Iron and the Anemic High-Nutrient-Low-Chlorophyll Oceanic Regions

Since the 1930s, scientists have been curious about the fact that in certain regions of the oceans, nitrogen and phosphorus are seemingly sufficient, but chlorophyll a does not accumulate. These regions—formally identified as the high-nutrient-low chlorophyll (HNLC) regions—are found only in the extremes: the Equatorial Pacific, the Subarctic Pacific, and much of the Southern Ocean. While there has been no lack of hypotheses to explain these regions, they are now known to be limited by iron. This chapter recounts how the iron hypothesis came to prominence, provides a brief review of cellular iron physiology, and provides a description of the major iron enrichment studies and their findings, as well as the controversies surrounding major ocean enrichment experiments.