Chemistry And The Aquarium: Iron In A Reef Tank
In many reef tanks, the only supplements added are calcium and a source of alkalinity (and all of the chemical impurities that come along with these additives and, of course, food). No others, it would seem, are mandatory for keeping many organisms. There is one other supplement, however, that has proven itself to be very useful in many reef tanks: iron. Specifically, it has proven itself to be useful in applications involving tanks with substantial growths of macroalgae.
The benefit of iron appears to be at least two-fold (and maybe three-fold). The main benefit is that at least some species of macroalgae grow faster, and appear a darker, more attractive green, when the tank is dosed with iron. In addition to the aesthetic benefits, this increased growth permits the macroalgae to be a better nutrient export system. A secondary benefit is that faster growing macroalgae may better compete with microalgae, which is often a source of frustration to reefkeepers. One more speculative benefit is that it may decrease the likelihood of caulerpa undergoing sexual reproduction, creating water quality problems.
This article outlines what iron is used for biologically, and makes some suggestions as to how to use it in a reef tank environment.
Iron in Biology
All living creatures need iron. Some get it in food and some by absorbing free iron from solution. Still others get it by secreting special molecules, called siderophores, which go out into solution, bind very strongly to iron ions, which are then actively reabsorbed when the species that released it comes into contact with the siderophore/iron complex. In many natural environments, ranging from parts of the ocean to the human intestine, iron can be in such short supply that it becomes a limiting nutrient for growth. As a consequence, organisms have developed elaborate methods of collecting iron from their environments. While detailing these mechanisms is beyond the scope of this article, the fact that such methods have been developed and are used at substantial energy cost to the organisms involved, shows that iron must clearly be of great value to them. But why?
It turns out that iron, Fe, has myriad uses in biological systems. One that is well known to many people is that iron is a critical constituent of hemoglobin. In that application, iron is held at the center of a large complex of organic molecules. It is the ability of that iron atom to bind oxygen, as O2, that makes it so valuable. Fish, for example, have large amounts of hemoglobin in their blood, permitting oxygen to be efficiently transported from the gills to peripheral tissues. Fish, however, supply their iron requirements from their food, so as long as they are getting appropriate foods, they should not suffer from iron deficiency regardless of the iron concentration in the water column.
There are many other applications for iron in biological systems, and many of these are carried out by organisms that do not generally consume "food," such as algae. It is those organisms that are most susceptible to iron shortages in the water. What do these organisms use iron for?
Iron can readily exist in two ionic forms, Fe2+ and Fe3+. This fact is not true of many other metals, and permits iron to be used by organisms in ways that other metals cannot. Iron is, in fact, used by all organisms in a great many proteins and other types of organic molecules for this very reason. Many of these applications take advantage of one direction or the other of the redox reaction between these two forms of iron (equation 1).
1. Fe2+ ⇔ e- + Fe3+
For example, if you need electrons (e-) to carry out a chemical transformation (i.e., if you want to carry out an electrochemical reduction of some molecule), you can simply start with a bunch of Fe2+ that is bound into a suitable protein. Then, when it contacts the molecule to which you want to transfer the electrons, it releases an electron and becomes Fe3+.
Likewise, if you want to oxidize something, you can start with the iron in the Fe3+ form, and take up an electron from the molecule of interest. Biological systems carry out many of these reactions, some of the most important of which are photosynthesis (the use of light to generate high energy molecules) and respiration (the metabolism of high energy molecules to generate energy).
Iron in Photosynthesizing Organisms
As mentioned above, iron is critical for photosynthesis, and this fact is at least in part responsible for the substantial need for iron by phytoplankton and macroalgae. Photosynthesis essentially splits carbon dioxide (CO2) into carbon (in the form of organics) and oxygen (O2) as shown in equation 2.
2. 6 CO2 + 6 H2O → C6H12O6 (glucose) + 6O2
The process is very long and complicated (and fascinating!), but in a simple sense, electrons need to be transferred from the oxygen atoms of the CO2 to the carbon atoms of the CO2, leaving behind oxygen and organic molecules. It is iron which in part facilitates this process through reactions similar to that in reaction 1.
Iron in the Ocean
Iron in the ocean is primarily iron(III) (Fe3+), because any Fe2+ that forms is oxidized back to Fe3+ by oxygen (O2) and other oxidizing species. The concentration of iron varies substantially with location and depth, and is depleted at the surface due to scavenging by organisms. Typical surface concentrations are on the order of 0.1 nM (0.000006 ppm). When not bound to an organic molecule, iron in seawater exists primarily as dissolved Fe(OH)3. Iron(III) is quite insoluble in seawater at pH 8.2 due to the formation of iron oxides (rust) of various compositions. In fact, it is one of the least soluble cations in seawater. So dumping in a lot of unbound iron into a reef tank may simply result in much of it precipitating onto the bottom.
