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What is a Trace Element and Why Should You Care About Them?

By Dr. Craig Bingman. Presented October 17th, 1999 on #reefs IRC.

Craig has been keeping marine aquariums since his undergraduate days at the University of Kansas, where he received a bachelor's degree in biochemistry. He continued to dabble in the hobby during graduate school at the University of Wisconsin at Madison, where he received a Ph.D. in Biochemistry in 1991. Sometime during his postdoctoral appointment at Columbia University, he became very interested in the chemical interactions in reef systems. He is a frequent contributor to Aquarium Frontiers.


The question posed in the title of this talk is double edged. While I will do my best to provide answers to the questions above, I'm also gently poking a bit of fun at aquarists who speak about how trace elements are or are not depleted by various practices. To them, I ask "what do you mean when you say trace elements? Have you ever measured them? And why are you talking about them as though they were functionally related?"

So, our first goal is to discover what scientists mean when they say "trace element." At this point, you would usually get some sort of a list of various elements, and you would either nod knowingly, or your eyes would glaze over. There is a wonderful tool that chemists use, called the periodic table or periodic chart, that groups elements according to their electronic and chemical similarities. So we are going to use the periodic chart as our ally in this discussion. It will help us stay organized and on track.

Therefore, let me introduce a close personal friend of mine, the periodic chart. There are a series of figures that accompany this talk, and you can go there by clicking: Craig's figures .

The first figure shows the periodic chart, and I've used colors to highlight the major divisions of it. In blue, we have the main group elements. In elemental form, all of these elements are completing either s or p shells of electrons. As you scan along from the left to the right, atomic number increases. You start at hydrogen, which is element number 1, go across to helium, element number 2. Hydrogen has one electron in the first s shell, helium has two electrons in that shell, and it is complete. Lithium starts filling the second s shell of electrons, beryllium completes that. Boron starts filling the 2p shell, and the p shell takes a total of 6 electrons to complete. So the first row has two elements, the second two plus six or eight elements. When we get to the fourth row, something new starts happening, and an entirely new type of shell of electrons, the d electrons, start filling up. The transition elements are elements that are filling these d shells of electrons. After Barium, element 56, the fourth and final type of electronic shell begins to fill, the f shell. The lanthanide and actinide elements are filling the f shells.

What does any of this mean to an aquarist? Hang in there. We will get to that. So now we know about the organization of the periodic chart, and we can look at it and tell whether a given atom is in the main group of elements, is a transition element, or a lanthanide or actinide. Keep that mapping in your mind as we proceed. We can say something about these elements now, and this will become more clear as the talk goes on. The main group elements contain the bulk elements that living things are made of. The transition elements have important roles in catalysis in biological systems, especially the first row of them. And there are no positive biological roles for the actinides or lanthanides.

The next series of figures maps the abundance of various elements in sea water onto the periodic chart. The first frame shows the major elements, which I've operationally defined as anything present at greater than one millimole per kilogram in full-strength (Salinity=35) sea water. What can we say about the major elements in sea water? Well, there are no transition elements in the major elements in sea water, nor are there any lanthanides or actinides. They are all main period elements. Looking at the atomic number of the major elements, they are all relatively light. There is nothing present past the very first part of the fourth row (Calcium.) OK, so the major elements are all main group elements, and they are all light. What about the minor elements? Well, our horizons broaden slightly, but not all that much. We make it down to the first part of the fifth row, strontium, element number 38. Still, there are no transition elements present.

It is only when we make it to the trace elements that the first transition elements appear. We still have no lanthanides or actinides present. That's interesting, isn't it? Only when we make it to the ultra-trace category do we start to see the lanthanides and actinides appearing.

