Solar UV-B radiations can have a negative impact on photosynthetic organisms leading to a reduction of the uptake of atmospheric carbon dioxide. Aquatic ecosystems absorb a similar amount of atmospheric carbon dioxide as terrestrial ecosystems, are estimated to produce half of the biomass on our planet and are a major source for human food supply. At the base of these ecosystems, phytoplankton are the main primary producers. Among them, diatoms have colonized all types of aquatic environments and are thought to be responsible for as much as 20 % of global primary productivity. Although diatoms do not have the plant UV-B photoreceptor (UVR8), they are able to accumulate UV-B absorbing compounds when exposed to UV-B, suggesting adaptive responses. However, when exposed to chronic UV-B exposure, diatoms exhibit a decreased growth rate and increased cell volume reflecting a decoupling between cell division and cell growth. The proposed project aims at defining the genetic basis of systemic and specific signaling pathways in response to UV-B in the diatom Phaeodactylum tricornutum. A high throughput approach will be used to unveil the transcriptomic reprogramming happening upon UV-B stress. DNA stress checkpoint and DNA repair genes will be further characterized for their response to UV-B as the encoded proteins are essential actors at the interface of environmental stress and the controlling of cell cycle blockage and re-entrance. Promoters of the highly UV-B regulated genes will be dissected to identify cis-elements and further used to identify potential UV-B specific transcription factors. The project aims at integrating my knowledge on light signaling in the cell cycle research work on diatoms performed in the group of Prof. Lieven De Veylder. Exploration of this novel interdisciplinary scientific niche, together with the acquisition of new knowledge and additional skills, will help me in obtaining a position as an independent researcher in the near future.