## 3 Results and Discussion
### Influence of Different Conditions on the Content of Photosynthetic Pigments  
#### 3.3.1 Effect of Light on Photosynthetic Pigment Production  
The growth of microalgae biomass and the build-up of valuable compounds are influenced by multiple elements, including illumination, nutrient levels, and cultivation techniques [71]. Considering the reliance of photoautotrophic microalgae on light for absorbing energy and chemically transforming, light acts as the primary energy source for their biochemical processes, greatly impacting the growth of algae [6; 71]. A distinct trend was observed in the build-up of fucoxanthin within microalgae, where the highest concentration of fucoxanthin ranged from 0.52% to 4.28% DW, under varying light intensities from 10 umol photons m\({}^{-2}\) s\({}^{-1}\) to 100 umol photons m\({}^{-2}\) s\({}^{-1}\)[6]. Nevertheless, the ideal lighting conditions for fucoxanthin production differ across various species [63; 72; 73; 74]. There was a notable disparity in the fucoxanthin production between the low-light and high-light intensity groups (Figure 6a), where every test group under low light exhibited conspicuous variances in fucoxanthin production relative to the control group (Figure S2). Comparable findings were observed in a different investigation on _Isochrysis zhangjiangensis_, where low light enhanced the accumulation of fucoxanthin, achieving 23.29 mg g\({}^{-1}\) DW [72]. The increase in light intensity, along with the decrease in fucoxanthin levels, might be due to fucoxanthin acting as a light-harvesting pigment in PSII photosynthesis, balancing out the reduced light intensity [74]. In terms of addition, the diadinoxanthin cycle can synthesize fucoxanthin, and higher light intensities result in the conversion of diadinoxanthin to diatoxanthin, which diminishes the biosynthesis of fucoxanthin [75]. It was established that the maximum concentration of fucoxanthin reached 7.76 \(\pm\) 0.30 mg g\({}^{-1}\) DW in high trace metals (HMs) concentrations, and these HM levels also enhanced the biosynthesis of fucoxanthin to a cell yield of 0.98 \(\upmu\)g 10\({}^{-7}\) cells (Table S2). The findings of our research indicated  
Figure 5: Changes in the maximum quantum efficiency of PSII (\(F_{\
u}/F_{\rm m}\)) of \(C\). _menghiniana_ under three light conditions and different nutrient conditions: three light conditions (**a**), different nitrate conditions (**b**), different phosphate conditions (**c**), high silicate (**d**), high trace metals (**e**), and lower concentration than F/2 medium (**f**). The vertical bars represent averages \(\pm\) STDEV (\(n\) = 3).  
that, under low-light culture conditions, high trace elements, high silicate, and high nitrate contributed to the growth of biomass and improved photosynthesis of _C. meneghiniana_ to a certain extent (Figure 4) while also encouraging fucoxanthin accumulation (Figure 6).  
Chlorophyll-a is vital as a pigment that absorbs light, crucial for absorbing and transferring the energy in algal cells throughout photosynthesis [76]. As a crucial carotenoid, \(\beta\)-carotene also plays a role in photosynthesis by absorbing light energy as a key concentrating pigment [77]. Our experimental findings revealed markedly higher levels of Chl-a and carotenoids in low-light environments as opposed to high-light settings. The cause of Chl-a accumulation may be related to the energy transmission routes of the fucoxanthin-chlorophyll protein (FCP) complex in _C. meneghiniana_[78]. The levels of \(\beta\)-carotene remained consistent across both high-light and low-light conditions, probably because low light is the optimal light intensity for _C. meneghiniana_ growth, so the accumulated content was relatively high. Yet, an abrupt exposure to intense light changed the physiological state of _C. meneghiniana_, altering its activity in the photosynthetic function and impacting the creation and accumulation of \(\beta\)-carotene [77].