## 4 Discussion  
### Salinity Modulates Growth of Spirulina  
The present study provides evidence of the salinity dependency of the growth of the _Spirulina_ strain. The highest maximum specific growth rate was observed at 10 ppt and then drastically decreased at 17, 40, and 45 ppt. This data shows that although the strain can tolerate hypersaline conditions, growth becomes severely impaired once salinity exceeds the optimal levels [25].  
The results are in agreement with earlier studies, which have shown that _Spirulina_ grows well in moderate salinity and is affected by stress when the salinity is above the tolerance range of the organism. In [PERSON] and [PERSON]'s study [11], _Spirulina major_ had the highest chlorophyll-a content and growth rates at salinities of 9-16 ppt, whereas at salinities of 25-40 ppt, growth rates were very low. [PERSON] et al. [6] also obtained similar results, as they reported that the protein content of _Spirulina platensis_ was the highest at a low salinity of 13 g/L and decreased at higher salt concentrations, which could be related to the sensitivity of nitrogen metabolism to salt.  
The current study showed that _Spirulina_ cultures grown at the intermediate salinities (22-36 ppt) did not have statistically significant differences in specific growth rates. This may indicate a physiological acclimation phase in which osmoregulatory mechanisms, such as the synthesis of compatible solutes or the expression of salt-stress proteins, may help to  
Figure 3: Effects of salinity on the size structure of the _Spirulina_ strain. (**a**) Stacked bar plot showing the density (no. of filaments mL\({}^{-1}\)) classified into six size classes across salinity levels ranging from 10 to 45 ppt. (**b**) Principal Component Analysis (PCA) biplot illustrating the variation in size class structure across salinity treatments. Arrows represent the contribution of each size class to the principal components, while salinity treatments are projected as scores.  
stabilize the cellular metabolism [26; 27]. Indeed, [PERSON] and colleagues [26] observed that morphological variants of _Spirulina platensis_ showed altered protein expression related to energy metabolism and translational machinery under different environmental stresses, including salinity. The molecular responses observed may explain the relatively constant growth rate observed between 22 and 36 ppt.  
The lower growth rates observed at 40 and 45 ppt are indicative of the upper limit of salinity that the _Spirulina_ strain can tolerate since, beyond this salinity, ion toxicity and osmotic imbalance will impede metabolic activity. High salinity may cause a reduction in photosynthesis and nutrient absorption and thus lead to reduced biomass production [12]. Furthermore, high salt concentrations can cause damage to the structure and functioning of the thylakoid membrane, which results in decreased light utilization efficiency and reduced cellular performance [14].  
However, the fact that the strain can still grow at salinities as high as 45 ppt is a clear indication of its eurylahine nature. This physiological plasticity could be due to the fact that _Spirulina_ is a salt-tolerant, alkaline lake algae that lives in environments with changing ionic conditions [1]. However, the results of this study also indicate that the salinity for the maximum growth rate of the _Spirulina_ strain is at the lower end of the gradient, particularly at 10 ppt.