BIOCHEMISTRY IN THE AQUARIUM
BY CRAIG BINGMAN, Ph.D.

The Halogens — Part III: Iodine

TERRY SIEGEL
Many reef aquarists have observed that corals, like the gorgonian (Plexaurella sp.) pictured here, benefit from regular iodine supplementation. This particular specimen is from one of Terry Siegel's reef tanks.
In the third part in this series we will cover the biochemistry and oceanography of iodine. From a chemical perspective, iodine is the most complex of the halogens. It has some of the most profound biological interactions as well. The redox states of iodine will be described, and the mechanism for interconversions between those states will be presented. Special attention will be paid to the biological roles of iodine. Finally, because many, if not most, reef aquarists use some type of iodine supplementation, this halogen is of particular interest to the reefkeeper.

Redox chemistry

Iodine has the most diverse and accessible redox chemistry of any of the halogens. The abundance of different redox states makes the quantitative analysis of iodine in an aquarium difficult, and this difficulty is compounded by the low concentration of iodine species in natural seawater.

Two redox states dominate in surface seawater — iodate and iodide. The iodine in iodate is in the +5 redox state, while in iodide it is in the -1 redox state. So, the iodine in iodate has 5 fewer electrons than an atom of elemental iodine, and iodide has 1 more electron than an atom of the free element. Intermediate oxidation states are not very stable, but are significant, as they are the active states in halogenation of organic molecules. Hypoiodate or hypoiodous acid is an activated form of iodine and has powerful antibacterial properties.

Iodine in the Ocean

The concentration of total iodine in the ocean is often given as 0.06 milligrams per liter (mg/L) of I, or about 5 x 10-7 molar, or 0.5 micromolar (see Footnote 1). Roughly two-thirds of the iodine in sea surface water is in the form of iodate, and one-third is in the form of iodide, although there is substantial variation in those ratios. The relative concentration of iodide decreases with depth in the ocean, and often iodide becomes undetectable at below a few hundred meters, with all the inorganic iodine in the form of iodate (Tian and Nicolas 1995). However, in anoxic sediments, iodate is converted to iodide. This may have important implications for various aquarium husbandry practices, and we will return to this subject later. Total dissolved iodine (the sum of all dissolved organic and inorganic iodine forms) follows a nutrient profile in the ocean, depleted by biological assimilation at the surface, increasing to a maximum value at a few hundred meters depth and then falling slightly to a nearly constant value through the rest of the water column (Tian and Nicolas 1995).

Biological relevance

It is almost universally stated that iodide is the form of iodine that is relevant for assimilation into biological molecules (Kirk 1991). There are well-described uptake mechanisms for iodide ion (Kirk 1991). Uptake mechanisms for iodate are somewhat more obscure, but it is possible that it can be concentrated by the nitrate uptake system in marine organisms. Some tracer work is consistent with iodate being essentially inaccessible to fish, with a concentration ratio of 1 (the ratio of iodate concentration within the fish and the concentration in the water). However, some marine algae appear to be able to concentrate iodate (Hirano et al. 1983).

Conversion from iodate to iodide: nitrate reductase

There is a deeper similarity between iodate and nitrate than the last three letters of their names. The “ate” ending usually refers to the highest, easily accessible oxidation state of oxyanions. Iodate and nitrate are the most oxidized common forms of iodine and nitrogen. More than that, IO3- (iodate) and NO3- (nitrate) are both flat, planar ions, with the iodine or nitrogen at the center, surrounded by three oxygens. Both ions have a net charge of -1. A common theme in biochemistry is that isosteric (same shape) molecules can compete for the same binding sites on enzymes. So, one might suspect some interaction of iodate with enzymes that metabolize nitrate and the other way around. As it turns out, this is precisely the case for this pair of anions.

Almost all reef aquarists have heard of the term “denitrification.” You probably also appreciate that this process is supposed to be happening in regions of the aquarium where the oxygen concentration is low — for example, within live rock and in sand beds in both marine and freshwater aquaria. This denitrification process is mediated by an enzyme within microorganisms called nitrate reductase.

