We examine how the transcriptome of Prochlorococcus strain NATL2A changes in response to extended light deprivation, both when grown alone and in the presence of a naturally co-occurring heterotroph, Alteromonas macleodii MIT1002. Overall design: Samples are collected over time from cultures experiencing either a 13:11 light:dark light cycle or that remain in the dark following the end of the expected night. Samples are collected from each of 3 biological replicate cultures of Prochlorococcus, grown either alone or in co-culture with a marine Alteromonas.