## 2 Materials and Methods
### Analytical Methods  
To determine the cell density of microalgae, this experiment conducted counts of microalgae suspensions at different concentrations under an optical microscope using a hemocytometer method, and measured the corresponding optical density values with a spectrophotometer, followed by regression analysis. This study fitted density curves for three wavelengths, as shown in Figure 3. The group with the highest R-squared value was selected, and the standard curve for cell density of _N. clostarium_ f. _minutissima_ at 750 nm was obtained as follows:  
Cell Density (cell/mL) = 2 \(\times\) 10\({}^{7}\) A\({}_{750}\) + 331,651 (R\({}^{2}\) = 0.9992)  
Figure 2: Modification of the photoreactor system. The microalgal growth rate (K) was calculated using the formula by [PERSON] et al. [20]:  
\[\text{K}=(\text{LnN}_{\text{t}}-\text{LnN}_{0})/(\text{Ln2}\times\text{t}) \tag{2}\]  
where  
K--Cell growth rate (1/d)  
N\({}_{\text{t}}\)--Final cell density  
N\({}_{0}\)--Initial cell density  
t--Time (d), where t = 1 for daily growth rate and t = 7 for weekly growth rate. The microalgal cultivation period was 7 days, with K\({}_{\text{i}}\) representing the daily growth rate on day i, K\({}_{\text{w}}\) representing the weekly growth rate (also the growth rate for the experimental period), and K\({}_{\text{max}}\) indicating the maximum daily growth rate. Biomass dry weight was measured on the final day of the electric stimulation experiment. Centrifuge tubes were pre-weighed (m\({}_{1}\)), and the volume of the algal suspension used for biomass measurement was recorded as V. After centrifugation at 4000 rpm for 10 min, the algal pellet was washed three times, the supernatant was discarded, and the pellet was dried in an oven at 80 \({}^{\circ}\)C for 24 h; the final weight was recorded as m\({}_{2}\)[21]. The biomass dry weight (M, mg/mL) was calculated using the following:  
\[\text{M}=(\text{m}_{2}-\text{m}_{1})/\text{V} \tag{3}\]  
Chlorophyll-\(\alpha\) content was measured at the end of the experiment. Two milliliters of the microalgae culture were collected into clean EP tubes, washed three times with distilled water, and centrifuged at 1000 rpm for 2 min. To extract chlorophyll-\(\alpha\), 1.6 mL of methanol was added, and the mixture was vortexed vigorously for 2 min to ensure adequate contact between the solvent and microalgae. The solution was incubated in darkness for 2 h, centrifuged to collect the supernatant, and the absorbance at 666 nm and 653 nm was measured. chlorophyll-\(\alpha\) content was then calculated using the following formula [22; 23]:  
\[\text{Chl a}\ (\text{\mu g}\cdot\text{mL}^{-1})=15.65\ \text{A}_{666}-7.34\ \text{A}_{653} \tag{4}\]  
To assess the polysaccharide and protein content, 50 mL of microalgae culture was centrifuged at 4000 rpm for 10 min, washed three times, and resuspended in 30 mL deionized water. The suspension was disrupted using an ultrasonic cell disruptor (500 W, 10 min, ice bath), and after a second centrifugation at 4000 rpm for 10 min, the supernatant was collected for polysaccharide and protein determination, and stored at 4 \({}^{\circ}\)C.  
Polysaccharides were quantified using the phenol-sulfuric acid method. A 200 \(\upmu\)L sample, 100 \(\upmu\)L of 5% phenol solution, and 500 \(\upmu\)L of concentrated sulfuric acid were mixed in an EP tube, sealed with parafilm, and incubated in a 90 \({}^{\circ}\)C water bath for 30 min. After cooling to room temperature, the absorbance at 490 nm was measured and polysaccharide content (\(\text{\mu g}/\text{mL}\)) was calculated based on a standard curve [24]. Protein content was determined using the Lowry method with the Beijing Solarbio Protein Assay  
Figure 3: Cell density standard curves of _N. clostrium_ f. _minutissima_, with optical density (OD) on the \(x\)-axis and cell count on the \(y\)-axis. (Regression analysis is performed using Excel, employing the method of least squares).