There is increasing evidence that resource availability significantly modulates the measured effects of Ocean Acidification (OA) on marine phytoplankton. In this context, the interactive effects of OA and irradiance have been proven especially important, yet most studies have applied constant light levels only. Whether such experimental results can be extrapolated to the ocean, where light intensities are highly dynamic, is currently unknown. To investigate the potential effect of dynamic light on OA responses, we grew the diatom Chaetoceros debilis under two pCO2 (390 and 1000 µatm) and light conditions (constant and dynamic). To characterise the respective responses a variety of parameters were measured (e.g. growth, elemental composition, primary production). Our results suggest that dynamic light strongly alters the effects of OA, as high pCO2 had a beneficial effect on primary production under constant but a negative effect under dynamic light. To understand these complex interactions, we assessed the underlying processes on different levels, obtaining results from transcriptomics, photophysiology as well as carbon fixation.