Furthermore, AUREO1a LOVa reacts with conformational changes in response to BL exposure allowing homodimerisation and enhanced DNA binding. Thus it seems likely that the observed light dependent physiological effects originate primarily from a BL induced functionality of AUREO1a. One important feature of our work is the finding that, although four different aureochromes are expressed in P. tricornutum, the silencing of a single aureochrome gene cannot be compensated. This indicates that the individual aureochromes might have discrete functions similar to the AUREO1 and 2 proteins of V. frigida. This is further supported by the observation of differential circadian gene expression patterns of the aureochromes of Thalassiosira pseudonana and by our finding of four distinct groups of aureochromes thar feature group specific homologous regions. Group 1 and 2 correspond to the respective aureochromes of V. frigida described by Takahashi et al., while the other two groups 3 and 4 are dominated by diatom aureochromes, raising the question whether diatoms might possess exclusive classes of aureochromes associated with diatom specific functions. This notion is also supported by the observation that P. tricornutum, like the diatoms P. multiseries and F. cylindrus, possesses at least one aureochrome of each group. If this pattern is confirmed in future research, a re-evaluation of the current aureochrome nomenclature might be appropriate. A preceding study on the physiological characterisation of P. tricornutum in response to different light qualities revealed that the quantum requirement of biomass production was significantly SU5416 increased in WT cells under ML RL conditions compared to all other tested culture conditions. Although monochromatic RL conditions are artificial and do not occur naturally, this demonstrated that chromatic acclimation can affect the overall cellular energy balance. In contrast to WT cultures, an increased quantum requirement under ML RL conditions compared to other applied culturing conditions was not detected in aureo1a cultures. This is in agreement with the apparent acclimation of the aureochrome 1a silenced strains to increased light intensities at ML RL conditions as indicated by elevated NPQ capacity, Pmax or xanthophyll cycle pigment concentrations. Generally, no obvious disadvantage of aureo1a cultures compared to WT cells was detected using our specific experimental setup with persistent exponential growth. However, WT and aureo1a cultures were additionally grown in batch cultures revealing that aureochrome 1a silenced strains show a prolonged lag phase during batch cultivation compared to WT cultures, while no significant differences of growth rates during the exponential growth phase were detected. This indicates that the suppression of AUREO1a in P. tricornutum disturbs the initial photoacclimation after changes of 
the ambient light conditions rather than the growth performance under steady state conditions. Thus, one role of AUREO1a could be the promotion of acclimation to fast changes of ambient light conditions, which are common in the natural habitat of diatoms. In this context it would be interesting to study the performance of aureo1a cultures under fluctuating light conditions. Daily changes in activity and feeding are driven by an internal timekeeping system to occur synchronously at the same time during the day. The phasing of these rhythms is dependent on the species, being increased during the night in nocturnal species and during the day in diurnal species. The internal timekeeping system drives daily rhythms in feeding and activity even under conditions lacking daytime signals, such as LY294002 citations constant darkness. Under these conditions, the rhythms exhibit approximately 24-hour periods and are thus called circadian. This mechanism not only drives the rhythms but also sets them to a proper phase.