OK, the next frame shows elements categorized by their depth profile in the ocean. If the oceans were well mixed and nothing was depleting the elements, then the sea would have a uniform composition from the surface to the bottom. As it turns out, the oceans aren't all that well mixed, and there are chemical, biological and physical processes either releasing elements into the ocean. As examples of physical processes adding elements, consider river inputs, or dust from the Sahara blowing into the Atlantic and bringing with it substantial quantities of iron. Biology powerfully shapes the depth profiles of some elements. For elements with positive biological roles, especially those present in low abundance, may follow what is called a nutrient depth profile in the ocean. Life depletes that element in the surface layer of the ocean. As organisms die and waste falls through the water column, these elements are released at intermediate depths, giving rise to what is called a mid-water maximum. Many but not all elements that show a nutrient profile in the ocean are biologically essential or very important. However, there are many other elements, for example, S, that is absolutely essential for biology, but because it is present in such high concentrations, biology does not appreciably disturb the depth vs. concentration profile for this element. Conversely, lead follows a nutrient profile in the ocean, but there is very scant evidence for positive biological roles for this element. It is usually considered just toxic to varying degrees.

So... we have examined depth profiles and raw abundance as tools of classifying elements that are important to biology and therefor of significance to reef aquarists. We have found both of those classification schemes wanting, so we will just make up our own synthetic classification of elements according to their biological importance. You can MY attempt at such a classification in the next frame. Most of my assignments are not contentious, a few of them may be. Most of the information in that slide is self-explanatory. All of the essential "bulk" elements are main group elements. Many of the essential trace elements are transition elements. There isn't much positive happening in the bottom part of the periodic table. You will recall that those elements have very low abundance in sea water. If you were an organism, and you were picking out elements that you would absolutely require to live, you might want to pick reasonably abundant ones. As it turns out, life has followed that logic very closely, making use of relatively abundant elements for the bulk construction, and using somewhat more rare elements for essential catalytic roles (the essential transition elements.) If there are questions about other odds and ends present in this classification, I'll be happy to answer them later.

So, why am I so down on the phrase "trace elements?" After all, there are some trace elements that are important. Yes, but there are others that have no positive role in biology. I prefer a more functional classification. There are some relatively low-abundance elements that are essential micronutrients, for example, iron. There are other elements that I think are very important for marine systems that are used almost completely for the formation of skeletons or tests (Sr and perhaps also F) or are modified and excreted to change the environment immediately around the organism (several of the halogen elements, especially Br and I.) I'll have more to say about those other roles of elements in biology, perhaps in a future discussion here. For now, you have a framework for thinking about various elements. Are the abundant? Would my tank be likely to run out of a major element? Is it likely that food will give enough of elements that have primarily nutritional roles? What about elements that are not very abundant but have important roles in test formation or important roles in modifying the environment around the organism? If an element isn't used mainly for incorporation into the substance of an organism, is it likely that food would provide an adequate supply of that element? Those are all interesting questions to consider, and we now have a chance to discuss them.

Congratulations to all who made it through that. ;-)

How do you feel about adding iodine or Iodide, and what products do you recommend?

The initial observation is that iodide is rapidly depleted, but some of that is converted into another form, iodate. Unfortunately, testing for iodide and iodate is difficult, and I have only recently found an assay that allows me to look at both.

What are the roles that chelated elements play in our saltwater tanks? (stuff like calcium, iron, copper, etc..)

A number of the transition elements (especially iron) can be present in organic complexes. They can be important in stabilizing iron in solution. It is interesting that iron was probably enormously more abundant in the ocean before the atmosphere of the earth become oxidizing (a couple of billion years ago.) Some people have speculated that life might not have chosen iron for as many roles as it did if the choice had been made in an oxidizing environment. OK, copper, calcium chelates: I don't know why anyone would want to try to chelate either in a reef tank. Corals don't need chelated calcium, and the form of copper that kills organisms is free copper.

How do you determine how much of which elements are added to the tank through feeding?

Basically, you don't. Operationally, people are just hoping that food supplies enough of the required transition elements that are used as nutrients. In most cases, they are probably correct, although some tanks might run a bit short on some things, depending on how strong the export mechanisms are in the tank. As far as elements that are mainly modified or excreted in other forms, like I and Br, there is scant hope that feeding will supply all of the needs of the system for these elements.

