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Background  Diatoms, which can accumulate large amounts of carotenoids, are a major group of microalgae and the dominant primary producer in marine environments.  Phaeodactylum tricornutum , a model diatom species, acquires little silicon for its growth although silicon is known to contribute to gene regulation and play an important role in diatom intracellular metabolism. In this study, we explored the effects of artificial high-silicate medium (i.e. 3.0 mM sodium metasilicate) and LED illumination conditions on the growth rate and pigment accumulation in  P. tricornutum , which is the only known species so far that can grow without silicate. It’s well known that light-emitting diodes (LEDs) as novel illuminants are emerging to be superior monochromatic light sources for algal cultivation with defined and efficient red and blue lights.    Results  Firstly, we cultivated  P. tricornutum  in a synthetic medium supplemented with either 0.3 mM or 3.0 mM silicate. The morphology and size of diatom cells were examined: the proportion of the oval and triradiate cells decreased while the fusiform cells increased with more silicate addition in high-silicate medium; the average length of fusiform cells also slightly changed from 14.33 µm in 0.3 mM silicate medium to 12.20 µm in 3.0 mM silicate medium. Then we cultivated  P. tricornutum  under various intensities of red light in combination with the two different levels of silicate in the medium. Higher biomass productivity also achieved in 3.0 mM silicate medium than in 0.3 mM silicate medium under red LED light irradiation at 128 μmol/m 2 /s or higher light intensity. Increasing silicate reversed the down-regulation of fucoxanthin and chlorophyll  a  under high red-light illumination (i.e. 255 μmol/m 2 /s). When doubling the light intensity, fucoxanthin content decreased under red light but increased under combined red and blue (50:50) lights while chlorophyll  a  content reduced under both conditions. Fucoxanthin accumulation and biomass productivity increased with enhanced red and blue (50:50) lights.    Conclusion  High-silicate medium and blue light increased biomass and fucoxanthin production in  P. tricornutum  under high light conditions and this strategy may be beneficial for large-scale production of fucoxanthin in diatoms.

Combined artificial high-silicate medium and LED illumination promote carotenoid accumulation in the marine diatom Phaeodactylum tricornutum