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Appl. Sci. 2020, 10, 2531 2 of 18 for 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]. López-Figueroa 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 Jungandreas 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. This study provides a unique strategy that can be useful for lab and large-scale cultivation. 2. Materials and Methods 2.1. Microalgae Strain and Culture Conditions The inoculum culture for all experiments was prepared the same: all the experiments were conducted into 250 mL Erlenmeyer flasks containing 50 mL of liquid media with and an initial inoculum size of 0.2 OD. Axenic cultures of P. tricornutum (Culture Collection of Algae and Protozoa CCAP 1055/1) were obtained from Western University, Canada. P. tricornutum cells were grown and maintained in L1 media without silica pH 8 (Artificial Sea Water) as described in [16] in 250 mL Erlenmeyer flasks (50 mL of culture) at 18 ± 1 ◦C. The experiments were conducted in the growth Chamber CMP6050 with light intensity 75 μE m−2 s−1 and a photoperiod of 16:8 h light/dark cycles. The light source was cool white light F54T5/841, which was kept 60 cm above the bottom of the culture flask. The light conditions were variable by ±1–2 μE m−2 s−1 in different corners of the rotational shaker. The humidity of the chamber was 50% with no additional air supply, and the rotary shaker speed was fixed at 130 rpm. For experiments, the color spectrum for red (680–700 nm) and yellow (570–590 nm) light was obtained by wrapping the Erlenmeyer flasks using colored cellophane sheets. The light intensity wasPDF Image | Red Light Variation Lipid Profiles in Phaeodactylum tricornutum
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