## 1 Introduction  
_Spirulina_ is a filamentous cyanobacterium and is among the most widely cultivated microalgae for its high nutritional value, resilience to environmental stress, and potential biotechnological applications [1]. _Spirulina_ is often called the'super food' due to the fact that it contains proteins, fatty acids, vitamins, pigments like phyocytanin and chlorophyll \(a\), and many minerals [2, 3]. In addition to the economic and dietary value of _Spirulina_, it also plays an ecological role as a primary producer in saline-alkaline aquatic ecosystems and contributes to the global carbon cycle and primary production [4, 5].  
Optimizing environmental conditions remains a key focus in _Spirulina_ research, particularly for improving biomass yield and metabolite production [6; 7]. Among the environmental variables, salinity is a critical factor that influences not only growth and photosynthesis but also morphological and biochemical traits [8]. Salt stress disturbs cellular homeostasis through the development of osmotic stress and ionic toxicity that impact metabolism and energy use [9].  
In recent years, an increase in the salinization of freshwater and coastal systems has been observed due to climate change, sea-level rise, decreases in rainfall, and human activities like irrigation and industrial discharge [10]. This environmental shift is important to address how photosynthetic organisms like _Spirulina_ will react to changing salt concentrations in their natural environment and in culture. Understanding these responses is essential for sustaining large-scale production in changing environments.  
There are conflicting findings on the effect of salinity on the growth and production of _Spirulina_. For example, [PERSON] and [PERSON] [11] found that _Spirulina_ major had higher chlorophyll \(a\) content at moderate salinities (9-16 ppt), but the highest salinities (\(\geq\)25 ppt) caused a significant reduction in biomass. On the other hand, other studies have indicated that _Spirulina_ is able to thrive in a broad range of salinities and can have enhanced photosynthetic rates under high salt concentrations [12]. However, much of the existing literature has focused narrowly on physiological parameters such as pigment concentration and nutrient uptake, with limited attention to morphological traits.  
_Spirulina_ has been observed to exhibit morphological flexibility in terms of filament length, arrangement, and coiling under stressors like temperature and light [13; 14; 15]. However, the influence of salinity on filament size structure remains poorly documented. Morphological characteristics can affect harvesting efficiency, settling behavior, and stress tolerance--factors that are highly relevant to scalable cultivation. A more integrated approach that considers both physiological and morphological responses is therefore essential to define optimal and resilient production strategies under variable salinity conditions.  
In this context, the present study aimed to test the effects of different salinity levels on a _Spirulina_ strain concerning three biological aspects: (1) growth, (2) productivity, and (3) filament size structure. We hypothesized that salinity alters not only the growth and physiological performance but also the morphological traits of _Spirulina_ in ways that influence its suitability for large-scale cultivation. This study provides insights into how salinity stress may shape _Spirulina_'s functional responses and informs strategies for improving production under increasingly saline conditions.