So from the practical aquarist perspective - which trace element additives should be used, how much, how frequently, and what effect does skimming really have on them?

Many of the transition elements, including iron, form tight complexes with organic molecules. There is little doubt that skimmate is very rich in iron. I've measured rather shocking Fe concentrations in the skimmate from my tank. So it definitely can be depleted via skimming. Unfortunately, I don't have an answer as to what trace element supplements you should use and how much, but I would make one suggestion, and that is, to do a bioassay on your tank. If you have problems with nuisance algae growing, don't add any of the so called trace element supplements to your tank! If you are concerned about the availability of iron, like perhaps you like macroalgae and you aren't getting as much growth as you would like, start adding perhaps 1/4 the rec'd dose of a trace element supplement, and then adjust up or downn depending ont he growth of problematic algae in yor system. As for what supplement to use, the only one that I really have looked at is Combisan, and I think it is probably the best in that class of products.

Do you know of any animals that need Sr supplementation?

There are some animals that require Sr during their development. I think that one type of snail has been shown to require it. There are also classs of organisms commonly called "radiolarians" that absolutely require Sr. And they move around enough Sr as SrSO4 to give Sr a slight nutrient depth profile in the ocean.

In your opinion which filtering would deplete more usable trace elements, skimmers or algae turf scrubbers?

I don't know which would deplete more, but obviously both will remove essential transition elements, and the bulk nutrients like N and P. There are some differences in the mechanisms for those two systems, and on some level, the ATS systems are more "elegant" but both will depleted "trace elements." I see that I haven't broken you folks of using that term yet. ;-)

How effective is a calcium reactor at adding trace elements?

I would expect a calcium reactor to add the elements that are removed by coral skeleton and coralline algae crust formation in the ocean, but they will not do much as far as adding elements depleted by bioassimilation. You would need to feed to get those elements. So, it might help for some things, like for example maybe F-, but I would not expect it to give the system adequate Fe or I-.

A while back, you predicted that unskimmed tanks would have a lot of problems, due to lack of phosphate export. Have you found that to be the case, or seen that happen in others tanks?

Well, I think that you can keep a system without skimming, certainly, but in the cases that I've looked at an heard about, if there is a substantial amount of feeding in the system, then you need to have some sort of export mechanism for P in place, or there are problems. And I've seen a lot of tanks that have had big P problems a year or a couple of years after they were estabilished.

What testing/studies have you done with ferric oxide as an algae control?

I haven't personally done any testing, except one very casual time when I was just starting out and bought one of these ferric hydroxide things for P control. I was later rewarded with one hell of a hair algae problem later, and I think that was because some of the Fe wound up in the system and fueled algal growth.

Do you think that heavy algal loads play a role in removing the nutrients that are of primary importance to inverts?

It depends on the element in question. If we are talking about Fe, Mn and other first row transition elements, then algal growth definitely removes them. Could you please clarify what invertebrate and what elements you are asking about?

Specifically soft corals. Nutrients along the lines of those added by other than food (in quantity) such as Sr, Va, I...

Strontium is something that is interesting, but at the present time, I can't really recommend that anyone dose it into their tanks. The dosing suggestions given by all manufacturers are completely irrational. It is a disaster and I intend to correct that soon. As far as V goes... it is used by many organisms in small qantities, by others in very large quantities. I was not aware that soft corals have a huge requirement for V. Some tunicates do sequester large amounts of V.

So if I understand your use of trace elements, it is not as "Elements that exist only as small traces" but rather the specific location in the periodic chart? What is a your term then for elements that must be replaced in the aquarium?

No, trace elements means elements present below a certain concentration level in the ocean. I don't like to use that phrase to refer to elements that are essential micronutrients. If you look at one of the later slides on my web page, you will see the synthetic classification scheme that I think works better than "trace elements."

If someone was going to "roll their own" reef mix for dosing elements what formula would you recomend?

