. 578 pp.

Ruppert, E. E, R. S. Fox and R. D. Barnes. 2003. Invertebrate Zoology, A Functional Evolutionary Approach. 7th Ed.  Brooks/Cole-Thomson Learning.  Belmont, CA. xvii +963 pp.+ I1-I26 pp

Strathmann, M. F. 1987.  Reproduction And Development Of Marine Invertebrates Of The Northern Pacific Coast. University of Washington Press. Seattle. 670 pp.

Tardent, P. and T. Holstein. 1982.  Morphology and morphodynamics of the stenotele nematocyst of Hydra attenuata Pall. (Hydrozoa, Cnidaria).  Cell and Tissue Research.
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Weast, R. C. 1966. Ed. The Handbook of Chemistry and Physics. 46th  edition.  Chemical Rubber Company.  Cleveland , Ohio.  pages B-104; 199-120.

Weber, J. 1989.  Nematocysts (stinging capsules of Cnidaria) as Donnan-potential-dominated osmotic systems.  European Journal of Biochemistry 184: 465–476.
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