Phytoplankton are the foundation of marine food webs, but their growth depends on supply of over a dozen nutrients to the upper ocean where photosynthesis can occur. Many of these are deemed ‘micronutrients’ and represent less than 1 part per billion of the atoms in seawater. Micronutrient scarcity has forced phytoplankton to develop unique strategies to acquire these resources, or to reduce their biochemical dependence on them, but these efforts are not always enough: throughout the oceans, micronutrient limitation of phytoplankton growth controls the size of marine ecosystems and influences the global carbon cycle. Many mysteries about these micronutrients remain, particularly for metals like iron, cobalt, and zinc whose chemistry in seawater is complex. My research focuses on the global cycles of these elements in the oceans, how they influence the growth and evolution of phytoplankton lineages, and how their distribution would change in past or future climates.