Iron Cycle

Iron is a key ingredient of many enzyme systems in the ocean, and thus plays a unique and important role in controlling its productivity. At the time marine life first evolved, iron was extremely abundant in the reduced form Fe( ii), but as photosynthetic organisms restructured the chemistry of the ocean through evolution of excess oxygen, it became less and less abundant because of the very low solubility of the Fe( iii) form. In the modern ocean, iron has become a rare trace element in spite of its huge relative abundance in the rocks and soils of the continents, with the result that continental weathering in the form of dust particles now represents the main external supply of this vital nutrient in the oceans. As a result of these self-engineered changes, marine phytoplankton had to adapt to a shortage of iron by developing much more efficient mechanisms for sequestering and using what little iron is available, and it has become one of the controlling factors for phytoplankton growth in large parts of the global ocean.