There are two possible motives that an organism would have for reducing nitrate. The first is that the organism is nitrogen-starved and is converting nitrate (oxidation state +5) to ammonia (oxidation state -3). The ammonia produced by this reduction is then incorporated into essential biological molecules like protein and nucleic acids. The nitrate reductase enzymes that accomplish this conversion are called “assimilatory nitrate reductase” molecules and are found in some bacteria, but are most common among plants and algae. This is an expensive reaction from a metabolic point of view, requiring 8 reducing equivalents per mole of nitrate reduced to ammonia. The reaction also absorbs protons, effectively generating alkalinity in the system. It occurs most commonly in oxidizing environments, and in such an environment the organism must supply electrons to push the reaction up an energy hill.

The second type of nitrate reductase is called a “respiratory nitrate reductase.” It is used by organisms when the oxygen concentration of the surrounding area is low (anoxic or anaerobic environments). When you consume organic molecules, your body combines organic molecules with oxygen and oxidizes them to carbon dioxide and water. Through some really elegant biochemistry, the organism hangs on to some of the energy liberated in that series of reactions so that it can perform useful work, like making chemicals it needs, or moving about. In this form of oxidative metabolism, oxygen serves a role called “terminal electron acceptor.” Humans and many other familiar forms of life are absolutely dependent on oxygen to serve this role in our biochemistry.

In a metabolic sense, some microorganisms are much more versatile than humans. Instead of being absolutely dependent on oxygen, they can use other oxidized molecules from the environment when oxygen is in short supply. Nitrate is an oxidizing ion, and respiratory nitrate reductases enable the organism to substitute nitrate for oxygen and otherwise carry on as usual. Nitrate is reduced to nitrite, and, further, to gaseous nitrogen oxides, and even elemental nitrogen in this process. Still other organisms can exploit other oxidized molecules, like sulfate, and reduce it to hydrogen sulfide. Generally, nitrate tends to be reduced before the microbial community switches to sulfate reduction.

Respiratory and assimilatory nitrate reductases can reduce iodate to iodide. There is excellent evidence that the facultative anaerobic marine bacterium Shewanella putrefaciens is competent to reduce iodate to iodide under conditions of low oxygen concentration (Farrenkopf et al. 1997). It is not unreasonable to assume that at least some other marine bacteria in the same metabolic class will be able to do the same. This process may be important in reef aquaria.

Non-biological interactions of iodine species, organic molecules and light

There is also evidence that iodate can be converted to iodide by purely chemical mechanisms. It has been demonstrated that photochemically activated reactions with iodate in near surface water produce some iodide (Spokes and Liss 1996). The same studies indicate that dissolved organic chemicals play an important role in these processes.

Of potential note to aquarists is the reaction of iodate with vitamin C (ascorbic acid or ascorbate). Some types of chemical analyses for measuring total iodine first convert iodate to iodide by reacting the solution with ascorbic acid under acidic conditions (Campos and Lucia 1997). Some individuals within the reef hobby have noted apparently favorable reactions in aquaria dosed with ascorbic acid or ascorbate. It is tempting to suggest that this may have less to do with the ascorbate itself being directly used by organisms in the tank, and more to do with the purely chemical conversion of iodate to iodide by ascorbate. Dosing aquaria with ascorbate is an experimental practice and it should be done with caution. It is also known that thiosulfate can reduce iodate (Maros et al. 1989).

It is worth noting that other studies have indicated that biological reactions, and not photochemistry, determine the iodine speciation at the surface of the ocean (Brandao et al. 1994). Photosynthesis produces some hydrogen peroxide. Marine algae may be using iodide ion and haloperoxidases to scavenge toxic byproducts of photosynthesis.

