The iron hypothesis of John Martin and the in situ iron fertilization experiments in the HNLC regions were the most exciting events in the 1990s. Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the “iron hypothesis”. For this reason, it is important to understand the response of pelagic biota and atmospheric carbon dioxide to increased iron supply. Three in situ iron enrichment experiments， IronEx1 and IronEx2 (in eastern equatorial Pacific), SOIREE (in Southern Ocean)， were carried out to test the contentious hypothesis. These experiments confirmed that the increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters. But downward export of biogenic carbon did not increased. Estimates of the CO₂ amount removed from the atmosphere by the iron-related phytoplankton bloom remained uncertain. The “iron hypothesis” is still a hypothesis.