In this study, we investigated the responses of <i>Phaeodactylum tricornutum</i> cells acclimated to 300 µmol m<sup>−2</sup> s<sup>−1</sup> photon flux density to an increase (1000 µmol m<sup>−2</sup> s<sup>−1</sup>) or decrease (30 µmol m<sup>−2</sup><sub> </sub>s<sup>−1</sup>) photon flux densities. The light shift occurred abruptly after 5 days of growth and the acclimation to new conditions was followed during the next 6 days at the physiological and molecular levels. The molecular data reflect a rearrangement of carbon metabolism towards the production of PEP and/or pyruvate. These intermediates were used differently by the cell as a function of the photon flux density: under low light, the photosynthesis was depressed while respiration was increased. Under high light, lipids and proteins accumulated. Of great interest, under high light, the genes coding for the synthesis of aromatic amino acids and phenolic compounds were upregulated suggesting that the Shikimate pathway was activated.This article is part of the themed issue ‘The peculiar carbon metabolism in diatoms’
