Many microorganisms produce resting cells with low metabolic activity that allow them to survive prolonged nutrient or energy stress. We show that Prochlorococcus , the dominant phytoplankton lineage in large regions of the nutrient-poor ocean, cannot survive extended stress alone. In late stationary phase cultures, some cells retain metabolic activity (single-cell carbon and nitrogen uptake), but these axenic cultures do not re-grow when transferred into new media. However, when co-cultured with a heterotrophic bacterium, Prochlorococcus survived nutrient starvation for months. This dependence on heterotrophic bacteria is not limited to nutrient starvation - up to 10% of the Prochlorococcus cells in the oceans live under conditions of light starvation, surviving by utilizing organic compounds released by other microorganisms. We propose that reliance on co-occurring heterotrophic bacteria or on the organic compounds they produce, rather than the ability to survive extended starvation as resting cells, underlies the ecological success of Prochlorococcus .