## 3 Results and Discussion
### pH and Salinity Stress Tolerance  
This experiment was conducted over a 12-day period under various pH and salinity conditions (Figure 2) to assess stress tolerance based on growth rates. The rationale behind testing these stress conditions was to evaluate the strain's physiological adaptability to environments with fluctuating pH and salt concentrations, which are commonly encountered in large-scale cultivation systems, including outdoor and industrial settings. Obtaining such baseline data is essential for identifying the environmental limits of the strain and assessing its feasibility for broader biotechnological applications. Among the tested conditions, the most acidic condition, pH 4, resulted in a rapid shift in the culture medium toward alkalinity (Table S2). Meanwhile, pH 12 led to the lowest growth rate, whereas pH 10 showed a similar growth pattern to the control. By day 12, the growth rate at pH 10 surpassed that of the control, making it the highest among all pH treatments--highlighting the strain's enhanced performance under slightly alkaline conditions. These results suggest that _Desmodesmus_ sp. KNUA231 has the ability to utilize bicarbonate (HCO\({}_{3}\)-) as an alternative carbon source under high-pH conditions, where free CO\({}_{2}\) is limited due to carbonate equilibrium. In alkaline environments, the concentration of hydrogen ions (H\({}^{+}\)) is low, and CO\({}_{2}\) in the medium tends to decrease as it shifts toward bicarbonate and carbonate forms. However, if KNUA231 efficiently converts HCO  
Figure 1: **(a)** Growth curve of optical density measured for 9 days at 25 \({}^{\circ}\)C, 200 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) light intensity, with a 16:8 light/dark cycle, agitation at 120 rpm, and 1–1.5% CO\({}_{2}\) conditions. **(b)** Results of chlorophyll \(a\), chlorophyll \(b\), and total carotenoid (\(\upmu\)g mL\({}^{-1}\)) after 9 days of cultivation. into intracellular CO2, it can maintain photosynthetic activity and sustain growth even as external CO2 diminishes. As more inorganic carbon is assimilated for photosynthesis, CO2 levels in the medium drop further, reducing H+ generation and causing a continued rise in pH rather than a decrease. This internal carbon acquisition mechanism likely provides a physiological advantage for growth in high-pH environments. Furthermore, previous studies have reported that _Desmodesmus_ species tend to exhibit relatively stable growth across a wide range of environmental conditions rather than displaying exceptionally high growth rates under specific conditions [41, 42]. Given these findings, _Desmodesmus_ sp. KNUA231 is more likely to maintain stable growth across diverse environmental fluctuations rather than exhibiting rapid growth under optimal conditions. This physiological adaptability enhances its potential for large-scale cultivation in various cultural environments, suggesting that it could be utilized in a broad range of industrial applications without being restricted to narrowly defined optimal conditions. To assess salinity tolerance, the growth rate at 5 g L\({}^{-1}\) NaCl exhibited a continuous increase over 12 days, although it remained lower than that of the control. Under 10 g L\({}^{-1}\) NaCl conditions, growth increased slightly for the first three days but stagnated thereafter (Table S3). At higher salinity levels, growth was entirely inhibited. These findings suggest that _Desmodesmus_ sp. KNUA231 can sustain growth under moderate salinity conditions but experiences significant growth inhibition at higher salt concentrations. Figure 2: Conditions for checking pH and salinity tolerance. Figure showing the (**a**) OD and (**b**) pH results of pH 4, 6, 8, 10, and 12 compared to pH 7, the control. Conditions for checking salt tolerance: NaCl 5, 10, 20, 30 g L\({}^{-1}\) (**c**) OD and (**d**) pH results compared to 0 g L\({}^{-1}\) as the control. As shown, _Desmodesmus_ sp. KNUA231 exhibits the broadest pH tolerance range (4-10), while maintaining moderate salinity tolerance (up to 10 g L\({}^{-1}\) NaCl).