Conversion from iodide to organic complexes and to iodate: haloperoxidases Haloperoxidases are enzymes that “activate” halide ions (Cl, Br, I) by oxidizing them with hydrogen peroxide. There is some disagreement in the literature as to exactly what happens on the surface of the enzymes when halides are oxidized (Frieden 1984), but the initial product is likely to be an X+ ion (Cl+, Br+ or I+, chlorodinium, bromodinium or iodinium ions respectively). Other possibilities include the elemental form of the halogen, or halogenium ions bound to the enzyme, like enzyme S-I complexes.

This activated form may then react with an organic molecule bound by the peroxidase, or it may be released and go on to form a hydrated product, ClOH, BrOH or IOH. There are at least two possible reasons for biological systems to make an activated form of these elements: Incorporation into organic molecules or their biocidal effects. Living systems exploit both possibilities of this chemistry.

Iodine has long been appreciated as an essential microelement for many groups of complex organisms. Initial evidence for the importance of iodine came from the realization that humans and other vertebrates fed an iodine-deficient diet developed several metabolic problems (hypothyroidism and goiter), and that these problems could be reduced by supplementing their diets with iodine.

Iodide is accumulated in the thyroid gland by the action of an “iodide pump.” The pump consists of two parts. The first is a transmembrane protein that co-transports a Na+ and I- ion across the membrane. The second part of the iodine pumping mechanism is a Na/K ATPase. This protein moves sodium ions out of the cell and moves K+ ions into the cell against concentration gradients of both ions. (In living cells, intracellular Na+ is usually low, intracellular K+ is high.) The first protein is able to exploit that concentration gradient of Na+ to concentrate I- inside the cell, by up to a thousandfold.

Once inside the thyroid gland of vertebrates, a special haloperoxidase enzyme activates it and iodinates tyrosine residues on a thyroglobin precursor protein. The thyroglobin protein is then processed proteolytically to make free thyroid hormones. The free thyroid hormones are then released into the bloodstream where they regulate a number of metabolic processes. There is a huge body of literature on the biological chemistry of thyroid hormones. Readers wishing to learn more about the primary literature behind the three preceding paragraphs are directed to reviews by Frieden (1984) and Kirk (1991).

Although invertebrates do not possess a vertebrate-like thyroid gland, they are able to accumulate iodide from the environment. They also make a wide variety of iodinated organic molecules, including iodotyrosine derivatives very similar to the thyroid hormones of vertebrates. The formation of iodoaminoacids is very common in the sea and it has been known since 1974 that cnidarians (the phylum that includes corals) manufacture iodotyrosines (thyroxines, thyroid hormones). Thyroxine was shown to promote early conversion of hydroids into free-living forms in jellyfish (Spangenberg 1974). Organisms from many other invertebrate phyla also produce iodinated amino acids (Gorbman 1978). The biological function of these iodinated amino acids is currently under investigation. Marlin Atkinson and co-workers are investigating the intriguing possibility that they regulate coral growth and perhaps other physiological functions.

There are also iodoperoxidases and other haloperoxidases on the surfaces of organisms (or outside of cells) that seem to activate iodine and other halides purely for their biocidal properties. Curiously, even terrestrial vertebrates have maintained such a system for activating halides for biocidal purposes as part of their immune systems. Certain types of white blood cells manufacture hypoiodite and hypobromite (Kirk 1991). Some of the same chemical tricks are used in the eternal war for space on the reef and the ongoing battle that your body wages to fend off microbial invaders.

Elemental iodine in the ocean and the aquarium, and Lugol's solution

Elemental iodine is unstable in seawater and is found in the elemental form in vanishingly low concentrations. It initially seems to undergo this reaction: I2 --> I- + I+. The I+ then undergoes a complex series of reactions.

