Strontium And The Reef Aquarium

Much has been written about the need for strontium in reef aquaria. Some authors have described it as an important additive,1 while others have described it simply as a poison.2,3 Why are there such divergent opinions on strontium? Partly this is because there have been few, if any, scientific studies of the effects of depleted strontium on organisms that most reef aquarists maintain. Partly too it may be because many aquarists confuse the idea that an organism “needs” an ion in solution with the idea of “needing a supplement.” Finally, I think many authors have, perhaps incorrectly, extrapolated observations from one or more reef aquaria to others that may have a totally different strontium balance.

This article on strontium is the second of several that delve into a variety of issues involving magnesium and strontium. Calcium, magnesium, and strontium are very similar chemically. So similar, in fact, that they get in the way of each other in a variety of situations, and that is part of the reason why these ions merit interest by aquarists. This article details the nature of strontium in seawater, and it describes which marine organisms are known to need strontium and for what purpose. It also describes which marine organisms are known to take up strontium (such as corals), whether they “need” it or not. Finally, it describes how strontium is added, measured, and removed from marine aquaria.

Next month I hope to extend these articles with an analysis of how much magnesium and strontium are delivered to aquaria in limewater (kalkwasser). My aquarium has not become depleted in strontium despite not having added strontium supplements for several years. Combine this practice with the fact that calcification is known to remove strontium from solution, and one can only conclude that strontium must be entering my aquarium in some way. Limewater seems the most likely way, and I’ll report on such matters next month.

Strontium in Seawater

In full strength seawater (S=35), strontium is present at approximately 90 μM (μM is short for micromolar, which is a measure of the actual number of ions present, as opposed to ppm (parts per million), which is a measure of the mass of ions present). By concentration, it is the fifth most abundant cation (positively charged ion) behind sodium (Na+; 469,000 μM), magnesium (Mg++, 53,000 μM), calcium (Ca++; 10,300 μM), and potassium (K+; 10,200 μM). In mass units, it is present in seawater at about 8 ppm.

The strontium concentration is largely constant across the oceans, with local variations of only 2-3% relative to calcium.4, 5 As salinity varies in lagoons and estuaries, of course, the concentration of strontium rises and falls accordingly. It was long thought that the strontium concentration was also unchanged with depth,6 but more recent measurements have shown that the strontium concentration rises somewhat with depth due to the action of acantharians that deposit strontium sulfate skeletons, slightly depleting the surface of strontium. As these free floating organisms die and settle to the bottom, the deeper waters become enriched in strontium as the skeletons dissolve.4

Strontium, with a molecular weight of 87.5 grams/mole is significantly heavier than calcium (40 grams/mole) or magnesium (24.3 grams/mole). In physical size, a strontium ion is about 13% larger than a calcium ion, and 70% larger than a magnesium ion (Figure 1).

Figure 1

Comparison of the size of various ions. Clockwise from the top left, these are magnesium (Mg++), calcium (Ca++), strontium (Sr++), carbonate (CO3–), and sulfate (SO4–) ions.

Strontium is present in seawater as the Sr++ ion, meaning that it carries two positive charges, just as calcium and magnesium do. Most of the strontium is present as the free ion, with only water molecules attached to it.7 A small portion (about 10%) of the strontium is present as a soluble ion pair with sulfate (SrSO4), and much smaller portions are paired with bicarbonate (SrHCO3+), carbonate (SrCO3), fluoride (SrF+), borate (SrB(OH)4+), and hydroxide (SrOH+).7 Habib Sekha (owner of Salifert) has suggested that strontium in seawater may be ion paired to sulfate to a much greater degree than suggested above, but I have not seen strong evidence to support that hypothesis.

The average residence time for a strontium ion in the ocean is on the order of 10-20 million years.7 That time is similar to magnesium (20-50 million years) and is substantially longer than that for calcium (a few million years) and aluminum (100 years). It is also substantially less than sodium (about 250 million years). In a certain sense, this is an indication of how reactive strontium is: it stays in seawater a long time because it’s fairly unreactive, but it does get taken out of solution through various biological and chemical processes more readily than sodium does.

