The diatoms Thalassiosira hyalina and Nitzschia frigida are important members of Arctic pelagic (open-water) and sympagic (ice-associated) microalgal communities. We investigated here the molecular mechanisms these algae apply to cope with abrupt high light exposure (Shift from 20 to 380 µmol photons m-2 s-1, resembling upwelling or ice break-up). Experiments were done under contemporary as well as future pCO2 (400 vs. 1000 µatm) to investigate whether [CO2] or pH modulate the reactions to high light intensities. After proper acclimation to the low-light regime, cells were sampled (= 0 hours), then exposed to the high-light treatment expression patterns were followed over 120h. Transcriptomic data were discussed also in the light of an accompanying physiological dataset from the same experiment (Ane Kvernvik et al. in prep).