## 4 Discussion
### Implications for Optimization of Spirulina Cultivation  
The integrated physiological and morphological responses of _Spirulina_ across a salinity gradient offer valuable insights for refining cultivation strategies, particularly in the context of maximizing biomass productivity and maintaining culture resilience. The highest specific growth rate was observed at 10 ppt, but the photosynthetic efficiency increased significantly at higher salinities, most notably at 27 ppt. This suggests that under moderately saline conditions, _Spirulina_ may shift its metabolic investment from rapid proliferation toward enhancing energy conversion efficiency and photophysiological performance. The salinity level of 27 ppt appears to represent a crucial physiological threshold--one that balances acceptable growth rates with superior photosynthetic performance and manageable morphological variation. At this level, cultures maintained substantial productivity while exhibiting intermediate cell sizes and moderate reductions in cell density. These characteristics are advantageous in large-scale cultivation, where consistent oxygen evolution and biomass composition are often prioritized over maximal growth rates alone. This interpretation is supported by the findings of [PERSON] et al. [6], who reported that _Spirulina platensis_ exhibited increased protein productivity at lower salinities when supplemented with carbon dioxide but also retained tolerance to higher salinity regimes. Moreover, [PERSON] et al. [12] demonstrated that salinity tolerance in biomomforming cyanobacteria is strain-specific, with certain species showing enhanced photosynthetic activity across a broad salinity range. These observations highlight the flexibility of cyanobacteria like _Spirulina_ in modulating productivity under salt stress and emphasize the potential for targeted environmental tuning in cultivation systems. Importantly, the capacity of _Spirulina_ to survive and maintain photosynthetic efficiency even at 45 ppt underscores its potential for cultivation in non-arable, salt-affected regions. This aligns with the growing interest in utilizing brackish water or wastewater as a cost-effective medium for microalgal production [17]. The organism's resilience to salinity also opens opportunities for integrating _Spirulina_ cultivation into sustainable aquaculture systems and land reclamation initiatives, where freshwater scarcity or salinization pose significant constraints. Furthermore, the morphological flexibility of _Spirulina_--manifested in its ability to alter filament size and structure under environmental stress--may contribute to its adaptive advantage in fluctuating habitats. Studies have shown that such morphological changesare not merely structural but may reflect deeper proteomic and metabolic adjustments [26], enabling the organism to maintain function despite suboptimal conditions. This adaptive plasticity is particularly relevant under climate change scenarios, where the increasing salinization of freshwater and estuarine systems is expected due to sea-level rise and intensified droughts [32]. In conclusion, cultivation at approximately 27 ppt may offer an optimal compromise between growth and productivity, while also taking advantage of _Spirulina_'s inherent tolerance to salinity-induced stress. Such strategic modulation of salinity could enhance the efficiency, resilience, and scalability of _Spirulina_ production systems, supporting their broader application in food, feed, and environmental biotechnology sectors. Conceptualization, I.K.M.L. and B.E. ; methodology, I.K.M.L. and B.E. ; formal analysis, I.K.M.L. and B.E. ; investigation, I.K.M.L. ; resources, I.K.M.L. and B.E. ; data curation, I.K.M.L. and B.E. ; writing--original draft preparation, I.K.M.L. and B.E. ; writing--review and editing, I.K.M.L. and B.E. ; visualization, B.E. ; supervision, B.E. All authors made equal contributions to this study. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.