How does the reactivity of strontium compare to calcium and magnesium? As mentioned in the introduction, they are all very similar chemically. Size is the primary difference, and that size difference actually causes some chemical differences, especially the solubility of certain strontium salts. Strontium carbonate (the primary crystalline form of which is called strontianite) is less soluble that calcium carbonate, which in turn is less soluble than magnesium carbonate. In part, this may be the reason that strontium gets into calcium carbonate skeletons of organisms, as is detailed more below.

Interestingly, strontium sulfate (also known as celestite) is much less soluble than calcium sulfate and magnesium sulfate. While none of these are actually saturated in seawater, strontium sulfate is close, at 30% of saturation.8 That difference is important, and in fact some organisms (such as the acantharians mentioned above) use strontium sulfate as a skeleton (which is detailed more below).

Organisms That Use Strontium: Acantharia

One of the main users of strontium in the ocean are the Acantharia.4, 9-16 These beautiful, free floating unicellular microorganisms are related to radiolaria. They have radiating spines of strontium sulfate that are largely external to the central cytoplasm. Inside the “body”, the spines are connected. The end result is something that looks like a microscopic sea urchin.

Acantharia live in the upper regions of the oceans where they deposit their strontium sulfate skeletons. Presumably, these skeletons are protected from dissolution somehow, perhaps with an organic coating as the radiolarian Sphaerozoum punctatum does, described in the next section. When Acantharia die and sink, the protection of the strontium sulfate is lost, exposing the strontium sulfate to the open water, resulting in dissolution. They are populous enough that they are an important part of the strontium cycle in the oceans (which also includes river and hydrothermal vent input), taking strontium from the surface and delivering it to depths of up to 900 m, where most have completely dissolved.14,15

Are there Acantharia in reef aquaria? I don’t know. In aquaria that use natural seawater they might be seeded into the aquarium every time there is a water change. If there are appreciable numbers in some aquaria, then they might well provide a significant sink for strontium in those aquaria. In fact, they could cause rapid depletion of strontium. In other aquaria, for example those that use heavy skimming and artificial salt water, the likelihood that they are present in important numbers is likely much smaller. This potential difference between aquaria is one of many ways that the strontium balance may vary between different aquaria.

Organisms That Use Strontium: Radiolaria

Certain species of radiolaria also use strontium sulfate, despite having silica skeletons. The radiolarian Sphaerozoum punctatum, for example, release flagellated swarmers during reproduction which contain crystals of strontium sulfate (celestite).17-19 These crystals are deposited inside a cytoplasmic vacuole, and have a 50-100 nm thick coating of organic material on it. This coating may reduce the likelihood of dissolution of the celestite, and may be a clue as to how the Acantharia keep their strontium sulfate spines from dissolving. What purpose these crystals serve to the S. punctatum is unknown, but it may be an antipredatory defense.

Organisms That Use Strontium: Gastropods

Some gastropods, such as the sea slug Aplysia californica, have a clearly defined requirement for strontium in the water. When they are grown in artificial seawater lacking strontium, they develop deformed shells and statocysts (that lack a statolith).20 Statocysts are balance organs in many invertebrates. They consist of a fluid-filled sac containing statoliths that stimulate sensory cells and help indicate position when the animal moves. Statoliths themselves are small solid granules, and are made largely of calcium carbonate in Aplysia californica.

These organisms are quite sensitive to reductions in strontium, with a drop of only 1 ppm causing a detectable difference in development. For this organism there is a critical window of strontium exposure around day 4 of its life. Strontium added after that point cannot make up for the poorly developed statolith, which causes erratic swimming.

How exactly the strontium is used in the deposition of shells and the statolith is not known, but it is apparently somehow related to calcification.21 The statoliths of A. californica raised in normal seawater have been shown to contain elevated strontium.22 In the absence of strontium, the calcification is decreased by 80%. At low strontium levels, calcification was not significantly reduced, but the shell and statolith were still defective. These researchers concluded “Although the role of strontium in embryonic calcification of A. californica remains enigmatic, these data suggest that strontium affects a highly discrete regulatory component because these more general indicators of calcification and differentiation are unaffected by its absence.”

