Major symbiotic events have occurred in aquatic systems, most prominent of which is the origin of the eukaryotic cell that created the ancestor of all animals, plants, fungi and protists. Understanding symbioses of aquatic organisms will enrich our grasp of the full range of symbiosis on our planet. The foundation is investing $140 million over nine years to support development of new tools, theory and concepts about aquatic symbioses and to bring different research communities together to learn how symbioses involving microorganisms function, evolve and serve critical ecosystem roles in marine and freshwater environments.