## 3 Results and Discussion
### Influence of Different Conditions on the Content of Photosynthetic Pigments
#### 3.3.2 Effect of Nutrient Conditions on the Content of Photosynthetic Pigments  
The nitrogen element serves an irreplaceable function in the enzyme that generates photosynthetic pigments, with its composition and quantity directly affecting the elements that foster algal proliferation and enzymatic metabolism, thereby impacting algal life cycles and nutrient renewal [79]. The findings revealed a notable diminution in all the pigment levels among all the MN groups (Figure S2) in LL conditions compared to the control group, with the HN group under LL exhibiting substantially increased levels of fucoxanthin (Figure S2a) and \(\beta\)-carotene (Figure S2d) than the control group. The maximum fucoxanthin yield was 5.66 \(\pm\) 0.17 mg g\({}^{-1}\) DW (under LL and HN) among the different levels of light intensities and N treatments (Figure 6a). This result is in accordance with other research indicating that the fucoxanthin content of _Thalassioira weissflogii_ at a nitrogen concentration of 300 mg L\({}^{-1}\) was about seven times greater than in cultures without nitrogen [80]. Figure 6: Fucoxanthin (**a**), \(\beta\)-carotene (**b**), chlorophyll-a (**c**), chlorophyll-c (**d**), and carotenoid (**e**) contents of _C. meneghiniana_ under three light conditions and different nutritional conditions. The vertical bars represent averages \(\pm\) STDEV (\(n=3\)). * \(p<0.05\), **\(p<0.01\), *** \(p<0.001\). Similarly, a study found that nitrogen-enriched conditions (150 mg L\({}^{-1}\) NaNO\({}_{3}\)) were more favorable for diatoms to accumulate fucoxanthin under autotrophic conditions, which is consistent with the findings of this paper [63]. This suggests that photosystems are extremely susceptible to the nitrogen supply, and that the production of fucoxanthin, one of the most essential pigments in photosynthesis, is vulnerable to nitrogen limitation or enrichment [81]. The existing studies have reported that high concentrations of N (\(\geq\)300 mg L\({}^{-1}\)), although not exceeding 9 g L\({}^{-1}\), are favorable for microalgae to accumulate more fucoxanthin [6, 32, 82]. The chlorophyll-a content (0.35 \(\pm\) 0.02 ug 10\({}^{4}\) cells\({}^{-1}\)) was measured in a culture containing both low light (LL) and high trace metals (HM), succeeded by the MP group (0.30 \(\pm\) 0.04 ug 10\({}^{4}\) cells\({}^{-1}\)) (Figure 6). The scarcity of nitrogen resulted in reduced pigmentation, suggesting a decelerated photosynthesis process owing to nitrogen deficiency, consequently diminishing the Chl-a content [83]. Furthermore, under low-light conditions, suitable phosphorus enrichment (MP) benefits the production of fucoxanthin (Figure S2a) and \(\beta\)-carotene (Figure S2d) by algal cells, which may be related to the effect of the suitable nitrogen-to-phosphorus ratio on the physiological status of microalgae. In addition to nitrogen, factors such as silicate and light intensity also play an important role in the biosynthesis of the microalgae fucoxanthin [17]. The growth of diatoms heavily relies on silicates as vital nutrients, which are crucial for the formation of the rigid outer shells of frustules. Due to the potential disruption of the cell growth and structure by a lack of silicate, this research employed nutrient-rich environments rich in silicate to cultivate cells, aiming for enhanced growth and increased fucoxanthin output [84]. As the concentration of silicate escalates, the number of cells (Figure 4d) and the fucoxanthin (Figure 6a) and \(\beta\)-carotene (Figure 6b) contents increased, and the fucoxanthin content of the cells cultured under high-silicate conditions was higher than that of the control group under the same light intensity (Figure S2a-c), in which the highest fucoxanthin content was found under low light (Figure 6a). The results of a study examining the cellular response of the marine diatom _Navicula laevis_ to different concentrations of silicate showed that the biomass was highest on day four (2.41 g L\({}^{-1}\)) at a silicate level of 480 mg L\({}^{-1}\).