## 3 Results and Discussion
### Effect of Current Intensity on AXT and Other Carotenoids  
Algal carotenoids are classified into caroten and xanthophylls. Carotenes contain only hydrocarbons in their structure (e.g., \(\beta\)-carotene), while xanthophylls are oxygenated carotenoids, which contain various functional groups such as hydroxy (zeaxanthin) and keto (AXT and canthaxanthin) groups [34]. AXT is closely tied to the formation of \(\beta\)-carotene, a primary precursor that undergoes enzymatic transformations involving intermediates during carotenogenesis. In _H. lacustris_, the trajectory from \(\beta\)-carotene to AXT involves a biochemical route via a canthaxanthin intermediate. Zexaxanthin is also synthesized as a primary carotenoid, similar to \(\beta\)-carotene [35; 36]. Figure 4a-d illustrate the relative content (%) of AXT and other carotenoid pigments (i.e., \(\beta\)-carotene, zeaxanthin, and canthaxanthin) in electrotreated _H. lacustris_ under different current intensities (5, 10, 20, and 30 mA) compared to that of the untreated controls. The treatment time was fixed at 4 h based on the results from the previous 10 mA experiment (Figure 3). Increasing the current intensity from 5 to 10 mA led to a parallel upward trend in the levels of the three other carotenoids, similar to the AXT pattern. At 10 mA, the relative contents of AXT, canthaxanthin, zeaxanthin, and \(\beta\)-carotene in electrotreated cells were 121.8 \(\pm\) 4.2%, 123.4 \(\pm\) 7.0%, 111.1 \(\pm\) 2.9%, and 116.1 \(\pm\) 5.0%, respectively, representing the highest percentage increases compared to the contents of untreated cells. This indicates that a carefully tuned electrostimulation strategy can potentially stimulate multiple enzymatic pathways for the production of desired carotenoids in microalgae. Figure 4: Effect of current intensity on the relative contents of \(\beta\)-carotene (**a**), zeaxanthin (**b**), canthaxanthin (**c**), and astaxanthin (**d**) of _Haematococcus lacustris_ electrotreated for 4 h under different current intensities (5, 10, 20, and 30 mA) compared to those of the untreated controls. The relative content (%) of each carotenoid pigment is presented compared to that of the untreated control (100%). The error bars represent the standard deviations of four samples. * \(p\) < 0.05 and ** \(p\) < 0.005. However, at higher currents of 20 and 30 mA, the contents of all quantified carotenoids in _H. lacustris_ gradually decreased with increasing current intensity compared to those of the untreated controls (Figure 4a-d). This trend corresponds to the time-dependent profile of AXT biosynthesis shown in Figure 3 and aligns with the previously reported build-up of neutral lipids upon electrotreatment in _Chlorella_[28]. These findings suggest that a threshold exists at which the positive effects of electrostimulation on microalgae are inhibitory. Therefore, to maximize AXT electrostimulation, it is crucial to properly manage the electrochemical conditions while simultaneously ensuring the vitality of the microalgae. This necessitates a comprehensive exploration of various bioelectrochemical engineering aspects, such as determining the ideal duration and magnitude of the electric current; selecting appropriate operation modes such as continuous, intermittent, or gradual applications; ensuring effective supplies of CO\({}_{2}\) and light; devising optimal electrode materials and structures; and even considering the use of exogenous electron mediators. Notably, under identical electrotreatment conditions of 10 mA for 4 h, the relative enhancement in AXT content differed significantly in different experiments: 34.9 \(\pm\) 4.3% (Figure 3) and 21.8 \(\pm\) 4.2% (Figure 4d) compared to that of their respective untreated control groups. This variability could potentially be attributed to the utilization of initial greenish-brown palmella _H. lacustris_ cells that exist in distinct developmental and physiological states.