PAR regulation of photoprotection in Phaeodactylum tricornutum (Bacillariophyceae): Roles of doses and irradiances

Photosynthetic diatoms are exposed to a rapid and unexpected variations in light intensity that can modulate the contents of photosynthetic pigments and the carotenoid profile. In this study, the influence of light availability on photoprotection in the diatom Phaeodactylum tricornutum was investigated. Experimental cultures were exposed to three irradiances (100, 200 and 400 µmol photons m −2 s −1 ) and two light: dark cycles (12:12 and 24:00 h) that resulted in different light energy daily doses (D = 4.32 mol photons m −2 ), where the treatments were named in ND xh (N = amount of daily doses D, and x = photoperiod in hours). The samples treated at different doses were compared in terms of cell growth, chlorophyll- a fluorescence, pigment content and gene transcription levels. Specific growth rate was 1.9-fold higher in 8D 24h daily dose (34.56 mol photons m −2 ) with 3 times higher light absorption in comparison to the lowest light energy dose. Also, at higher light intensities the content of chlorophyll- a , fucoxanthin and diadinoxanthin in P. tricornutum was lower, while the regulation of the xanthophyll cycle was achieved by the highest light energy doses. The transcriptional profiles of ZEP1 , ZEP2 , VDL1 and VDL2 genes were influenced by the highest light energy doses, on the other hand VDE and ZEP3 genes were poorly regulated by light. In addition, a similar transcription pattern was found for two isoforms of ZEP genes as well as in VDL genes. This study demonstrated that light energy doses and irradiances affect the photoacclimation and photoprotection responses.