One of the iodine additives promoted for reef aquarium use is Lugol’s solution, which in veterinary practice is sometimes used as a topical disinfectant. Iodine is quite insoluble in water, but in the presence of excess I-, the I2 is converted to I3-. This trick allows one to get a substantial amount of what is essentially elemental iodine into a purely aqueous solution. Lugol’s solution consists of 10 grams of KI and 5 grams of I2 per 100 milliliters final volume. This makes the solution almost exactly 1 molar total iodine, with 0.602 molar I- and 0.394 molar I atom equivalent. It has been shown that when elemental iodine is added to actual seawater samples containing naturally occurring dissolved organic substances, the minority of elemental iodine (17 percent) was oxidized to iodate, the remainder being reduced with the involvement of organic substances dissolved in the water (Truesdale et al. 1995). It is also interesting to note that inorganic iodine was not conserved in these experiments, so presumably some iodinated organic compounds were formed directly in the water. So, the expectation is that with the conversion of the elemental iodine in Lugol’s solution mainly to iodide through the series of intermediates that formed, over 90 percent of the total iodine in Lugol’s should ultimately become iodide.

The disappearance of molecular iodine is rapid. It has been observed that the apparent rate constant for reduction of I2 added to seawater varies from 0.030 and 2.31 per minute. The ephemeral existence of the yellow-colored molecular iodine casts some doubt on its ability to provide any significant or meaningful photoprotection for light-shocked animals. The anecdotal evidence suggesting that addition of Lugol’s solution to aquaria aids in photoadaptation (Delbeek and Sprung 1994) must find another mechanism other than simple light absorption by molecular iodine or triiodide ions. For example, iodide is known to react efficiently with some toxic byproducts of photosynthesis. This might acount for the anecdotal evidence mentioned by Delbeek and Sprung (1994).

Although the elemental iodine should rapidly become iodide and intermediates, it may take some time for these intermediates to clear from the water. Some of those intermediates are toxic. Based on Marlin Atkinson’s presentation at the 1997 Western Conference, and his suggestion that it may be prudent to add only iodide salts to reef aquaria, a few aquarists have offered observations leading them to believe that Lugol’s solution may be quite detrimental to some organisms, Xenia being the most often cited example. None of these observations have been controlled experiments, nor were they conducted in replicate.

Gradual Dosing

At the 1997 Western Marine Conference, Marlin Atkinson presented a seminal overview of iodine and its interactions with marine systems. There were several important aspects of this talk — a summary of iodine chemistry, rate constants for the conversion of I- to other species by reef benthos and the suggestion that it is a good idea to dose iodine gradually.

Sessile marine organisms have a problem. They need to be attached to a surface in some fashion, and there is only so much area to go around. In the ocean, as in human life, it seems that everyone is trying to grow on top of everyone else. If you look at a healthy reef community in the ocean, you will be struck by the fact that there seems to be some organism or another occupying what seems to be every square centimeter of the reef structure. The reef is a city with a nearly 100-percent apartment occupancy rate.

As we have seen, at least some of these organisms seem to be packing haloperoxidase enzymes on their surfaces. You might think of them as little guns and their ammo is bromide (see the previous column), and in some cases, perhaps also iodide ions. Aquarists who have studied the rise and fall of iodide ion concentrations in their reef aquaria with the Seachem iodide test kit know that over a week, and in some cases even over a few days, the iodide concentration in a reef tank can fall from natural seawater concentrations to quite low concentrations. When you add a big dose of iodide, all those little guns may fire all at once. This may be a problem for the organism.

Because the bromide ion participates in similar chemistry and seems to be used for similar purposes, aquarists seeking to raise the bromide concentration in their tanks suddenly to natural seawater concentrations may also run into the same sort of problem. So, gradual dosing of iodide and bromide makes a lot of sense. Because it has been shown that iodide and iodinated amino acids have a critical role in regulating some aspects of coral metabolism, it also makes sense to attempt to present these organisms with a constant level of iodide for that reason as well.

Connections with aquaria

The most simple conclusion that one can draw from this is that it is a good idea to have some sort of anoxic compartment in the aquarium. It may be that those individuals who report great success from plenum systems or systems with “live sand” are observing not only the denitrifying capacity of those areas, but also the reduction of iodate to iodide in those compartments. It is also possible that some of the lore that has arisen in the hobby regarding the perils of high nitrate concentration in reefs has more to do with the inadequate capacity of the system to reduce iodate back to iodide, or, as Marlin Atkinson suggested at the last Western Conference, some interference between nitrate and iodide uptake.