Organisms That Use Strontium: Cephalopods

As in Aplysia californica, strontium appears to be important to the development of at least 7 species of cephalopod.23,24 In all of these, the aragonite statolith develops properly in artificial seawater containing 8 ppm strontium, but not in artificial seawater from which strontium is absent. The end result for cephalopods raised in seawater lacking strontium is uncontrolled swimming due to the poor statolith development. For cuttlefish (Sepia officinalis) the cuttlebone also develops improperly in the absence of strontium.

Organisms That Use Strontium: Corals and Calcerous Algae

Organisms that calcify (that is, deposit calcium carbonate skeletons) are known to incorporate strontium into them. This deposition may be:

Intentional for positive reasons. That is, the strontium serves a useful purpose. Perhaps the purpose is to initiate or maintain calcium carbonate precipitation in some fashion. Some organisms described above clearly fall into this category.
Intentional for negative reasons. That is, the organism wants to get rid of potentially toxic strontium and depositing it into a CaCO3 skeleton is one way to accomplish that task).
Accidental. That is, the deposition of strontium serves no real purpose for the organism, but is simply the result of the fact that strontium looks rather like calcium, and gets into pathways intended for calcium and becomes deposited into the skeleton. This is obviously the way that strontium gets into abiotically deposited calcium carbonate.

In the abiotic precipitation of calcium carbonate from seawater (by the slow addition of carbonate), strontium is incorporated into the growing crystal at almost the same ratio to calcium that is present in the seawater. That is, about 103:1, Ca:Sr (at 25 °C).25 Interestingly, he amount of strontium incorporated is slightly lower at higher temperatures (105:1 at 30 °C), and slightly higher at lower temperatures (97:1 at 10 °C).13,25

This temperature dependence has lead many researchers to investigate, and find largely true, the idea that the strontium to calcium ratios in corals might be used as a temperature probe for ocean temperatures. Most interestingly, it has been extended into ancient coral skeletons, where actual temperature measurements are lacking.26 Many factors have complicated these studies, such as

Different corals incorporate different ratios at the same seawater temperature and strontium levels,
The same corals can deposit different ratios during the day and the night
Different parts of the skeleton of a single coral may have different ratios
The presence of zooxanthellae can significantly perturb the ratio.25
The incorporation of strontium is strongly dependent on the strontium concentration in solution (which may vary over geologic time, with depth, and with salinity)

Other related measurements (such as the incorporation of various oxygen isotopes into the calcium carbonate) may ultimately turn out to be more useful for temperature estimation. The ratio of the different strontium isotopes (molecular weight 87 and 86) in deposits has also been suggested to be a measure of the weathering of land, since the ratio in rivers is different than in the ocean.27

In any case, there are many fascinating studies reported related to strontium in corals and other calcium carbonate deposits. While many of these are only tangentially related to aquarium issues, some of the basic issues are important for understanding the strontium balance in aquaria.

In an old (1957) study28 of 900 samples of calcium carbonate from many different organisms, it was noted that:

“Analyses of limestones, reef cores, limestone precursors, and fossils indicate that replacement and recrystn. lower the Sr/Ca ratio”. This fact is important because if calcium carbonate is the ultimate basis for calcium additives to aquaria (such as CaCO3 in CaCO3/CO2 reactors, or CaCO3 heated to form lime that is later used to make limewater), then the amount of strontium present in these ancient deposits will control the amount getting into aquaria. If these deposits are deficient in strontium, then an aquarium using them may also become deficient.
“… reef corals, and green aragonite algae all have high Sr/Ca ratios that may be related to rapid deposition of carbonates associated with photosynthesis processes”. Again, if reef corals are not the ultimate source of CaCO3 used to make calcium supplements, then the levels in the aquarium may drop over time, just as in (1).
“The Sr/Ca ratio is characteristic of a species or a taxonomic group….” In this case, if the organisms used to make the original CaCO3 are not the same species that are depositing CaCO3 in the aquarium, then the relative amount of strontium in the supplement may be too high or too low, allowing the aquarium to become enriched or depleted over time. The same could be said for the deposition temperature.

