## 3 Results and Discussion  
### Effect of Light Colors on Biomass and Lipid Yield in Mixotrophic and Autotrophic Conditions  
For the mixotrophic conditions, two different carbon sources were used by supplementing the standard L1 media with 1% glucose (W1, Y1, and R1) or 1% glycine (W2, Y2, and R2). The highest biomass was achieved under the red light condition compared to white or yellow light (Figure 1A). Specifically, the level of biomass productivity obtained in autotrophic growth (R) was significantly higher by 1.16 and 1.05-fold compared to W and Y autotrophic conditions, respectively. Our results are in agreement with multiple studies that have reported an increased higher production rate of _Scenedesmus_ sp. under red and blue light [25] and _Nannochloropsis_ sp. under red and yellow light [26]. No additional biomass was obtained in R1 or R2 conditions compared to R, suggesting that mixotrophic conditions are less interesting for an industrial application. Figure 1: Analysis of _P. tricornutum_ biomass (**A**) and lipid yield (**B**) in mixotrophic and autotrophic conditions under different colors of light. Abbreviations for the conditions are W: white, R: red, and Y: yellow, W1, R1 and Y1 refer to L1 media supplemented with 1% glucose, whereas W2, R2, and Y2 refer to the L1 media supplemented with 1% glycine. Each bar represents the average of three replicates, and error bars represent standard deviation. Means with different letters above the bars correspond to significant differences calculated using the multiple t-test. The columns identified with different letters (a–c) are significantly different using one-way analysis of variance (ANOVA) followed by a pairwise mean comparison Tukey’s test where differences were detected (\(n=3\), \(p<0.05\)). The lipid production of _P. tricornutum_ was only different, and significantly higher, in red light autotrophic (R) or mixotrophic supplemented with glycine (R2) conditions (Figure 1B). The latter was expected, as studies have reported an increase in biomass and lipids by multi-fold under mixotrophic conditions in different species [27,28]. Furthermore, many studies have shown a positive trend of red light with an increase in the specific growth rate and lipid production in _Chlorella_, _Botryococcus_, and _Nannochloropsis_[7,26,29]. Another study showed a positive trend of the combined impact of red and blue light on biomass productivity and fucoxanthin production in _P. tricornutum_[30]. However, contrasting results have been reported where they observed high productivity under yellow light as compared to red and blue light for _Chlamydomonas reinhardtii_[13]. However, these experiments were conducted in turbidostat controlled lab-scale panel photobioreactors and reported that in mass culture, productivity and biomass specific light absorption are inversely correlated [13]. Furthermore, in the same study, they have reported that the supplementation of blue light to yellow light is better for stable cultivation. The growth of microalgae in mixotrophic conditions is an interesting choice for enhancing the productivity of particular metabolites, which will vary according to the product of interest wanted [31]. However, there are cost-effective and economic advantages to growth in mixotrophy if the carbon source is derived from cheap waste resources such as agricultural and municipal wastewater that are rich in micronutrients and carbon sources [32]. There has been in-depth research conducted on the impact of varied carbon source such as glucose, fructose, and glycerol on enhancing the biomass of _P. tricornutum_[31,33]. A relevant point to consider when comparing results from red light with those of red light combined with blue, or other wavelengths is that the monochromatic red light corresponds to the absence of the others.