Writing this series of columns has been something of a long “strange trip” through the literature. I’d love to be able to offer you ultimate conclusions. Some aspects of iodine chemistry and the aquatic chemistry of the halogens are clear. I’ve tried to present them in a concise form.

But, what I’ve personally wound up with is a set of questions that I hope the aquarium community will find answers for soon. This is an occasion where home aquarists can make potentially very interesting observations on their home systems. The questions in my mind as I close this column are as follows.

Some people use thiosulfate to dechlorinate water for use in their reef aquaria. As we have seen, thiosulfate can react with iodate and reduce it to iodide. The reef community has moved toward highly purified water and dechlorinator is no longer necessary. But, might the addition of small amounts of sodium thiosulfate reduce some iodate to iodide and make it more available? I’ve not tried it yet. Is it possible that too much dechlorinator might cause a large conversion of iodate to iodide, and cause problems for the organisms? It can be fairly said that we do not know the answer to this question at present.

Some people have advocated the use of ascorbic acid (vitamin C) in reef aquaria, and have their own set of anecdotal observations to support that use. Might it be that the ascorbic acid is actually converting some iodate to iodide and this is the benefit? Why is it said by the advocates of vitamin C that it is important to slowly dose the aquarium with vitamin C in the beginning? Perhaps a massive conversion of iodate to iodide? We don’t know the answer to this question at present.

Many people swear by the use of live sand in reef aquaria. The potential for a live sand bed or a plenum to reduce nitrate in a reef tank is fairly well established in the reef community at this point. But, does an anoxic compartment also facilitate the reduction of iodate to iodide? Does the presence or absence of a live sand bed have a significant effect on the apparent rate of conversion of iodide to iodate? Do these systems tend to maintain a more constant concentration of iodide than systems without live sand? Are reef aquaria that run higher in organics more tolerant toward the addition of Lugol’s solution? And, does Lugol’ss solution really have an adverse effect on corals in aquarium situations? A highly motivated aquarist with a lot of Xenia might consult with a friendly biologist as to how to construct a statistically significant test of that hypothesis.

Do those of you running ozone see significant changes in the rate of iodide decrease in your reef tanks with and without ozone? What about those of you running intense ultraviolet sterilizers?

And, what about those denitrating filters that seem to have decreased in popularity? Has anyone measured the iodide concentration entering the filter and exiting it? If there is nitrate reduction, there should be conversion of iodate to iodide, so the answer might surprise you.

How much iodine is removed from marine aquaria by foam fractionation? Is there significant escape of iodine through the gas phase? These chemical measurements will be made more difficult by the fact that iodate should be converted to iodide in skimmate, because it has a low oxygen concentration and usually has a rich bacterial population. There may be significant conversion of iodate to iodide in skimmate, so a total iodine method should be used for these measurments.

The scientific community will continue to address the larger issues of how iodine interacts with marine systems, its role in regulating growth and reproduction in invertebrates, and the role of iodide and haloperoxidases in providing protection against toxic products of photosynthesis. However, reef aquarists have questions specific to their systems, and your careful observations may be of use to other aquarists and may open windows to some aspects of iodine chemistry and interactions with other husbandry practices in closed marine systems that are poorly understood at present.

Footnote 1: To convert from grams per liter to moles per liter, divide the concentration of the chemical in grams per liter by the molecular or atomic mass in grams per mole. For example, iodine is 0.00006 grams per liter divided by 126.9045 grams per mole of I = 4.7 x 10-7 moles per liter.

References

HOME Table Of Contents BIOCHEMISTRY IN THE AQUARIUM

©Copyright 1997-98, Aquarium Frontiers and Fancy
Publications Inc. All rights reserved.