Biochemical Profile of Limnospira platensis Cultivated Under Elevated CO2 with the Addition of Sodium Selenite

Selenium (Se) biofortification of microalgae is a promising strategy for producing functional biomass with enhanced nutritional value. This study evaluates the effect of sodium selenite (Na2SeO3) supplementation to the culture medium on the biochemical profile of Limnospira platensis cultivated under intensive growth conditions with an elevated CO2 concentration (3 vol.%). Two consecutive cultivation experiments were conducted using Na2SeO3 supplementations of 10, 20, 40, and 80 mg/L along with a control (0 mg/L). In the second experiment, biomass previously cultivated at 20 mg/L served as the inoculum to evaluate the response of a pre-adapted culture. Biomass productivity, protein, carbohydrate, lipid, pigment, CHNS composition, Se accumulation, and macro- and microelement contents were determined. Maximum biomass productivity (0.369 g·L−1·day−1) was obtained at 10 mg/L. The most severe disruptions to biochemical and elemental parameters occur at 80 mg/L. This concentration triggers a statistically significant reduction in protein content (down to 46.98% in the first experiment), an accumulation of mercury, an elevation in carbohydrates, and a corresponding shift in the C/N ratio. The second experiment demonstrated enhanced metabolic stability of the pre-adapted culture, including preservation of protein content under high selenium stress and normalization of copper accumulation, indicating the adaptive capacity of L. platensis to prolonged selenium exposure. The concentration of Se in the biomass grown under 0 (control), 10, 20 and 80 mg/L of Na2SeO3 was 0.5, 88.7, 394.0 and 762.0 mg/kg, respectively. Overall, a method for producing L. platensis biomass with a controlled Se content has been demonstrated. Based on these findings, Na2SeO3 concentrations of 10–40 mg/L are recommended for producing Se-enriched biomass, balancing productivity, nutritional quality, and elemental safety. The nutritional efficacy of the biomass obtained using this method should be the subject of future research.