## 2 Materials and Methods
### Electric Stimulation of AXT Biosynthesis  
Figure 1 illustrates the photosynthetic electrochemical system used to stimulate AXT biosynthesis in \(H\). _lacustris_. The system comprised a two-chamber H-type electrochemical reactor, a power supply, two white LED lamps, and aeration control. The custom-made H-type reactor consisted of two glass bottles (working volume: 200 mL; Corning, Armonk, NY, USA) linked by a glass tube and separated by a proton-exchange membrane (PEM, PFSA D 125-U; Fuel Cell Store, Bryan, TX, USA), as described by [PERSON] et al. [27]. The PEM was positioned in the middle of the tubes and firmly clamped. We used plain carbon rods (diameter, 6 mm; length, 50 mm; Shinsung Carbon, Gwacheon, Republic of Korea) connected to titanium wires (diameter, 1 mm; length, 12 cm; Seoul Titanium, Seoul, Republic of Korea) using an electroconductive silver paste (Elocat P-100, Cans, Tokyo, Japan). The distance between the cathode and anode was maintained at 15 cm. In previous experiments using the same microalgal strain, severe cell damage was induced even at a low current intensity of 3 mA in the cathodic chamber of an electrochemical system [25]. Therefore, in this study, mild electrical stimulation of the microalga was performed only in the anodic chamber. For the electric stimulation experiment, greenish-brown palmella cells of \(H\). _lacustris_ (0.8 \(\pm\) 0.1 g/L), collected from a 4-day bc-PBR culture, were directly transferred to the anodic chamber of the H-type electrochemical reactor under clean bench conditions. A sterile NIES-C medium fortified with 11.2 mM phosphate buffer (pH 7.0) was introduced into the cathodic chamber to prevent substantial pH fluctuations. Currents of 4.8 \(\pm\) 0.3, 10.0 \(\pm\) 0.6, 20.1 \(\pm\) 0.8, and 29.9 \(\pm\) 0.4 mA were applied to the microalgal cells for up to 12 h by adjusting the voltage to 2, 4, 6, and 8 V, respectively, using a direct current power supply (GPD-43035, GWINSTEK, New Taipei City, Taiwan), as outlined by [PERSON] et al. [28]. For convenience, each current value was designated as 5, 10, 20, or 30 mA on the main scale included in the relevant figures. Cyclic voltammograms of the carbon electrodes in the presence and absence of \(H\). _lacustris_ were obtained using  
Figure 1: (**a**) Schematic diagram of photosynthetic electrotreatment and (**b**) illustration of the photosynthetic two-chamber electrochemical cell system used in this study, equipped with a power supply, light source, magnetic agitation, and aeration control to stimulate astaxanthin accumulation in _Haematococcus lactisris_. The diagram was modified from [PERSON] et al. [25]. DC, direct current; LED, light-emitting diode; ROS, reactive oxygen species. a single-chamber electrochemical reactor with three electrodes [25]. Notably, different oxidation and reduction current densities were observed in the microalgal cells. To maintain the metabolic activity of the photosynthetic microalgae during the electrochemical stimulation experiments, continuous CO\({}_{2}\) and light supply were provided. Both anodic and cathodic chambers were aerated with 5.0% CO\({}_{2}\) (\(v/v\)) in air at a flow rate of 40 mL/min through stainless-steel needles (size 21G, LK Lab Korea Co., Namyangju, Republic of Korea) after being filtered using a 0.2 \(\upmu\)m polyvinylidene fluoride air filter (Biofact Co., Daejeon, Republic of Korea), incorporating the same aeration control system as used for microalgal cultivation (see Section 2.1). Additionally, both chambers were independently stirred at 250 rpm using magnetic stirrers (Color Squid, IKA, Staufen, Germany) with polytetrafluoroethylene magnetic bars (size 30 \(\times\) 8 mm; Bel-Art Products, Wayne, NJ, USA) to prevent the _H. lacustris_ cells from settling. The electrochemical reactor received illumination with a light intensity of 200 \(\pm\) 10 \(\upmu\)mol photon/m\({}^{2}\)/s in the same temperature-controlled room (approximately 25 \({}^{\circ}\)C) used for bc-PBR culture.