## 3 Results
### Effect of Electrical Treatment on Microalgae Growth in the Photobioreactor  
This study aims to evaluate the effectiveness of direct current (DC) stimulation for large-scale microalgal feed production. The photobioreactor facility used in this experiment was established between February and March 2023 and has developed into a fully operational system capable of producing over 4 tons of microalgal feed per day, primarily used for aquaculture species. Wastewater from the aquaculture park is stored in a reservoir, and excess microalgae can utilize the nutrients in the wastewater to aid in water purification. Currently, under the photobioreactor's cultivation conditions, the maximum achievable microalgal cell concentration is limited to 1 \(\times\) 10\({}^{7}\) cells/mL. Based on prior experimental results, voltage gradients of 0.5 V, 1 V, and 3 V were selected for the electrical stimulation experiments conducted in a modified photobioreactor. Nutrient input was replenished after 48 h to ensure a continuous supply in the system. The results, as shown in Figure 9, indicate that _N. clostarium_ f. _minutissima_ exhibited significant differences (_p_ < 0.05) in maximum cell density and dry weight under different voltage stimulations. Specifically, the final cell density at 1 V increased significantly (_p_ < 0.05), with a 9.97% increase compared to the control, while the dry weight reached 0.574 \(\pm\) 0.012 mg/mL, showing a 5.00% increase over the control (_p_ < 0.05). No significant changes were observed in cell density or dry weight at 0.5 V. However, at 3 V, the final cell density decreased significantly (_p_ < 0.05) by 20.37% compared to the control, and the dry weight dropped to 0.495 \(\pm\) 0.013 mg/mL, representing a significant reduction of 9.50% (_p_ < 0.05). These results suggest that 1 V DC stimulation significantly promotes microalgal growth in the photobioreactor setting, while 3 V exerts a strongly inhibitory effect, and 0.5 V has no significant influence. As shown in Table 3, the specific growth rate (K\({}_{1}\)) at 1 V was higher than in the other groups, representing the highest daily growth rate within 72 h. However, K\({}_{2}\) was lower than in the other groups, possibly due to the initial growth promotion by electrical stimulation, followed by slower growth as the cell density reached higher levels. At 3 V, K\({}_{3}\) was markedly higher than in the other groups, representing the highest daily growth rate within 72 h, while the K\({}_{3}\) value at 1 V was also higher than K\({}_{2}\), potentially due to the nutrient addition at 48 h, which may have promoted further growth. Figure 8: Cyclic voltammetry (CV) curves before and after inoculation with culture medium (**a**), CV curves of _N. clostarium_ f. _minutissima_ under different voltages of short-term (**b**) and continuous (**c**) DC stimulation. To further explore the synergistic effects of electrical stimulation and nutrient addition on microalgal growth in photobioreactors, the experimental results were compared with the growth curve of _Nitzschia closterium_ f. _minutissima_ under standard production conditions (Figure S1). It was observed that all treatment groups experienced an additional growth phase after 48 h and entered the plateau phase earlier, at around 60 h, ultimately achieving higher cell densities. Regarding the maximum cell density, both nutrient addition and electrical stimulation had significant effects on microalgal growth (\(p\) < 0.05). Compared to the standard conditions (10.51 \(\pm\) 0.39 \(\times\) 10\({}^{6}\) cells/mL), nutrient addition at 48 h resulted in a 5.76% increase in the maximum cell density, reaching 11.10 \(\pm\) 0.53 \(\times\) 10\({}^{6}\) cells/mL. The combination of 0.5 V stimulation and nutrient addition led to a 4.45% increase (10.96 \(\pm\) 0.52 \(\times\) 10\({}^{6}\) cells/mL), while the 1 V + nutrient addition treatment saw a more substantial increase of 16.31%, with a maximum cell density of 12.17 \(\pm\) 0.49 \(\times\) 10\({}^{6}\) cells/mL.