In most of the oceans, the growth of phytoplankton is limited by nitrogen sources (typically nitrate). In some places, however, where there is adequate nitrogen, phosphorus, and silica (if we are referring to diatoms), the growth of phytoplankton is believed to be limited by the availability of iron. Experiments have, in fact, shown that growth can be increased in some of these areas through addition of iron to the ocean. Many of these experiments are summarized by Frank Millero in his book "Chemical Oceanography" (second edition; 1996).
One of the facts that arises from these studies involves phosphorus. The preferred solution ratio of iron to phosphorus is between 1:100 and 1:1250 for coastal species of phytoplankton, and about 1:10,000 for open ocean species, suggesting that the open ocean species have developed better mechanisms for collecting and/or using iron. I mention this fact not because we can use it quantitatively to know if we have enough iron in our systems, but rather to demonstrate that different organisms have different abilities to fulfill their iron requirements, and that iron may be limiting the growth of one organism, while in the same tank, nitrogen, phosphorus or silica may be limiting to another.
Note that I stated that if the iron is not bound to an organic it would primarily exist as soluble Fe(OH)3. However, in both the oceans and in reef tanks, there are uncounted hordes of organic molecules that bind iron quite strongly. Unfortunately, while the speciation of some metals has been well studied in some fresh water systems (such as copper in certain lakes), the speciation of iron in seawater has not generally been elucidated. I expect this lack of information stems largely from the difficulty in identifying all of the organic species present, in knowing which ones are binding iron, and in the fact that the nature of organic molecules in seawater will vary from location to location, from season to season, and likely even with the time of day.
Iron in Reef Tanks: Does Supplementation Make Sense?
If iron can become a limiting nutrient in some parts of the ocean when adequate nitrogen and phosphorus are present, then it makes sense that the same might be true in our tanks, where nitrogen and phosphorus are typically present in substantial excess over the oceans. There is little published data, however, on iron levels in reef tanks, and even if overall iron concentrations were available for reef tanks, one might still be mislead if the iron were not present in a readily bioavailable form (such as in inorganic particulates or strongly chelated by organic molecules).
Since the foods delivered to reef tanks contain a large amount of iron, how could the water column ever be "low" in iron? In the case of iron, there are several potentially important export mechanisms from reef tanks. Iron bound to organic molecules may be readily skimmed, depending on the nature of the organic. Iron is also taken up by the many organisms in the tank. Also, iron in the water column may simply not be bioavailable when chelated to certain organics (as mentioned above and discussed in more detail below). Finally, iron may precipitate in any of the varied environments present in a reef tank. These include high pH environments where certain additives are introduced (like limewater), potentially causing rapid formation of iron oxides and hydroxides. It is also possible that iron binds onto calcium carbonate surfaces, both those present as sand and rock, and those being created as coral skeletons and other biological structures.
It is not my purpose to prove here that iron is limiting in many or even in a single reef tank, only that it is plausible, and that the experimental evidence provided by hobbyists that iron supplementation has significant effects is not at odds with the scientific literature or with common sense.
Iron in Reef Tanks: When Could Supplementation be Most Beneficial?
In many reef tanks, it is not apparent that iron supplementation is beneficial. Perhaps it would be in all reef tanks, and we simply have not yet carried out appropriate experiments to show the value. Likewise, perhaps it is detrimental in many, and again, we have not carried out experiments that show this fact to be true. There are, however, some situations where iron supplementation seems beneficial, and one where it would be detrimental.
Let's start with the latter. In a reef tank without macroalgae, or without adequate quantities that the macroalgae can be considered a significant sink for nutrients such as nitrogen and phosphorus, addition of iron may actually exacerbate an existing microalgae problem. It might also tip the balance toward a microalgae problem if iron were limiting microalgae growth. In these cases, I would either not add iron, or add it with an eye to stopping the addition if microalgae growth worsened.
Situations that might benefit from iron would be those where there is substantial macroalgae growth, with or without a microalgae "problem". In the case without any microalgae concern, the macroalgae may simply grow faster and thereby lower the nutrient levels in the tank that are otherwise undesirable (such as phosphate which can inhibit calcification by corals). Macroalgae growth is, in fact, one of the best phosphate export mechanisms in a reef tank, and optimizing this method may be very beneficial.
I have also heard from many hobbyists whose macroalgae (such as species of Caulerpa ) have grown pale over time and were not growing well. On iron addition, the macroalgae greened up considerably, were much more attractive in the eyes of the hobbyists involved, and were growing substantially faster. I have also personally never had my Caulerpa racemosa undergo sexual reproduction despite years of vigorous growth, but that may or may not have anything to do with the iron additions (see discussion below).
When microalgae is a concern, as it was in my refugium a few months after I first set it up, iron may also help. After adding several varieties of macroalgae to a well-lit refugium, all was well until the microalge algae eventually began to coat everything in the refugium. Finally, someone in an on-line forum suggested that I use iron to boost the macroalgae growth, and that the iron would help it out-compete the microalgae. Low and behold, it worked; after a few months, the microalgae had almost totally disappeared and the macroalgae were growing like gangbusters (at least the two species that survived the microalgae plague: Caulerpa racemosa and Chaetomorpha sp.).
