## 3 Results and Discussion
### Growth Curve Analysis of P. tricornutum during Red Light Shift  
Studies have reported that light color and intensity bring changes in the photon flux density, which have a strong impact on growth and metabolic pathways [32,33]. Photoautotrophic conditions create certain limitations when grown in bioreactors because self-shading by microalgal cells in high-density cultures limits the light available for growth, which can be confirmed in lab-scale experiments by observing the growth curve. Therefore, to reduce the impact of either loss of light as heat or self-shading, the strategy of light shifting from one spectrum to another can trigger different photoreceptors to activate different light-signaling pathways, allowing a better conversion of light energy [34,35, 36]. Therefore, we investigated the effect of red light shifts on _P. tricornutum_ growth during different growth phases under batch cultivation for 11 days. _P. tricornutum_ grown in red light until the end of exponential phase (from day 1 to 7) and then shifted to white light (RS) showed the greatest growth compared to all other conditions tested (Figure 2, Appendix A--Figure A1). In addition, RS is significantly better to promote growth compared to continuous white (W) or red light (R) (Figure 2). _P. tricornutum_ cultured cells were adapting to each condition in a similar manner during the adaptation and exponential phases (Figure 2). It should be noted that there seems to be a crucial lag between day 4 and day 7 (exponential to stationary), where it is not clear whether cell growth continued before slowing down (Figure 2). Additionally, the slight difference between R and RS at day 7 is unexpected, since they were under the same conditions. It is possible that these small differences are due to replicate variation and growth chamber light variations. Indeed, when we checked the light intensity using the flux meter, we observed a variation in light intensity of 1-2 uE m\({}^{-2}\) s\({}^{-1}\) between the different corners of the growth chamber, which could explain the variation between R and RS at day 7 and the width of the error bars. However, major differences in growth were observed at the end of the exponential phase and during the stationary phase, where RS clearly promoted growth compared to other treatments (Figure 2; Appendix A--Figure A1). Indeed, at day 11, the RS growth was significantly higher with 22% and 14% more cells than continuous W and R light, respectively (Figure 2). The cell growth observed under R and RS treatment can be explained, in part, by the presence of a wide range of photoreceptors with the ability to sense red/far-red wavelengths and their potential to prevent photoinhibition [30; 32]. A similar study has reported the increasing growth of _P. tricornutum_ under defined and efficient combinations of red and blue lights [30]. Another study has shown the positive impact of supplementing different light colors on _Chlamydomonas reinhardtii_ culture [13]. Through various studies, it was suggested that different classes of photoreceptors have specific responses to specific light mediated by different light-signaling pathways that are yet to be completely elucidated [2; 37; 38]. The other physiological phenomenon that can change the culture conditions is the rate of photoinhibition, which seems to be higher in W as compared to R. The energy of each photon is inversely proportional to the wavelength. For example, R (\(\lambda\) = 700 nm) is 1.8 eV, whereas blue light (\(\lambda\) = 450 nm) is 2.8 eV. The W or full spectrum (\(\lambda\) = 400-700 nm) tends to cause photoinhibition, inducing cell damage and reducing the overall time of stationary phase as compared to R (Figure 2). Since the cumulative wavelength of red in R is lower than that in W, it may prevent photoinhibition and consequently lead to higher growth and overall biomass [39]. Similar results have been reported for _Chlorella_ sp. [39] and _Scenedesmus_ sp. [25].