## 1 Introduction  
Diatoms (Bacillariophyta) are among the most abundant phytoplankton species on Earth [1]. They play vital roles as primary producers in aquatic food webs, being responsible for half of the organic materials in the ocean and up to 20% of the Earth's oxygen produced by photosynthesis [1]. Their high productivity has attracted scientific study related to their biotechnological potential, e.g., as biorefineries [2]. Diatoms have thereby mainly been exploited by the nutraceutical, fuel and aquaculture sectors, emerging as novel bioresources for the production of bioenergy, food and aquaculture feed, and as supporter of wastewater bioremediation [3]. Besides the most recent approaches to improve microalgae value-added yield production, using genetic engineering and molecular biology approaches [4], the manipulation of culture conditions (e.g., the light availability) has also been investigated [5]. The unique products of diatoms originate from their photosynthetic metabolism. Fucovanthin (Fx) is a primary carotenoid with expanded light absorption in the cyan and green spectral range of light. It has drawn attention due to its antioxidative properties, as potential antiobesity, and potential anticancer compound as well as some suggested effects to mitigate [PERSON]'s disease [6; 7; 8]. Today, the main commercially available source of Fx is brown seaweeds (Stramenopiles), a phylogenetic group to which the diatoms also belong [9]. Diatoms store photosynthetic energy foremost in form of fatty acids, with high proportions of polyunsaturated fatty acids (PUFAs), namely \(\omega\)3 fatty acids such as eicosapentaenoic acid (20:5, EPA) ([PERSON] et al., 1993). Additionally, diatoms show high amounts of storage lipids (triacylglycerglycerols, TAG) [10], with high nutritional value [11] and are useful for aquaculture feeding and potentially biofuel production [12]. Diatoms require light for their growth, as their metabolism bases on photosynthesis as primary energy source [13]. Some common cultivation systems involve light emitting diodes (LEDs) to support algal growth in cell cultures at the maximum light absorption of the main photosynthetic pigments, in the blue and red spectral range of light at ca. \(\lambda\approx\) 450 nm and \(\lambda\approx\) 650 nm, respectively [5]. The narrow-band wavelengths of LEDs can also be applied to test physiological implications of particular spectral parts of light and metabolite production under more environmental-like conditions, where light intensity as well as light spectral composition are affected by the wavelengths dependent light attenuation characteristics of water [14]. Individuals near the surface might thereby receive significantly higher amounts of light, compared to cells located at subsurface layers in the water column. In addition, light wavelengths of higher energy in the blue spectral range can penetrate deeper in the absence of dissolved organic matter and microscale particles [15]. In consequence, individuals of the same species can be projected to significant differences of available light during their lifetime. The different wavelengths to which diatoms are subjected in their natural environment and at different life stages are key factors shaping the metabolic activity of the cells [16]. The presence of blue and red light photoreceptors, i.e., cryptochromes and aureochromes, and phytochromes, respectively, allows diatoms to sense their position in the water column and to perform morphological and physiological alterations to adapt to changes in environmental conditions [17; 18]. In the present work, we studied the effects of narrow-band illumination with blue and red light LEDs compared to red-green-blue (RGB) LEDs upon photo-pigment, lipid and fatty acid production in the diatom _Phaeodactylum tricornutum_. Different pulse amplitude modulated (PAM) chlorophyll fluorescence techniques were used to determine the energy flux and dissipation along the photosynthetic transport chain under these illumination conditions.