Iron is limiting in todays oceans yet many organisms require it for enzymes and most importanly photosynthesis

Question

how did this come about?

Answer 1

Well, really, the problem is that iron is not soluble in water under oxidizing conditions (it makes "rust"-iron oxide and hydroxide minerals that precipitate out of the water and drop into the sediments).

In reality, there is always some limiting nutrient.  In fresh water lakes, for example, phosphorus is often the limiting nutrient, which is why adding phosphorus to the water (from fertilizers and laundry detergents for example) leads to uncontrolled growth of plantlife and basically the death of the lake - all those plants suck out the oxygen at night and kill the lake for any animals (fish).

When something isn't limited, life tends to take over until there is something limiting.

Answer 2

This is an interesting question.

It is thought that before the Cambrian period, about a billion years ago, there was an amount of iron dissolved in the seawater.  At that time, primitive life had just started on earth, and there was no free oxygern in the atmosphere.  However, cyanobacteria lived on nitrogen, combining it with carbon dioxide and water to form organic substances (cyanobacteria are still alive today).

As a waste product, they produced oxygen (in a similar way that plants do).  This free oxygen was very reactive, and combined with the dissolved iron (as well as other dissolved minerals), and precipitated out insoluble iron oxide, which fell to the bottom.  This material formed a particular type of rock called 'banded iron formation', because it is made up of layers of iron oxide and silica.  These rocks can be found in some old precambrian sediments today.

Eventually, most of the iron was precipitated out of the water, and only then was there free oxygen available for the atmosphere.

These banded iron formations were sometimes subjected to further changes by circulating groundwaters, which concentrated the iron oxide into some of the large iron ore deposits currently being extracted (eg, in Australai).