I think it would depend on what is in your tank, and how you are feeding it. For micronutrients, I personally have just been adding greewater to my system. There are plenty of micronutrients in the algae, they go into the system when the algae are eaten or die, and then everyone else gets a shot at them before the skimmer has a chance to pull them out.

How do you feel about iodine in the reef tank? Do you promote Iodine use for xenia?

I don't have any specific dosing suggestions for I- right now. It is something that I am currently working on, but I'm not done with that work yet.

What trace elements are we adding that are not needed, and conversely, are we lacking additives?

There are major deficiencies in the formulations of some salts. I think that Br- should be present at NSW concentrations in any salt mix used in reef tanks, provided that the owner doesn't use ozone. It is also true that reef tanks very rapidly deplete F-. I think that I'm one of the first if not the first person to notice that. So far, it looks like the half-life of F- in a reef tank is about 4 days. As far as things like Fe goes, it is difficult to test for that, and I am in the group of people who find that there must be adequate Fe entering my system since there are always a couple of small tufts of algae around for the snails and urchins to munch.

How does one determine the amount of Br to add when there are no tests kits presently available? What role does Br have in the reef tank?

From my studies so far, it looks like you would probably keep adequate Br in the tank if your salt mix had Br to start out with, at NSW concentrations, and if you did an occasional water exchange. A test kit should not be required. I know that some people are adding the ESV Br/F supplement, but I don't think it is correctly formulated.

Did you have anything further to say about this subject at MACNA? Is another balanced additive in the works? Or is a balanced additive even possible?

An additive for Br or F? I think it needs to be two separate additives. Ultimately, I want to pull everything that is functionally related into supplements, and then people will be able to dose a couple or perhaps three things and keep their tanks on target. But first we have to understand functional relationships between different elements. For example, I expect Sr and F to be removed from the tank at a rate that is dependent on the calcification rate. So, yes, I do think that some sort of "balanced" supplements will be possible, but we are not there yet. And they aren't on the market yet.

Which salt mixes have NSW levels of Br? what salt do you find has the proper levels of trace elements for the reef aquarium?

I haven't looked at all of the salts yet.... I think that Tropic Marin might have NSW Br-. I know that Instant Ocean does not. If the company is chasing public aquarium contracts, they won't have Br- present, because they all use ozone. The salt won't have Br present, since the aquariums all use ozone.

Should we need to add any micronutrients if we are dosing kalk and feeding a diverse diet?

You will probably get enough iron and related micronutrients, but the tank won't necessarily be getting enough halide ions and some other things that don't come in via food in adequate quantities. For example, no one would suggest that a tank should get all the calcium and alkalinity it needs from feeding. ;-)

Why is copper so extensively used when it is known to break down the animals immune system? Also, in conjunction, with that doesn't putting fish in coppered water cause them to absorb this heavy metal and it can ultimately build up to toxic levels?

You can most certainly kill fish with copper, and you can cause liver damage in fish and immune suppression in fish even with theraputic concentrations of copper. Why is copper used? Because it works against a fairly wide range of external parasites. I actually think that it is used too often. Formalin can also be very effective against ectoparasites.

I know certain seaweeds contain iodine. Should we keep them in a metal halide lit sump? Would they release correct concentrations?

Well, if the seaweeds are alive, they will be absorbing iodine, not releasing it. ;-)

Do you think the daily addition of B-ionic is good or an overkill on the addition of elements?

I think that B-Ionic can be a very very good calcium and alkalinity supplement. but it isn't going to be a great source of transition elements, nor will it give the system enough F-, I-. It might be helpful in Br-, though.

Ok, so please answer the question we all came here to ask. Should we add trace element suplements to our tanks or not, and what additives do you use in your tank?

If you are keeping an average sort of mixed soft and stony coral tank, and you have quite a few fish, then your tank is probably getting more than enough Fe. Mn, and a bunch of other elements from that. So you are probably in fine shape as far as nutrients go. However, there will be some things that you didn't start out with, like Br-. And other things that are rapidly depleted, like F-, I-. Unfortunately, I don't know of any commercial supplement that really does a good job with that stuff, and there are no commercial Sr supplements that have correct dosing instructions.

