Chemistry Corner: MAGNESIUM
Broach the subject of chemistry with most folks, and you can actually see their attention flat-line. No one knows this better than me; after years of mind numbing college chemistry courses, I was there in body, but not necessarily in spirit. Yet, I am happy I made it through because that education has made me a better aquarist. Knowing a little about the chemistry of your marine aquarium pays huge dividends in the long run. Fortunately, there is no need to understand the vagaries of titrations, stoichiometry, or the construction of the period table. Just check out this new column, and I will break down all you need to know about the chemical constituents of your saltwater.
In this piece, the importance of magnesium is detailed. This little molecule plays a critical role in making calcium available for uptake by corals and other invertebrates. In fact, it is the unsung hero in this process, and keeping tabs on it will ensure long-term success. Corals, clams, arthropods, and coralline and calcareous algae will all reap healthy benefits from adequate magnesium levels.
Other than sodium and chloride, no element is more prevalent in seawater than magnesium. It is a stable molecule that tends to remain in the environment for an exceedingly long timeup to tens of millions of years! In the sea, magnesium exists as the cation Mg2+, at an average of 1288 mg/L (53 mM). The concentration, by mass, is approximately 3 times that of calcium (412 mg/L, 10 mM). Amazingly, at 100 gallons, an aquarium actually contains around 1 pound of magnesium, by weight.
Basically, the job of magnesium is to replace the calcium in a calcium carbonate bond. Since calcium also forms an ion with a plus-2 charge (Ca2+), the fit is perfect. The calcium is then free to be taken-up by marine animals and algae, and the new magnesium carbonate molecule can still be broken easily enough to allow carbonate to act as a buffer. Moreover, the typical pH range of seawater (8.1 to 8.4) would allow significantly less carbonate to exist than does, when magnesium is in solution. Therefore magnesium not only controls calcium availability, but alkalinity to some degree as well.
It seems simple enough, and it is. However, the importance of this relationship cannot be understated. In seawater, nearly ten times more carbonate is bonded to magnesium than calcium. That means that there are ten times more calcium ions available for direct uptake by invertebrates, as well as indirect uptake by vertebrates.
In the aquarium, magnesium levels are often forgotten about, because short-term testing usually reveals no changes. This stability gives the false impression that magnesium never falls. Nonetheless, magnesium gets slowly exported by the same organisms that export calcium. For example, photosynthetic organisms use magnesium in the creation of chloroplasts, and as an enzyme activator. Corals and other animals, on the other hand, deposit low concentrations of magnesium in their skeletons. Even algae get in on the action. In fact, coralline and calcareous algae may sequester up to 4.4% in their skeletons, which is much more than most organisms. By comparison, many coral species use less than 1% magnesium in their skeletons.
If over time, the magnesium levels in an aquarium are allowed to fall too low, calcium will become less available to livestock. Aquarists will often notice weak or slowed skeletal growth and attribute it to inadequate calcium levels. When they check, they are surprised to find that the calcium concentration is inexplicably fine. They might even find that levels are difficult to stabilize. This kind of problem is indicative of a low magnesium ratio, and the solution is relatively simple. First, doing regular water changes of 10% weekly or 20% biweekly, using a high-quality synthetic saltwater mix, is a must. Be aware that some mixes tailored for fish-only tanks may not have enough magnesium, and so the element should be supplemented if using these salts for reef tanks. Second, use foods that are high in magnesium such as nori, plankton, silversides, and lancefish. Third, the use of a calcium buffer which contains a known concentration of magnesium is critical. Some buffers include magnesium as an additive, while others do not. Using kalkwasser, or limewater, can provide adequate amounts of magnesium, but again, every mixture is different. Even the calcium carbonate used in calcium reactors presents a range of magnesium proportions. In the event that you discover a dangerously low magnesium level, it may be necessary to try to elevate the concentration quickly. There are several lines of commercial magnesium supplements available on the market, including those by E.S.V., Kent, Seachem, and Tropic Marin. A good all-in-one micronutrient additive will also help maintain magnesium levels, though they may be inadequate for quickly bringing up low concentrations.
I have, without question, oversimplified the issue of magnesium, not only in how it affects calcium availability and alkalinity, but also in how it interacts with the other elements in seawater. Nonetheless, I think its importance is clear. Like any good friend, you should not take it for granted simply because it is reliable. Though you may never notice diminished magnesium levels in your reef tank, this is not because magnesium is unimportant. That proportions of the ion stay relatively stable is indicative of its low reactivity, and is a double-edged sword of sorts. One the one hand, you may get away with testing magnesium levels as infrequently as once-a-month, or less. On the other hand, you may find yourself forgetting about the element altogether, only to end up with a mysterious calcium problem down the road. Alas, have no fear if this happens to you! Now you know where to check, and what to do with the results. Magnesium is like fresh airyou rarely notice it, but when its not there you sure miss it!
Works Cited
Holmes-Farley, Randy. Do-It-Yourself Magnesium Supplements for the Reef Aquarium. Reef Keeping Magazine Online. July 2006, 9:6. 2006. < http://reefkeeping.com/issues/2006-07/rhf/index.p...>
Holmes-Farley, Randy. Chemistry and the Aquarium: Magnesium in Reef Aquaria. Advanced Aquarist Online Magazine. Oct., 2003. 2003. URL: < http://www.advancedaquarist.com/issues/oct2003/ch...>
Holmes-Farley, Randy. Chemistry And The Aquarium Magnesium And Strontium In Limewater. Advanced Aquarist Online Magazine. Dec., 2003. 2003. URL: < http://www.advancedaquarist.com/issues/dec2003/ch...>
Brightwell, C.R. Marine Chemistry Neptune City: T.F.H Publications, 2007.