Open-ocean regions rich in nitrate and phosphate were thought to contain enough iron that iron deficiency could not explain their low phytoplankton productivity.

Some parts of the open ocean contain an odd combination: plenty of nitrate and phosphate, yet relatively little phytoplankton.

Phytoplankton are microscopic photosynthetic organisms that form the base of much of the marine food web. Like plants on land, they need major nutrients such as nitrogen and phosphorus. But they also need much smaller amounts of trace elements, including iron.

Iron limitation had been proposed for decades, but seawater measurements seemed to show enough iron that the idea was difficult to prove.

The measurement itself turned out to be part of the problem. The natural amount of dissolved iron in remote surface water can be extraordinarily small. A little contamination from a ship, metal equipment, dust, or a sample container can overwhelm the real signal.

Cleaner sampling in the 1980s revealed that some open-ocean waters contained far less dissolved iron than older measurements suggested.

Researchers then tested whether that shortage mattered biologically. Adding tiny amounts of iron to seawater samples produced much stronger phytoplankton growth and greater use of the nitrate already present. In the 1990s, open-ocean experiments added iron directly to nutrient-rich waters and produced clear blooms.

Iron is not the only control on ocean productivity. Light, grazing, nitrogen, phosphorus, silicon, and other factors can dominate in other places. The correction is that abundant nitrate and phosphate do not guarantee that phytoplankton have every nutrient they need.