Zitations from
ALLISON, Nicola, et al. Strontium in coral aragonite: 3. Sr coordination and geochemistry in relation to skeletal architecture. Geochimica et Cosmochimica Acta, 2005, 69. Jg., Nr. 15, S. 3801-3811.:
"Our data indicate
that the Sr composition of COCs is affected by biological or
kinetic processes, as well as temperature."
"The Sr/Ca composition of skeletal COCs is significantly
different from that of adjacent fasciculi (Cohen et al., 2001;
Allison and Finch, 2004) and Cohen et al. (2001) suggest that
biological and kinetic effects may be minimised in the COCs.
In this study, we observe Sr heterogeneity within both COCs
and fasciculi and we conclude that there are similar challenges
in reconstructing SST records from both of these skeletal
structures."
COC = centres of calcification
SST = sea surface temperature
Figures in the article show higher Sr/Ca ratios in COCs. The ratios vary from ca. 11.2 mmol/mol at COCs to 8.3 mmol/mol for Porites lobata in Fig. 5. and from ca. 10 mmol/mol at COC to 9.3 mmol/mol for Acropora palmata in Fig. 3.
The Sr/Ca ratios of Table 5 are 8.9 to 10.2 mmol/mol, average 9.2 mmol/mol
Pavona: Sr/Ca 9.0 mmol/mol at fasciculi - 10.2 mmol/mol at COC, avg. 9.4 mmol/mol
Montastrea: Sr/Ca 9.0 mmol/mol at fasciculi to 9.4 mmol/mol at COC, avg. 9.2 mmol/mol
Porites: Sr/Ca 8.9 mmol/mol at fasciculi to 9.2 mmol/mol at COC, avg. 9.0 mmol/mol
From Millero I have calculated a Sr/Ca ratio 8.7 mmol/mol, from data of the
PTEO of the MBARI I also get a Sr/Ca ratio of 8.7 mmol/mol for NSW.
This means Sr/Ca ratios of analysed skeletons are slightly higher (or strontium is elevated) relative to NSW ratios, especially at the COCs.
Reefers that did experimentation with strontium chloride addition reportet increased (longitudinal?) growth with strontium addition but brittle growth tips when Sr++ additions where high. If I recall it right Treuheit reported this in a German marine aquarium magazine in the early 90s I think.
@Randy Holmes-Farley , is there any hard evidence that Sr++ and Ba++ have no effect?
Several comments,
1. First is the idea that both you and Lasse keep throwing out that because the ratio of Sr to Ca in a skeleton is seemingly similar to the ratio in the water, or maybe higher than the ratio, that somehow that implies it is needed or useful. I fail to understand that logic at all. The incorporation of strontium (and even different isotopes of strontium) into precipitating calcium carbonate is known and published to depend on many factors, including the strontium concentration, the temperature, and the rate of calcium carbonate precipitation. The published incorporation rates are about the same level in corals and in abiotic precipitation.
If you wanted to help support a claim that Sr was being intentionally deposited because it was useful, I think that you would need to show that it is significantly higher in a coral skeleton than in abiotic precipitation taking place under similar conditions.
This entire discussion is not new. I published this exact discussion in 2003, and while there have been a huge number of studies on the Sr/Ca ratios (even ratios of individual isotopes), the goal has almost always been to get a paleo thermometer from fossil corals, and not answering whether Sr is useful.
This article on strontium is the second of several that delve into a variety of issues involving magnesium and strontium.
reefs.com
from that article of mine:
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.
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