In this project, transcriptomic data were produced for model microbial systems between marine bacteria and eukaryotic phytoplankton. These co-culture systems consisted of the model Roseobacter bacterium Roseovarius nubinhibens ISM and two different model phytoplankters, the diatom Thalassiosira pseudonana and the dinoflagellate Alexandrium tamarense. In both 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 diatoms and dinoflagellates often dominate primary production in coastal surface ocean waters.