Iron is a micronutrient indispensable for life, enabling processes such as respiration, photosynthesis, and DNA synthesis. Iron availability is often a limiting resource in today’s oceans, which means that increasing the flow of iron into them can increase the amount of carbon fixed by phytoplankton, with consequences for the global climate.

Iron ends up in oceans and terrestrial ecosystems through rivers, melting glaciers, hydrothermal activity, and especially wind. But not all its chemical forms are “bioreactive,” that is, available for organisms to take up from their environment.

Here we show that iron bound to dust from the Sahara blown westward over the Atlantic has properties that change with the distance traveled: the greater this distance, the more bioreactive the iron.