*4.3. The Effect of Different LED Colors on the Protein and Carbohydrate Content of I. Zhangjiangensis*

The effect of light intensity on microalgal cultures has been extensively studied, and it has been demonstrated that light intensity controls not only the growth rate, but also the lipid storage [17,18,20,21,25,41], structural distribution [66], cellular composition (such as proteins and essential fatty acids) [8], and pigment synthesis [67]. While most research has been focused on the effects of light intensity, it has been shown that light quality also plays a key role in algal metabolism and the effects of light wavelengths on growth are species-specific because of the differences in metabolic pathways, pigmentation, and photoreceptors between species [8]. Moreover, the biochemical composition is a pivotal factor in determining the nutritional value of microalgae. In this investigation, the protein and carbohydrate contents of *I. zhangjiangensis* were influenced by the different LED colors used in the indoor cultures (Table 2). The maximum protein content was measured in the microalgae cultured under white light, followed by the yellow and blue lights, and the lowest level in the red light. Similar results have been reported in microalgal conglomerates of *Chlorella variabilis* and *Scenedesmus obliquus* when the protein content of the microalgal consortia was highest under a cool-white light [68]. Contrasting results have been observed when blue light fluorescent tubes were closely related to protein enhancement in *Isocrhysis* sp. cultured in a bioreactor [42]. However, cell concentration and productivity did not change substantially upon changing the light spectrum during steadystate growth. In addition, experiments conducted with *Tisochrysis lutea* (previously named *I.* aff. *galbana*) under white, blue, green, and red fluorescent lamps in batch cultures revealed that the growth rate and cell density were highest with white light, followed by blue light. Meanwhile, cells under green light had a greater dry weight during exponential growth in comparison with the other light colors, and this monochromatic light also increased the eicosapentaenoic acid and protein contents [69]. In axenic cultures of *Dunaliella tertiolecta* and *Thalassiosira rotula*, blue light also allows higher photosynthetic carbon incorporation into protein than white light [70]. It can be inferred that the optimal light color for the cultivation of *Isochrysis* varies depending on the algal strains and light sources used.

In this study, carbohydrate content was higher in the blue light and the lowest content was in the yellow light. These results concur with the one reported for *A. platensis*, where the highest carbohydrate content was also measured under blue light [28]. However, there are conflicting results on the influence of blue light on microalgae carbohydrate content: In *T. lutea*, it did not change [69], but it decreased in *Isochrysis* sp. [42]. Blue light also enhanced dark respiration, as previously reported for *Scenedesmus obliquus* [71], *Rhodomonas salina* [72], or *D. tertiolecta* and *T. rotula* [70], confirming a higher rate of carbohydrate degradation under blue light. The evidence presented in this study confirms that light quality can affect the biochemical composition of microalgae cells of *I. zhangjiangensis* when they are cultured under different light colors.
