Exudate Stimulates Heterotrophic Bacterial Competition with Rival Phytoplankton for Available Nitrogen.

The marine cyanobacterium numerically dominates the phytoplankton community of the nutrient-limited open ocean, establishing itself as the most abundant photosynthetic organism on Earth. This ecological success has been attributed to lower cell quotas for limiting nutrients, superior resource acquisition, and other advantages associated with cell size reduction and genome streamlining. In this study, we tested the prediction that outcompetes its rivals for scarce nutrients and that this advantage leads to its numerical success in nutrient-limited waters. Strains of and its sister genus grew well in both mono- and cocultures when nutrients were replete. However, in nitrogen-limited medium, outgrew but only when heterotrophic bacteria were also present. In the nitrogen-limited medium, the heterotroph Alteromonas macleodii outcompeted for nitrogen but only if stimulated by the exudate released by or if a proxy organic carbon source was provided. Genetic analysis of suggested that it outcompetes for nitrate and/or nitrite, during which cocultured grows on ammonia or other available nitrogen species. We propose that can stimulate antagonism between heterotrophic bacteria and potential phytoplankton competitors through a metabolic cross-feeding interaction, and this stimulation could contribute to the numerical success of in nutrient-limited regions of the ocean. In nutrient-poor habitats, competition for limited resources is thought to select for organisms with an enhanced ability to scavenge nutrients and utilize them efficiently. Such adaptations characterize the cyanobacterium , the most abundant photosynthetic organism in the nutrient-limited open ocean. In this study, the competitive superiority of over a rival cyanobacterium, , was captured in laboratory culture. Critically, this outcome was achieved only when key aspects of the open ocean were simulated: a limited supply of nitrogen and the presence of heterotrophic bacteria. The results indicate that promotes its numerical dominance over by energizing the heterotroph's ability to outcompete for available nitrogen. This study demonstrates how interactions between trophic groups can influence interactions within trophic groups and how these interactions likely contribute to the success of the most abundant photosynthetic microorganism.