## 2 Materials and Methods
### Culture Treatment, Observation, and Data Analysis  
Non-invasive and fast microscopic observations were performed to characterize cultivation states (cell concentration and proliferation) under different nutrient composition and cultivation. To characterize the microalgal cell growth in a NaHCO\({}_{3}\)-exclusive biochemical condition, we examined three different experimental treatments (cases) in the laboratory: (1) the modified standard SOT in which SOT-cultivated _S. maxima_ were injected, (2) sodium bicarbonate (NaHCO\({}_{3}\))-free SOT with _S. maxima_ cultivated in the modified SOT, and (3) sodium bicarbonate (NaHCO\({}_{3}\))-free SOT with _S. maxima_ cultivated in sodium bicarbonate (NaHCO\({}_{3}\))-free SOT. Note that the semicontinuous operation mode was considered; to control the nutritional and light environment, a small volume of seed cultures was incubated first in batches and fed into large media later. To prepare for the inoculum cultures, _S. maxima_ was sterilized for 15 min at 121 \({}^{\circ}\)C and transferred in a 250 mL conical flask with \(>\)-3000 lux LED irradiated to provide a suitable photoautotrophic condition for 30 days. A _spirulina_ seed batch of 50 mL was injected afterwards into 1500 mL of different suspension media each (-3.33%) at an initial pH of 8.44 in Case 1 and 7.15 in Cases 2 and 3, respectively. The salinity was 1.23% (492 mg) in Case 1 and 0.41% (164 mg) in Cases 2 and 3. The cultivation cylinder vessels were individually and equally aerated at 1.5 L/min through \(\phi\) 4 mm air tubes in the lab (Figure 5). Then, we identified the influence of NaHCO\({}_{3}\) in _S. maxima_ cell growth in a room environment for 2 weeks. Viable cell sizes, structures, and populations were examined by imaging magnified at \(\times\)120-7000 (BK53M,Olympus, Tokyo, Japan). Culture samples for the analysis were taken aseptically from the top surface of the suspended solutions. The pH values were obtained using a calibrated Benchtop pH Tester, PM-3 (CAS, Seoul, Republic of Korea) (Figure A2).  
On the other hand, air diffusion in the test chamber was simulated to justify the positions of sensor installation and measurement of gas concentrations. Ansys Fluent (ver. 2022 R2) software was used for computational fluid dynamics (CFD). For data analysis results, the mean value of CO\({}_{2}\) concentration per each treatment was compared using the Mann-Whitney U test and the variance using [PERSON]'s test. The Kruskal method was applied to compare removal efficiencies upon CO\({}_{2}\) reinforcement. [PERSON]'s test was  
Figure 4: Test equipment setup (IAQ test air chamber): (**a**) dimensions of equipment, (**b**) air chamber— inlet view (right side), (**c**) air circulation scheme, (**d**) air chamber—outlet view (left side).  
Figure 5: Lab facility: cultivation of _S. maxima_ in different medium.  
followed as a post hoc analysis for multiple pairwise comparison. Data difference was considered significant at _p_-value \(\leq\) 0.001. All statistical analyses were performed using R\({}^{\text{TM}}\) via RStudio 1.3.959 (RStudio, Inc., Boston, MA, USA). The _nortest_ and _lawstat_ packages were used to carry out the Anderson-Darling (AD) normality and Brown-Forsythe variance test, respectively, in R\({}^{\text{TM}}\).