IRON or What a Lonely Planet We Really Are

"The oceans are often thought of as teeming with life, but this is true of only a few regions, and these tend to be over-fished as a consequence. More than 80% of the boundless ocean is empty. In the mid-1980s, John Martin of the Moss Landing Marine Laboratories, in California, put forward the theory that it was the lack of iron in the upper levels of the sea which prevented plankton from growing, and without plankton to feed on, other forms of marine life have no food supply to support them.

Ten years later Martin's idea was tested by a joint USA-UK research team, which used a solution of iron sulfate to fertilize 60 square kilometers of the Pacific Ocean, West of the Galapagos Islands. The results were dramatic. Within a week this barren span of ocean bloomed and turned green with plankton, proving that it was simply lack of this metal that was limiting their growth.

Suddenly, it was realized that the oceans could one day be fertilized, perhaps with ferrous sulfate, which is easily and cheaply made from rusty old iron and sulfuric acid. The seas could bloom again, as they must once have done when the world was young, and when cellular organisms first flourished and learned how to make use of this versatile metal."

from one of my favorite books "Nature's Building Blocks: An A-Z Guide to the Elements" by John Emsley