## 4 Discussion
These results align with the analysis of the biomass cultured in the laboratory and outdoors, revealing insignificant differences. The reason that _Leptolyngbya_ sp. KIOST-1 is beneficial for outdoor cultivation, similarly to _Arthospira_, lies in its growth characteristics. _Leptolyngbya_ sp. KIOST-1 was first isolated in nutrient-depleted SOT medium with a salinity and pH of 13.9 psu and pH 10.3, respectively [12]. As microalgae commonly found in soda lakes exhibit salt tolerance and alkalinity [44], we expected that _Leptolyngbya_ sp. KIOST-1 would also exhibit these characteristics. As shown in Figure 7, this strain exhibits saline tolerance and alkaliphilic properties. Cyanobacteria grow better than other phytoplankton under alkaline conditions. Although _Leptolyngbya_ sp. KIOST-1 required time to adapt to the high-salinity conditions, similar to other cyanobacterium species [45], it was able to achieve a maximum biomass concentration at up to 50 psu salinity. Species with these characteristics are advantageous for biomass production. The artificial addition of salts and pH adjustment have also been suggested to restrict the growth of contaminants introduced during the culture process in large-scale cultivation [46; 47]. We found that _Leptolyngbya_ sp. KIOST-1 cells grew normally under a wide range of nutritional conditions (Figure 6). Carbon and nitrogen are considered major nutrients for cyanobacterial growth and biochemical composition because they are essential for the formation of proteins, chlorophyll, and nucleic acids in cells [48; 49]. The main components of the SOT medium are also carbon and nitrogen, provided as sodium bicarbonate and sodium nitrate, respectively. Based on our results, _Leptolyngbya_ sp. KIOST-1 reached the stationary phase without a significant difference in growth, even at 1/5 concentrations of sodium bicarbonate and sodium nitrate in the SOT medium (Figure 6). In our study, cell death in _Leptolyngbya_ sp. KIOST-1 was observed in low-initial-sodium bicarbonate concentrations (Figure 6a) on days 8 and 12. Upon the initiation of cell death, the pH values of the media exhibited a sharp increase up to 11.0. Such cell death has also been observed in other _Leptolyngbya_ sp. and _Arthospira_ cultures [50; 51]. This sharp increase in pH was related to bicarbonate consumption. Although carbon dioxide (CO\({}_{2}\)) is considered a major carbon source for microalgal growth, most cyanobacteria actively take up bicarbonate (HCO\({}_{3}\)\({}^{-}\)) [52]. In general, HCO\({}_{3}\)\({}^{-}\) accumulates in the cytoplasm and is converted into CO\({}_{2}\) shortly before photosynthesis [52]. In addition, it leads to an increase in the pH of the culture medium because hydroxide ions (OH\({}^{-}\)) are used from HCO\({}_{3}\)\({}^{-}\) as cells grow. In this situation, residual HCO\({}_{3}\)\({}^{-}\) in the culture medium reacts with OH\({}^{-}\) to form carbonate (CO\({}_{3}\)\({}^{2-}\)) and acts as a pH buffer to slowly increase the pH [53]. At initial bicarbonate conditions below 6.72 g L\({}^{-1}\), the pH buffer mechanism did not work properly. This interpretation can explain the sharp increase in pH as well as the cell death. Therefore, although _Leptolyngbya_ sp. KIOST-1 is capable of sufficient growth at low carbon concentrations, caution should be exercised to prevent culture failure. While it has been confirmed that significant differences in biomass production can occur even with low concentrations of carbon and nitrogen sources, this study did not include a composition analysis in this regard. Therefore, it could be suggested to reduce nutrient concentrations for economical biomass production, but any resulting changes in composition should be investigated and confirmed. Interestingly, our results demonstrated that the strain continued to grow until the final day under nitrogen-limited conditions (Figure 6b).