Impact of salinity on morphology, growth, and pigment profiles of Scenedesmus obliquus HTB1 under ambient air and elevated CO2 (10 %) conditions

Certain microalgal species, such as Scenedesmus obliquus strain HTB1, thrive under high CO₂ concentrations, making them promising for carbon sequestration to mitigate climate change. Isolated from the Baltimore Inner Harbor, HTB1 grows faster with 10 % CO₂ than with ambient air. To investigate its responses to salinity and elevated CO₂, two experiments were conducted. In the first, HTB1 was cultured at seven different salinities (0, 17.5, 20, 22.5, 25, 27.5, and 30 ppt) (parts per thousand) under ambient air. Higher salinity caused cell shrinkage, color changes from green to pale white, reduced pigments like zeaxanthin, lutein, and chlorophyll b, but increased canthaxanthin. Growth declined significantly above 22.5 ppt. The second experiment compared HTB1's response to salinity (0, 10, 20 ppt) under air and 10 % CO₂. Cultures under 10 % CO₂ showed minimal color changes, while those under air shifted from green to brown, with salinity having less inhibitory effects on growth under elevated CO₂. Interestingly, lutein and canthaxanthin levels rose with salinity in 10 % CO₂. These findings indicate that elevated CO₂ mitigates salt stress in HTB1, reducing its impact on growth and promoting adaptive pigment changes. This study sheds light on how salinity and CO₂ interact to influence HTB1's morphology, growth, and pigment composition, enhancing our understanding of its resilience and potential applications.