Blue light is essential for high light acclimation and photoprotection in the diatom Phaeodactylum tricornutum.

The objective of the present study was to test the hypothesis that the acclimation to different light intensities in the diatom Phaeodactylum tricornutum is controlled by light quality perception mechanisms. Therefore, semi-continuous cultures of P. tricornutum were illuminated with equal amounts of photosynthetically absorbed radiation of blue (BL), white (WL), and red light (RL) and in combination of two intensities of irradiance, low (LL) and medium light (ML). Under LL conditions, growth rates and photosynthesis rates were similar for all cultures. However, BL cultures were found to be in an acclimation state with an increased photoprotective potential. This was deduced from an increased capacity of non-photochemical quenching, a larger pool of xanthophyll cycle pigments, and a higher de-epoxidation state of xanthophyll cycle pigments compared to WL and RL cultures. Furthermore, in the chloroplast membrane proteome of BL cells, an upregulation of proteins involved in photoprotection, e.g. the Lhcx1 protein and zeaxanthin epoxidase, was evident. ML conditions induced increased photosynthesis rates and a further enhanced photoprotective potential for algae grown under BL and WL. In contrast, RL cultures exhibited no signs of acclimation towards increased irradiance. The data implicate that in diatoms the photoacclimation to high light intensities requires the perception of blue light.