In this project, transcriptomic data were produced for model microbial systems between eukaryotic phytoplankton and Roseobacter clade members. In these systems, the phytoplankton relied on the bacteria for a source of vitamin B12, in exchange for fixed carbon. Members of the model systems represent taxa important in the ocean. Bacteria in the Roseobacter lineage are ubiquitously distributed in surface waters and consistently found in association with eukaryotic phytoplankton, while eukaryotic phytoplankton often dominate primary production in nutrient-rich surface ocean waters.