Photosynthetic phytoplankton are the foundation of marine ecosystems. Their growth in the sunlit ocean depends on ample supply of over a dozen essential elements. Of these elemental nutrients, the metal cobalt is found at the lowest concentrations in seawater, but it is unknown whether cobalt scarcity impacts phytoplankton growth. We have measured minimum cobalt requirements of the photosynthetic bacterium Prochlorococcus, which flourishes in nutrient-poor regions of the ocean where many other phytoplankton cannot survive. Prochlorococcus can grow with less than 50 cobalt atoms per cell, an extraordinarily small requirement that explains how this organism can persist in low-cobalt environments. These results enable predictions of how marine ecosystems respond to climate-driven changes in nutrient supply.