Photoacclimation in prokaryotic phytoplankton species commonly found in the ocean is not as well documented as for eukaryotes. Variations in the pigment cellular content and composition were observed in Synechococcus1 and Prochlorococcus2 in response to changes in light conditions. These variations lead to changes in the magnitude and spectral shape of light absorption capabilities3•4• Partensky et al. 2 reported a significantly higher photosynthetic capacity in Prochlorococcus grown under high light. Besides these signs of the capability of prokaryotic phytoplankton to photoacclimate, it is known that they possess no flagellae and do not operate a xanthophyll cycle5. So, keeping in mind the specific character of prokaryotes, it could be thought that the number of photoacclimation strategies is smaller compared with eukaryotes. Nevertheless, prokaryotic phytoplankton dominate large parts of the ocean, especially oligotrophic systems where it grows at high rates6. In the present study, we aim at understanding why prokaryotic phytoplankton have such an ecological success in the ocean. Specifically, our objectives are ( 1) to clearly identify the photoacclimation strategies deployed by these organisms, significant in terms or carbon fixation, and (2) to quantify the timescales on which they are deployed. Finally, we discuss how these strategies are effective in the ocean. To reach our objectives, we studied the photoacclimation kinetics of Prochlorococcus sp. We chose this species firstly because it often dominates the prokaryotic phytoplankton communities of oligotrophic systems7 . Secondly, there is a need for a better documentation on this recently discovered species, which was shown to account for up to 50 % of net primary production6 in oligotrophic systems.