A major uncertainty in our understanding of clouds and climate is the sources and properties of the aerosol on which clouds grow. We found that aerosol containing organic matter from fast-growing marine phytoplankton was a source of ice-nucleating particles (INPs). INPs facilitate freezing of ice crystals at warmer temperatures than otherwise possible and therefore change cloud formation and properties. Our results show that ecosystem processes and the properties of sea spray aerosol are linked.