Iron in Reef Tanks: Inhibition of Sexual Reproduction of Caulerpa ?
Since the Caulerpa racemosa in my tank has never experienced sexual reproduction, and the attendant water quality problems and die-off, I have often speculated as to why it happens to some and not others. One possibility is that iron dosing is important. After all, it makes sense that if the Caulerpa is stressed, it may want to "relocate" to a better environment, and one quick way to accomplish that task is to put its energy into sexual reproduction.
I recently ran a poll on one of the on-line reefkeeping forums. I provided eight possible responses and asked people to check all that applied. Here are the most pertinent ones to this article (the "none of the above" term means no iron, iodine, or bromide).
|I dose iron and grow Caulerpa , and it has never gone sexual||9|
|I dose iron and grow Caulerpa , and it has gone sexual||1|
|I dose none of the above, and grow Caulerpa , and it has never gone sexual||31|
|I dose none of the above, and grow Caulerpa , and it has gone sexual||20|
I find it interesting that only 10% of those who dose iron had a sexual event, while 39% of those not dosing iron had such an event. The numbers are clearly small, but running a chi-square test on the data shows it to be significant at p=0.038. In other words, there is a 96% chance that there is a real difference between the iron dose group and the no iron group with respect to Caulerpa undergoing a sexual event. However, we must recognize that those who dose iron may be the same folks who dose other things (like iodine, which also was statistically significant against no dosing), or do something else in common, potentially confounding a statistical test. Nevertheless, the difference is intriguing, and worthy of additional study. In my opinion, it is also good enough evidence for those plagued with such events to try iron dosing.
Iron in Reef Tanks: How Much and What Form?
Deciding how much iron to add is fairly easy because, in my experience, it doesn't seem to matter too much. Presumably, once you add enough to eliminate iron as a limiting nutrient, extra iron does not apparently cause harm (at least that I've detected in my tanks or heard of from others). I selected a dose of about 0.1 to 0.3 mL of a solution containing 5 g of iron (as 25 g of ferrous sulfate heptahydrate) in 250 mL of water containing 50.7 g of sodium citrate dihydrate. This liquid is dosed 2-3 times per week to my system with a total water volume of about 250 gallons. This iron(II) citrate has turned brown and cloudy since I first made up the bottle years ago, suggesting that it is oxidizing to iron(III) and some is precipitating from solution, but I still use it. Over the past 4 years, I've dosed nearly all of the 5 grams of actual iron to my tank.
Now that may sound like a huge amount, and it is. It's enough to bring 800 million gallons of completely depleted seawater up to the 0.000006 ppm level that I mentioned earlier for natural sea surface water. Still, I've not noticed any problem, do not know the steady state concentration, do not know how high of a solution concentration is actually optimal for my tank, do not know how much is biologically available by the mechanisms mentioned below, do not know how fast it is removed by skimming and other mechanisms, and do not know what would happen if I cut it back by a factor of 1,000.
All that I know is that microalgae has never been a problem since starting the iron, and I've not noticed anything negative that I could attribute to the iron (nor have I heard of any from others doing similar dosing). Still, I don't keep all organisms available to the hobby, and if you do seem to get a negative reaction from something, I'd advise backing off on the dose or stopping completely.
Since many hobbyists do not have access to the chemicals required to make iron(II) citrate, I'd advise buying a commercial iron supplement. There are a number available that seem appropriate and are not very expensive. Some commercial supplements combine manganese with iron (such as Kent's product), presumably because the scientific literature has demonstrated that phytoplankton also scavenge manganese from the water column. I've not experimented with manganese, but it is probably fine to use if you cannot find a pure iron supplement.
I'd also advise using only iron supplements that have the iron chelated to an organic molecule. The iron sold for freshwater applications is sometimes not chelated because free iron is more soluble in the lower pH of freshwater tanks. I'd avoid those products for marine applications. It will likely still work (as many of the studies in the scientific literature use free iron in seawater), but probably not as well because it may precipitate before it has fully fortified the system with iron.
In many cases of iron intended for the marine hobby, the product may not tell you what the iron is chelated with, in order to protect proprietary formulations. I don't actually know if it matters too much. Very strong chelation by certain molecules will actually inhibit bioavailability by not permitting release of the iron without completely taking apart the chelating molecule, but I expect that manufacturers have avoided those molecules. EDTA and citrate, and some others, actually degrade photochemically, releasing small amounts of free iron continually. It is believed to be the free iron that is actually taken up by many organisms, and likely iron(II), though some organisms may be able to convert iron(III) to iron(II) before uptake (the detailed absorption mechanisms are generally not known). There is a more detailed discussion of this degradation and uptake in "Captive Seawater Fishes" by Stephen Spotte (1992).
So good luck with iron dosing, and happy reefing!