## 3 Results  
### Effects of Electrical Treatment on N. closterium f. minutissima Growth  
_N. closterium f. minutissima_ was subjected to short-term direct current stimulation (D group) at voltage levels of 0 V, 1 V, 3 V, 5 V, 10 V, and continuous direct current stimulation (C group) at 0 V, 0.05 V, 0.5 V, 1 V. Since the electrodes remained in the algal culture for an extended period in the continuous stimulation experiment, and the 0 V short-term stimulation group closely mimicked the natural growth conditions of the microalgae, the 0 V group in the short-term stimulation was designated as CK, and the 0 V group in the continuous stimulation was labeled C-0 V, serving as the control for the short-term group. The experiment lasted for seven days, with daily sampling to measure optical density at 750 nm, from which growth curves were constructed and growth rates calculated. Biomass dry weight was measured at the end of the experiment. The results showed that short-term direct current stimulation had a significant effect on the maximum cell density of _N. closterium f. minutissima_ (_p_ < 0.05), though it had a limited impact on biomass dry weight accumulation (_p_ > 0.05). In contrast, both the maximum cell density and biomass dry weight significantly differed among the continuous stimulation groups (_p_ < 0.05). By comparing growth curves (Figure 4a) and growth rates (Table 2) for the different experimental groups, the growth dynamics of the microalgae were observed. Unlike the short-term stimulation groups, continuous stimulation resulted in poor growth across all groups, with no clear S-shaped growth curve, and the K values were generally lower than those of the short-term stimulation groups (Figure 4a). The D-1 V group maintained a high growth rate throughout the experiment, achieving the highest overall growth rate (K\({}_{w}\)) of 0.68 \(\pm\) 0.01 and reaching a plateau on day 6, similar to the CK and 10 V groups. The D-3 V and D-5 V groups exhibited the highest daily growth rates (K\({}_{max}\)) of 0.84 and 0.83, respectively, on day 2, but then entered the plateau phase earlier than the other groups. In the continuous stimulation groups, growth trends were nearly identical, peaking around day 4, followed by a negative growth phase. The C-1 V group showed the highest negative growth rate on day 5 (\(-\)0.41 \(\pm\) 0.05) compared to the other groups. The maximum cell densities of _N. closterium f. minutissima_ in the D-1 V and D-3 V groups significantly increased by 13.14% and 6.64% compared to CK (_p_ < 0.05), with averages of 11.80 \(\times\) 10\({}^{6}\) cell/mL and 11.20 \(\times\) 10\({}^{6}\) cell/mL, respectively. However, the variations in the D-5 V and D-10 V groups were not significant. The average maximum cell density across the groups initially increased with voltage but then decreased (Figure 4b), with no significant change in dry weight across the groups (Figure 4c). These results suggest that short-term stimulation at appropriate voltage levels promotes the growth of _Nitzschia closterium_, primarily reflected in increased cell density. Although ANOVA analysis indicated that short-term stimulation significantly affected cell density (_p_ < 0.05), further statistical analysis revealed no significant differences exist (Figure 4b). The biomass dry weight in the C-0.5 V group significantly increased by 7.20% compared to C-0 V (_p_ < 0.05), the highest among the continuous stimulation groups, but was significantly lower than CK and the other short-term stimulation groups (Figure 4c). Notably, the growth curves, maximum cell densities, and biomass dry weight in the short-term stimulation groups were superior to those in the continuous stimulation groups (_p_ < 0.05), suggesting that prolongedcontact between the algae and the electrodes significantly inhibited growth, with substantial background inhibition present in all continuous stimulation groups. Therefore, the effects of continuous stimulation on _N. closterium_ f. _minutissima_ growth remain inconclusive.