Alteromonas macleodii vesicle diameter measured during Cryo-EM micrograph experiment

The cyanobacterium Prochlorococcus has a conspicuously reduced genome causing it to require help from co-existing organisms for survival under a variety of stressful conditions. Here we showed that Alteromonas facilitated greater exploitation of medium resources by Prochlorococcus, allowing them to grow to greater cell densities, delay entry into stationary phase, and avoid cell death in batch co-cultures, potentially by degrading autotoxic substances produced in dense Prochlorococcus cultures. We further showed that heat-labile high-molecular weight exudates from Alteromonas were responsible for this effect. Alteromonas exudates contained a wide variety of proteins and demonstrated enzymatic activities, and both the composition and activity of the exudates changed after 500 generations of adaptation to a changed environment, suggesting some form of genetic regulation. Some of these proteins and activities may have been packaged within extracellular membrane vesicles, which we identified within Alteromonas exudates and found capable of physically associating with Prochlorococcus cells. Many of the functionalities observed in Alteromonas exudates (e.g., increasing phosphate availability, degrading hydrogen peroxide) were consistent with leaky Black Queen processes, which are defined as services provided by one organism that benefit the entire community and favor the evolution of interdependencies in microbial communities. This dataset contains manual measurements of extracellular vesicle diameters from Alteromonas macleodii obtained from cryo-electron micrographs. The images used for these measurements are also included in the supplemental material.