## 1 Introduction  
Diatoms _Phaeadactylum tricornutum_ have an important industrial potential for the production of biofuels, nutraceuticalals, cosmetics, valuable lipids, and bioactive molecules. However, the commercial use of diatom biomass is dependent on input energy and nutrient cost. The large-scale cultivation for a single cell organism at the industrial scale is a challenge considering the variables involved (e.g., energy, nutrients, and contamination issues). Diatoms are known to grow in both benthic and planktonic environments, making them capable of adapting in wide range of light spectra [1; 2; 3]. Light is the regulating parameter that controls the photosynthetic machinery of diatoms and regulates the range of light-induced physiological responses [3]. The systematic use of light energy affects the cellular metabolism, which is crucial to enhance the quantity and quality of diatom biomassfor the high production of desired bioactive molecules. Therefore, light should be provided with specific spectral components, intensity, and duration. Multiple ecophysiological studies have been published reporting on the distinct behaviors of different diatoms and algae in response to variations in light [4; 5]. It has been shown that light quality has an impact on chloroplast migration and on the light acclimation reactions of photosynthesis [6; 7]. Red light has been linked to stimulating growth, ethylene production [8], lipid accumulation [7], and increasing thylakoid stacking [6]. [PERSON] et al. [9] have suggested the involvement of phytochromes in increasing the intracellular nitrate content on exposure to red light for short periods altering the overall cellular nitrate content. Recent discoveries in genomics have revealed exciting information on phytochromes, blue-light sensing cryptochromes, and aureochromes [10]. In a study by [PERSON] et al. [11], it has been reported that _P. tricornutum_ cells acclimated to both red and blue light have comparable metabolite profiles, but drastic changes in metabolites and carbon partitioning was observed under red to blue light shift. In addition, various interdisciplinary studies have shown that light perceived by the red-light receptor (phytochromes) can regulate nutrient metabolism, cellular events, and signaling cascades [12]. These discoveries are still in progress and much remains unknown, but these studies can be explored for application-based diatom experiments such as developing low-cost cultivation technology or efficient bioreactors. In both culture medium and natural water bodies, light (intensity, distribution, photoperiod) may vary and will not behave similar to the dissolved salts or nutrients in culture medium. Therefore, the light intensity and color will have different impacts based on the volume of culture, geometrical shape of the reactor, shaker speed, aeration, composition of the culture media, etc. [13]. Considering all these factors and the available literature on the photosensory abilities of _P. tricornutum_, it is of paramount importance to create an effective light distribution system that can be used for commercial applications while reducing cost of input energy. Some studies have investigated the production of mutants of different antenna size to create an effective strain for efficient light utilization, but this process resulted in a reduced fitness of strains [14; 15]. In this study, we aimed to assess if specific shifts in light color enhance the biomass and total lipid content. Thus, we tested the impact of different light colors on _P. tricornutum_ in different growing conditions. Furthermore, we expanded our investigation on timing of red-light exposure and light shift impact on wild-type _P. tricornutum_ growth by analyzing its biomass, lipid, and fatty acid composition. Our research has shown the importance of light color, light availability, and timing of different red light (R) exposures, which can be explored as a strategy to enhance the industrial production of wild-type _P. tricornutum_ biomass and lipid content.