Updated project metadata. Prochlorococcus is the most abundant photosynthetic organism on Earth, contributing significantly to the global primary production and playing a prominent role in biogeochemical cycles. Prochlorococcus requires the capability to accommodate to environmental changes in order to proliferate in oligotrophic oceans, in particular regarding nitrogen availability. A precise knowledge of the composition and changes of the proteome is necessary to gain fundamental insights into such response. Here we study the effects of extreme nitrogen limitation, a feature of the oligotrophic oceans inhabited by this organism.