What's the truth behind silica and why is there conflicting reports over silica in the industry? How would you suggest to deplete a tank to a tolerable level of silicate, even when using RO/DI makeup water?

As far as what I use in my tank.... it is an experimental tank, and you probably would not want to do that to your system. I like to see some diatom growth in a tank, because they are very important in the food chain. Interrstingly enough, a great deal of the nitrate reduction that happens in the ocean is mediated by diatoms. So, because there was little silicate in the water of my system, and because I probably bought too many snails, I intentionally added Si to my aquarium. It did boost the diatom growth, as predicted, and I think that the snails thanked me for that. Some people have pathologically high Si in their tap water, and they really do have a reasonable concern about Si. However, I think that the hobby in general has swung to the opinion that Si is a bad thing to have in your aquarium, and you should try to limit the growth of diatoms as much as possible. I think that is a big mistake. If you are shutting down diatoms, you are also going to negatively affect many types of sponges.

What uses Br? Why should Br not be used with ozone?

Br is incorporated into many "natural products" or organic compounds synthesized by marine organisms. It is also converted to oxidized forms, and used as an "area denial weapon" by some marine organisms.

What effect does Ozone usage have on Br?

Oh, when you ozonize water with Br- in it, it is converted to toxic forms. Which is what some organisms do with Br as well, they just use peroxide as the oxidant, rather than ozone.

Can you suggest any books on marine elements and their usage/value to the aquarist? What is the address of your web page, Craig?

http://fpage1.ba.best.com/~cbingman . If there is enough interest, I might teach an online course at some point in the future. As far as a reference book, "Chemical Oceanography" by Millero is a good place to start.

Would some of the "buffer" compounds that incorporate bromine salts add enough Br?

Yeah, the two-part liquid calcium and alkalinity supplements eventually build up a non-trivial Br concentration in the water. How high depends on how fast calcification is, and how you do water exchanges, or if you just perform dilutions.

What problems was you refering to earlier about non-skimmer tanks? For a non-filtered nano tank w/ no skimmer what elements should I be concerned with besides calcium?

The big issue with tanks that don't have a good nutrient export mech is P and possibly also N and Fe accumulation. Please let me be clear about this, though. You can keep a wonderful non-skimmed tank. The only issue there is that you really need to be careful with feeding, or that you need to have ATS or activated carbon export going. If you don't, then you will eventually get hit with the high P whammy. It might take a year or two, but it will happen. There is no vapor phase chemistry for P as there is for N. It can't just leave the system into the air.

What's a good 'natural' method of removing phosphates in a reef tank?

There are a number of them... ATS works very well at removing P, harvesting macroalgae works, growing and removing for sale large amounts of soft corals also works well.

Are reefs in their native environment limited to any of the nutrients you have spoken on today (limiting growth factor)?

Well, depends on what you mean by limiting. If the question is do they actually run out of any given element, then the answer is no, but only because they are connected to the rest of the vast ocean. If you built a wall around some corals on a reef, the first thing that they would run out of is carbonate alkalinity. Just like in our tanks. They would eventually run out of other things as well.

Do the silica sands leach Si back into a system or are they stable?

If the sand is made out of crystalline quartz, then it will not be very soluble at all. If it has a lot of fine stuff in it, with a lot of biogenic opal (diatom skeletons, spong spicules) then they are more soluble than quartz or float glass, and you will get some Si release. For that matter, mainly calcareous sands can contain a non-trivial amount of biogenic opal.

Of the trace elements that were depleted in the system you were testing, where do you postulate them going? Skeletal deposits? Bio-incorporation?

F- is going into skeletons, I- is going into organic molecules and converting to iodate.

In many oligotrophic environments (where I think a decent proportion of the water which feeds reefs comes from), there is usually a limiting factor such as low P (Sargasso sea for instance). Is that the same on a reef?

