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The micronutrient iron is now recognized to be important in regulating the magnitude and dynamics of ocean primary productivity, making it an integral component of the ocean's biogeochemical cycles. In this Review, we discuss how a recent increase in observational data for this trace metal has challenged the prevailing view of the ocean iron cycle. Instead of focusing on dust as the major iron source and emphasizing iron's tight biogeochemical coupling to major nutrients, a more complex and diverse picture of the sources of iron, its cycling processes and intricate linkages with the ocean carbon and nitrogen cycles has emerged.
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Ocean primary production is important for the operation of the Earth System, underpinning the functioning of the global carbon cycle, air-sea CO2 exchange and marine ecosystems1. In the past three decades the micronutrient iron has been established as a key elemental resource that shapes the magnitude and dynamics of primary production in the global ocean. Oceanographers first became interested in iron in the 1930s, hoping to explain why the major nutrient inventories (nitrogen and phosphorus) were not fully depleted by primary producers in surface waters of much of the Southern Ocean2-4 (the 'Antarctic paradox'). Indeed, early laboratory studies showed that iron enrichment stimulated the growth of phytoplankton5, providing evidence that iron could control phytoplankton growth in sea water6,7. However, owing to the low solubility of iron in the modern oxic ocean8 and the tendency for iron to be 'scavenged' from the water column by sinking particles9, dissolved iron was probably a rare commodity for ocean biota. By the early 1980s the crucial role for iron as a co-factor in many cellular enzymes, particularly those linked with photosynthesis, respiration and nitrogen fixation had been identified10,11. Yet, despite this body of knowledge, it was not until the early 2000s that the global importance of iron to ocean productivity and biogeochemistry became widely recognized and included in global ocean models12.
Emergence of the 'iron hypothesis'
A key issue hindering the study of iron-phytoplankton interactions in the ocean was the need for contamination-free sampling and accurate measurements at the required picomolar to nanomolar levels (10-12 to 10-9 moles per litre). Only in the late 1970s did development of exacting trace-metal clean sampling protocols13...