## 2 Materials and Methods  
The experimental set-up, with a gliding arc discharge plasma reactor, was developed at the Lithuanian Energy Institute. A schematic view of the gliding arc discharge (GAD) plasma set-up is presented in Figure 1. The constructed plasma set-up includes the plasma reactor and power supply including the variable frequency and voltage generator (AC Power) and high-voltage transformer (Figure 1) and air supply systems. The air supply system includes a compressor, mass flow controllers, metal tubes, plastic pipes, and steel connections. Compressed air was used as the plasma-forming gas. The air was injected into the plasma reactor at room temperature from the top of the electrodes, with a flow rate of 13.39 L/min, and tangentially, with a flow rate of 9.44 L/min, before applying a high voltage. This combination of air flow rates made it possible to obtain the same temperature in the plasma over the entire surface area of the Petri dish. Flowmeters were used to control and maintain a stable air flow rate during the discharge process. The G2000 high voltage/medium frequency generator (Redline Technologies Elektronik GmbH, Baesweiler, Germany) was used to generate the gliding arc discharge at atmospheric pressure. Discharge voltages were obtained using a Rigol DS4012 oscilloscope (Rigol Technologies, Inc. Suzhou, China) with a high-voltage passive probe, Tektronix P6015A (Tektronix, Co., Ltd. Shanghai, China). The current during discharge was measured with a current probe, Pintek PA-699 (Pintek Electronics Co., Ltd. New Taipei City, Taiwan). Two \"knife-like\" electrodes with a length of 52 mm and width of 25 mm were used, and the gap between the electrodes at the top was kept at 1 mm. The output voltage (V\({}_{\text{G}}\)) of the AC generator was changed from 50 V up to 250 V keeping the frequency at 270 kHz and the signal duty cycle at 0.35 during discharge. The turn ratio of high-voltage transformer was 1:33. Figure 1: A schematic view of the GAD plasma system. 1—high-voltage transformer; 2, 3—air flowmeter controllers; 4—electrodes; 5—high-voltage probe; 6—current probe; 7—Petri dish with BG-11 medium; 8—plasma reactor. Determination of the evolution and duration of the arc discharge was performed by analyzing the slow-motion images captured by a high-speed camera (Phantom Miro M310, Wayne, NJ, USA). A fast, \(\sim\)5260 frames per second, 12-bit, 25.6 mm \(\times\) 16.0 mm CMOS sensor camera composed of a zoom lens and a neutral UV filter was used to visualize the gliding arc distributions at various voltages. The emission spectra of the air plasma were measured using a flame-emission spectrometer, Flame UV-VIS (Ocean Insight, Orlando, FL, USA), in a wavelength range of 200-1000 nm. The integration time was 0.375 s, the number of accumulated scans was 10, and each air plasma spectrum was measured 3 times to calculate its average value. Also, air plasma composition was determined using an acousto-optic emission spectrometer, IFU AOS4 (in the range of 250-800 nm), which operates based on a tunable acousto-optical filter (AOTF). The spectrometer had a solid-state monochromator/grating (with a spectral resolution: 0.05 nm at 250 nm and 0.5 nm at 800 nm). The emission spectra of the air plasma were measured in the area between the electrodes, at 35 mm from the top, and a total of 3 scans were performed. In order to evaluate the effect of the developed plasma generator, BG-11 freshwater microalgae growth medium was chosen as a test sample, and was collected after 7 days of the _Chlorella vulgaris_ cultivation process [36]. Changes in the physical (temperature, conductivity) and chemical (radical composition) properties of the medium were monitored before and after treatment with a wide range of plasma voltages and a treatment duration of 5 min. During the experiment, 10 mL of BG-11 medium was injected in a Petri dish located in a position 30 mm below the electrodes. During the plasma processing of the medium, temperature variations were measured using two chromel-aliumel thermocouples.