Phytoplankton, unicellular photosynthetic microbes, play a fundamental role in the global carbon cycle and fuel marine food webs. Globally, phytoplankton productivity is regulated by the availability of essential nutrients such as nitrogen and iron. Researchers at the GEOMAR Helmholtz Centre for Ocean Research, Kiel, have now been able to show that the growth of phytoplankton over large extents of the ocean is not limited by a single nutrient, but by multiple nutrients simultaneously. The study has been published today in the top scientific journal Nature.