When people speak of limiting nutrients, usually what they mean is that something is limiting further algal growth. So even if a coral is sitting in water that has limiting P or limiting Fe, that doesn't mean that its growth is limited. Waters that flow over coral reefs in the pacific and indian oceans, if there is a limiting nutrient, it tends to be Fe or P. There is usually enough nitrogen for further algal growth.

What is the end point of chemistry for phosphates in the marine environment? And at what point does this become bio-available pH wise?

Phosphate winds up in sediments. It winds up in dead animals falling through the water column into deep waters. It is cycled by erosion from land (uplift eventually brings the bottom of the ocean to dry land in some places) and there are also upwellings of deep water to the surface. That's largely the story.

Does biogenic decalcification occur in marine enviroments or just in freshwater enviroments?

I think that you could fairly call what coralline algae, calcareous green algae, and reef-building corals do "biogenic decalcification" although we usually just call it calcification.

Isn't Iodate bio-unavailable? Or at least have long times before depletion?

Iodate is not unavailable. Actually it is recycled to iodide by nitrate reductase. Remember that diatoms are the main source of nitrate reduction in the ocean, and you may see another reason why they might be more friendly than some folks give them credit for. But iodide is the form most commonly used for bioassimilation.

How concerned should we be about Dissolved Organic Carbons (DOCs) as compared to phosphates?

Well, DOC is an enormous bag of very different things. Some DOC is beneficial. because say vitamins are dissolved and they are made of organic carbon and some other things. If we are talking about a coral defensive secretion that is an organic molecule, then we'd probably like to adsorb that to some carbon.

You recomend adding trace elements in Redfieldian ratios. That would work for bio-assimilation but what about export by skimming or other filters. That's not guaranteed to be removed in those same ratios right?

This is the big issue with skimming and activated carbon that I see. I honestly don't know how whacked out things get. They are not redfieldian export mechanisms. Mainly at this point we just chuck food in and cross our fingers. (Redfield was a scientist who observed that phytoplankton growth removes N and P in more or less constant ratios.)

Do you recommend continuous carbon use? Forced flow? And how much carbon? I use it continuously, and I'm using it in a forced-flow arrangement right now.

Have you done any work with biological oxygen demand?

No, although I have made some dissolved oxygen measurements at various times. BOD is difficult to measure in salt water. Well, interpreting it can be somewhat difficult.

Getting back to P in sediments, we are mainly talking about orthophosphates at lower pH that become bio-avilable. When the pH swings higher, the P is converted to another form that is less bioavailable. Considering that the pH level goes down the deeper you go in a sandbed, why do you feel that we should then be concerned with phosphate removal?

I'm not sure that I completely understand the question, but operationally, older systems seem to run out of places to hide the P, and it winds up being available again.

What is the bulk of the Phosphate in sediments? Calcium Phosphate?

Probably. I've just made measurements that show that it is present. None of the mesurements that I'v made tell me exactly what chemical form the P is in.

If you do a 10% water change each week would this keep the trace elements in check?

Depends on what you mean by a trace element. ;-) If you are talking about Fe, Mn, etc, then your system was probably getting more from food than you will get from a water exchange. If you mean F-, no way. If you mean I-, nope. If you mean Br-, then there is just a trace of it in most salts.

Considering that it becomes less soluble as pH goes down, why then worry about it? 'It' being calcium phosphate.

It is the other way around. It becomes MORE soluble as the pH goes down.

What are your thoughts on salts that are just dryed from ocean water?

Well, it seems to be more complicated to get something that resembles natural seawater using evaporated salts than it is to just start from purified components. I know that the Red Sea salt that Marlin and I looked at was a little funny in a couple of major ions.

Well, that looks like it for the discussion tonight. I for one would like to be the first to thank Dr. Bingman for coming. Excellent talk Craig!

Thanks to all of you for your excellent questions! -Craig

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Last modified 2006-11-26 04:29
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