From Randy
"Iron in the ocean is primarily iron(III) (Fe[sup]3+), because any Fe[sup]2+[/sup] that forms is oxidized back to Fe3+[/sup] by oxygen (O[sub]2) 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[/sub]. 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.
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."
Entire article
http://www.advancedaquarist.com/2002/8/chemistry
