## 3 Results and Discussion
### Effect of Current Intensity on Cell Morphology, Viability, and ROS  
Figure 5a shows the cell viability rate of _H. lacustris_ at various current intensities (5, 10, 20, and 30 mA) after 4 h of electrotreatment. As expected, microalgal cells subjected to very mild currents of 5 and 10 mA maintained high cell viability rates, ranging from 98.1% to 99.1%, which were similar to those of the untreated controls (99.0%). However, at current intensities of 20 and 30 mA, the cell viability rates significantly decreased to 84.2% and 65.6%, respectively. To evaluate the impact of 4 h of electrotreatment at different currents (5, 10, 20, and 30 mA) on AXT biosynthesis and cell damage, the relative generation of ROS in electrotreated _H. lacustris_ cells was compared with that in untreated cells (Figure 5b). As the current intensity increased from 5 to 20 mA, the relative ROS level (%) in the electrotreated microalgal cells progressively increased from 17.1% to 34.9% above those in the untreated controls. However, at the highest intensity of 30 mA, the ROS content decreased to a level similar to that of the control group. Based on these results, we propose an ROS-mediated induction hypothesis to explain the enhanced AXT accumulation in _H. lacustris_ cells during electrotreatment. Given the prevailing electrochemical conditions coupled with a continuous air supply, various ROS species, including H\({}_{2}\)O\({}_{2}\), -OH, O\({}_{3}\), and -O\({}_{2}\)\({}^{-}\), were likely generated [26]. Notably, ROS can exert dual and contrasting effects on photosynthetic metabolism in microalgae. In particular, ROS can function as effective signaling molecules that promote the synthesis and accumulation of lipids and secondary carotenoid pigments such as AXT [37; 38]. However, when present at excessive concentrations, ROS can damage crucial cellular components, including lipids, proteins, and DNA, potentially leading to cell death [39; 40]. Our results suggest that _H. lacustris_ has evolved to synthesize AXT in response to oxidative damage, which acts as an antioxidant and efficiently scavenges ROS [41]. The relationship between AXT synthesis and ROS concentration in \(H\). _lacustris_ is complex. The concurrent increase in carotenoid production (Figure 4) and ROS levels (Figure 5b) in the microalgal cells treated with 5-10 mA compared to those of the controls suggests that ROS might promote the synthesis of AXT and the three other carotenoids. However, at higher ROS levels, specifically 20 mA, a threshold for cell damage was reached, leading to reduced AXT biosynthesis (Figure 4) and cell viability (Figure 5a). Notably, at the most stressful current of 30 mA, a lower ROS level was observed compared with that of the untreated control. This decrease could be attributed to the severe disruption of cellular homeostasis and essential enzyme systems, including esterases, involved in the measurement of ROS. Various biochemical analyses, including the determination of photochemical activity and the ratio of chlorophyll to carotenoid pigments [42], are required to understand the physiological and biochemical responses of microalgae during electrotreatment. Multiple factors, including ROS formation, acidification, and electrolysis of medium components, can affect the electrostimulation of microalgae. Furthermore, it is also necessary to elucidate the exact bioelectrochemical mechanism through additional studies, such as those involving the external addition of hydrogen peroxide. Figure 5c shows the changes in the mean cell diameter of \(H\). _lacustris_ after 4 h of electrotreatment at different current intensities (5, 10, 20, and 30 mA). In normal photosynthetic cultures, beyond approximately 20 days, _H. lacustris_ palmella cells subjected to nutrient limitation and aging gradually undergo volumetric expansion, accumulate AXT, and eventually transform into dormant cysts [16].