For this purpose, the intracellular localisation of aureochromes was studied by employing full length protein-GFP fusion proteins of three P. tricornutum aureochromes. Furthermore, AUREO1a silencing cell lines were generated and their physiological responses to cultivation under limiting and moderate intensities of BL and RL were investigated. To differentiate between light intensity and light quality driven reactions, the applied experimental design ensured that identical amounts of quanta were absorbed by the cells under BL and RL WY 14643 PPAR inhibitor conditions, respectively. High light BEZ235 acclimation in diatoms is typically associated with a decrease of the cellular Chl a content and enhanced maximum photosynthesis rates. Furthermore, the photoprotective potential is clearly increased which is usually accompanied by an increased pool size of xanthophyll cycle pigments and an accelerated deepoxidation of Ddx to Dtx under excess light conditions. High light acclimation is further connected to the up- or down-regulation of the expression of specific genes involved in photoprotection or light harvesting as well as other processes such as the carbon metabolism. Similar to high light acclimation, also the re-acclimation to illumination after prolonged darkness is accompanied by extensive transcriptional changes. In a previous study we have shown that RL is not a trigger for light acclimation in WT cells of P. tricornutum and that the formation of an apparently high light acclimated phenotype is mediated by the absorption of BL despite moderate light intensities at cultivation. Based on these results it was suggested that aureochromes might play an important role in the process of BL perception and that the active form of one or more of the aureochromes might act as transcription factors and induce or enhance the acclimation to higher light intensities. The predicted nuclear localisation of all four aureochromes and the confirmation by successful GFP fusion experiments for three of them further support a role of the aureochromes as transcription factors. Interestingly, AUREO1a was detected both in the nucleus and in the cytoplasm, indicating a different functionality compared to the other aureochromes, possibly involving shuttling between cytoplasm and nucleus. To study the influence of AUREO1a on the photoacclimation of P. tricornutum, we cultivated the aureochrome 1a silenced strains at BL and RL of different light intensities. Surprisingly, the results suggest a regulation of BL mediated light acclimation which stands in clear contrast to our expectations. At LL conditions, WT cells and the aureochrome 1a silenced strains showed very similar physiological properties under illumination with both BL and RL. This indicates that AUREO1a is not of major importance for the photoacclimation of P. tricornutum at low light intensities. Instead, the observed differences between the BL and RL acclimated phenotypes of the WT might be mediated by other BL receptors like other aureochromes or members of the cryptochrome family. At ML conditions, the physiological response of AUREO1a silencing cell lines showed explicit characteristics of acclimation to increased light intensities irrespectively of the applied light quality, including a reduction of the cellular Chl a content and the cellular dry weight as well as increased photosynthesis rates and an enhanced photoprotective potential. Hence, it can be stated that aureochrome 1a silenced cultures were ��hyper�� acclimated under ML illumination suggesting that AUREO1a is involved in the photoacclimation of P. tricornutum. Considering the lacking influence of AUREO1a on the phenotype of LL cultures, this indicates the presence of a light intensity perception mechanism which is negatively affected by aureochrome 1a and which may comprise the reduction states of stromal compounds or the reduction state of the PQ pool.