Many recent studies have shown the Ca:Sr ratio for many corals at typical tropical reef temperatures and at normal seawater strontium concentrations to be in the range of 100:1 to 120:1. When looked at very closely, it has been noted that corals do actually seem to incorporate slightly more strontium than happens abiotically under the same temperature and ambient strontium conditions (about 103:1 (at 25 °C).25 Is that because the corals “want” the strontium? Or just an artifact of the pathway that corals use to get calcium deposited29 into calcium carbonate? The answer is unknown, but there are a few clues buried in other studies, and these are described below.

Do Corals Need Strontium?

Over the past decade, a number of advanced aquarists have indicated that certain calcifying corals in their aquaria have responded positively when they have added strontium. Julian Sprung, for example, indicated that in one of his aquaria, strontium boosted growth considerably. Unfortunately, he does not report strontium levels from that aquarium.1

The several possible reasons for corals to take up strontium were presented earlier in this article. These reasons range from needing strontium for some purpose, through not caring one way or the other about strontium, to depositing it only as a means of eliminating it from their bodies. What does the scientific literature have to say on this subject? Unfortunately, not enough. There has never been a published study that showed exactly what happens to the health of corals (as measured via calcification rates or any other means) when strontium is absent.

There have been numerous studies that have shown what happens when strontium is raised above natural levels. In those studies, the amount of strontium incorporated rises linearly with strontium concentration up to at least 300 ppm in Stylophora pistillata.30 It has also been shown that calcium and strontium uptake may use the same pathways, since they are both inhibited by the same organic molecules that block certain protein transporters.30 Depending on the conditions, calcification can increase on adding extra strontium, as it does with added calcium.31,32 Nevertheless, these studies say little about whether natural levels of strontium are important for normal calcification.

Another study examined in microscopic detail where the strontium is located in Galaxea fascicularis.33 They found that strontium was significantly enriched in the mucous layers. They propose that the strontium is used to neutralize (thanks to its +2 charge) the highly anionic (that is, negative charged) individual mucin glycoproteins. Once neutralized, the mucin can condense into functional mucin granules. These glycoproteins are highly sulfated, and if one were selecting a cation to bind to and neutralize the mucin, one might well select a divalent ion with a preference for binding sulfate. That is exactly what strontium provides, recalling the fact mentioned above that strontium sulfate is much less soluble than either calcium or magnesium sulfate.

The authors also go on the state that the mucin layer itself may play an important role in ensuring that calcium is delivered to the ectodermal cells. That is, it helps maintain an artificially high calcium concentration near the surface of the ectodermal cells. This process would help what is otherwise believed to be a limiting factor in calcification: the active transport of calcium. Whether this hypothesis is valid or not remains to be established. Nevertheless, it has, for the first time, provided a plausible mechanism for corals to benefit from strontium.

Toxicity of Elevated Strontium

There have been relatively few studies on the toxicity of elevated strontium to most marine organisms. In most studies, researchers have found strontium to not be very toxic at levels that might be attained in a reef aquarium. In one study, researchers looked at developing mussel embryos (Mytilus californianus) for effects from barium and strontium. For strontium, they concluded that there were no effects at environmentally relevant levels, although barium was quite toxic at even fairly low levels.34

The table below summarizes some of the toxicity data that is known for strontium to wholly marine organisms (that is, those that do not spend any part of their lives in freshwater where they might be more sensitive to ionic toxins. For example, certain freshwater organisms are killed by strontium at levels far below those found in natural seawater). The most sensitive organism in table 3 appears to be a crab, with 38 ppm strontium reported to be lethal. If that experiment is valid, then aquarists would be cautioned to keep strontium from rising much above natural levels.