## 3 Results and Discussion  
### Effect of Light Intensity on H. pluvialis Growth and Biomass Production  
The growth of _H. pluvialis_ was monitored for 6 days using undiluted BWW and two blended media, BWW-BG11 1:1 and BWW-BG11 1:3, as detailed in Table 2. A regular BG11 medium served as the control. To assess the impact of light on biomass production, two light intensities were adopted for 6 days in the first two experiments, 35 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) in Exp. 1 and 140 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) in Exp. 2. In Exp 3, lasting 10 days, cultures were exposed to 35 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) for the first 5 days, followed by 140 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) for the next 5 days. Under 35 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\), all mixotrophic systems achieved greater biomass than the control (0.57 g L\({}^{-1}\)), with the highest (4.61 g L\({}^{-1}\)) in BWW-BG11 1:1 (Figure 1a).  
At 140 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\), the BG11 control yielded the most biomass (3.21 g L\({}^{-1}\)), while the top-performing mixotrophic medium was BWW-BG11 1:1 (3.13 g L\({}^{-1}\)) (Figure 1b). This result aligns with previous studies on _H. pluvialis_ cultivated in synthetic BWW mixed with a 50% BBM control medium [34]. When the light intensity increased from 35 to 140 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) after five days, BWW-BG11 1:3 produced the highest biomass (5.46 g L\({}^{-1}\)); while BBW alone had the lowest (3.50 g L\({}^{-1}\)) (Figure 1c).  
Overall, _H. pluvialis_ biomass in the BWW-BG11 mixtures exceeded that of the control, likely due to the elevated organic carbon content in BWW, which is rich in organic compounds like BOD and COD as well as essential nutrients for microalgae growth such as ammonium N (N-NH\({}_{4}^{+}\)), N, and P (Table 3).  
Although 100% (v v\({}^{-1}\)) BWW resulted in the lowest growth among the concentrations tested under mixotrophy across all three light conditions, the growth improved significantly when BWW was blended at 50% and 75% with the control medium. The limited availability of crucial substrates and nutrients, such as N and P in BWW (Table 3), may have restricted the growth of cells and the synthesis of nucleic acids, proteins, and photosynthetic pigments [23]. Our findings indicate that supplementing BG11 components with BWW enhances the nutrient supply, supporting _H. pluvialis_ growth. The synergistic effect of organic carbon sources and light generally boosts biomass production [35]. It has been proved that low light intensity favors _H. pluvialis_ growth up to the achievement of the optimal cell density, after which high irradiance acts as a stressor to trigger secondary carotenoids accumulation.  
In our study, biomass accumulation in _H. pluvialis_ ranged from 3.18 to 4.61 g L\({}^{-1}\) when grown mixotrophyvally at 35 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\), nearly doubling the biomass observed at 140 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\), which ranged from 1.94 to 3.13 g L\({}^{-1}\). Similar results were reported by [22], who observed the highest cell densities, 3.8 \(\times\) 105 cells mL\({}^{-1}\) and 2.9 \(\times\) 105 cells mL\({}^{-1}\), in WC and NPK media under 20/4 h and 24/0 h light/dark cycles, respectively, in the mixotrophic cultivation of _H. pluvialis_. [PERSON] et al. found that continuous light exposure (24/0 h with no dark period) was optimal for cell growth, lipid synthesis, and astaxanthin production _in H. pluvialis_[36]. The decision to conduct these experiments under continuous light was influenced by evidence that alternating dark and light periods can inhibit growth and reduce cell performance compared to continuous light exposure [37]. Additionally, alternating light and dark cycles may also decrease the growth efficiency [38].