## 2 Materials and Methods
### Light and Dark Bottle Experiments  
The light and dark bottle experiments were used to determine the primary productivity of the cultures under various light intensities across different salinity treatments. Each bottle received 22.5 mL of the salinity-adjusted Zarrouk medium, followed by the addition of 2.5 mL of undiluted stock culture. Plastic films with different opacities were used to control light intensity, simulating ambient light availability at 0%, 25%, 50%, 75%, 90%, and 100%, equivalent to 0, 13.1, 26.3, 39.4, 47.3, and 52.5 \(\upmu\)mol photons\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\), respectively. The experiments were conducted with three replicates per salinity treatment.  
Prior to the 24 h incubation, the initial dissolved oxygen (DO) concentration of this mixture was assessed using a dissolved oxygen sensor linked to a PASCO interface. The sensor probe was calibrated for precision by being placed in clean, distilled water for several minutes both before and after the DO measurements were taken. This calibration step made certain that the sensor readings were correct and consistent.  
Final DO readings were taken at the end of the incubation period. To prevent any artefacts (such as oxygen from the atmosphere) entering the sample, the DO readings were taken as soon as the bottle covers were removed. Rapid measurement was important to make sure that the DO values were a true reflection of the metabolic activities of the culture and not some form of interference.  
The changes in DO concentration over the incubation period were used to compute respiration (\(R\), mg O\({}_{2}\cdot\)L\({}^{-1}\cdot\)d\({}^{-1}\)), Net Primary Productivity (\(NPP\), mg O\({}_{2}\cdot\)L\({}^{-1}\cdot\)d\({}^{-1}\)), and Gross Primary Productivity (\(GPP\), mg O\({}_{2}\cdot\)L\({}^{-1}\cdot\)d\({}^{-1}\)) using Equations (2)-(4), respectively.  
\[R=DO_{initial}-DO_{final\ (dark)} \tag{2}\]  
\[NPP=DO_{final\ (light)}-DO_{initial} \tag{3}\]  
\[GPP=NPP-R \tag{4}\]  
where \(DO_{initial}\) is the initial dissolved oxygen concentration before incubation (mg O\({}_{2}\cdot\)L\({}^{-1}\)), \(DO_{final\ (dark)}\) is the final dissolved oxygen concentration in the dark bottle after incubation (mg O\({}_{2}\cdot\)L\({}^{-1}\)), and \(DO_{final\ (light)}\) is the final dissolved oxygen concentration in the light bottle after incubation (mg O\({}_{2}\cdot\)L\({}^{-1}\)).  
The relationship between light intensity and photosynthetic response was analyzed by fitting \(GPP\) against light intensity levels using a linear regression model (Equation (5)). The linear model's slope represented the photosynthetic rate at which \(GPP\) changed with each light intensity unit. The calculated rates from photosynthetic efficiency were used to create plots that evaluated how photosynthesis responded to different light intensities at various salinity levels.  
\[GPP=\beta+\theta\cdot I+\varepsilon \tag{5}\]  
where \(I\) is the light intensity (\(\upmu\)mol photons\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\)), \(\beta\) is the intercept of the regression model (mg O\({}_{2}\cdot\)L\({}^{-1}\cdot\)d\({}^{-1}\)), \(\theta\) is the slope, representing the rate of change in \(GPP\) per unit increase in light intensity (i.e., photosynthetic efficiency, mg O\({}_{2}\cdot\)L\({}^{-1}\cdot\)d\({}^{-1}\) per \(\upmu\)mol photons\(\cdot\)m\({}^{-2}\cdot\)s\({}^{-1}\)), and \(\varepsilon\) is the error term.