Many heterotrophic bacteria are known to release extracellular vesicles, facilitating interactions between cells and their environment from a distance. Vesicle production has not been described in photoautotrophs, however, and the prevalence and characteristics of vesicles in natural ecosystems is unknown. Here, we report that cultures of Prochlorococcus, a numerically dominant marine cyanobacterium, continuously release lipid vesicles containing proteins, DNA, and RNA. We also show that vesicles carrying DNA from diverse bacteria are abundant in coastal and open-ocean seawater samples. Prochlorococcus vesicles can support the growth of heterotrophic bacterial cultures, which implicates these structures in marine carbon flux. The ability of vesicles to deliver diverse compounds in discrete packages adds another layer of complexity to the flow of information, energy, and biomolecules in marine microbial communities.