Two new species of pennate Rhopalodiaceae diatoms (Epithemia pelagica and Epithemia catenata) were recently isolated from North Pacific Subtropical Gyre. The diatoms are unique as they contain nitrogen-fixing endosymbionts that have historically been characterized as independent unicellular cyanobacteria. Laboratory based culture measurements demonstrated that the evolutionary transition from a free-living cell to an endosymbiont has meant that nitrogen fixation is longer restricted to the night and can occur during both day and night. Analysis of global ocean sequence databases revealed Epithemia is widespread through the tropical and subtropical oceans. To learn more about the endosymbiosis, the host and symbiont genomes were recently sequenced as part of the Aquatic Symbiosis Genomics Project. Until now, the marine Rhopalodiaceae-diazotroph symbioses have been overlooked but their ubiquitous presence in the marine environment and their potential as cultured model organisms for the study of organelle evolution provides new insights into nitrogen fixation at the cellular and ecosystem scale.