Effects of Elevated CO2 Concentrations on the Biochemical Composition and Elemental Profile of Spirulina

Cyanobacterial cultivation under elevated CO2 concentrations represents a promising approach for coupling carbon dioxide mitigation with the production of biomass containing valuable biochemical compounds. In this study, we investigated the effect of CO2 concentration on the biochemical and elemental composition of spirulina (Limnospira platensis) cultivated in column photobioreactors. A culture previously adapted to elevated CO2 concentrations for six months was cultivated at 1.5, 3.0, and 6.0 vol.% CO2. The contents of proteins, lipids, carbohydrates, C-phycocyanin, chlorophyll a, carotenoids, and total phenolic compounds were determined, together with the CHNS elemental composition and C/N ratio. Increasing the CO2 concentration from 1.5 to 6.0 vol.% resulted in a slight increase in biomass concentration from 3.88 to 4.13 g L−1. Protein content remained relatively stable, with no statistically significant differences among treatments (one-way ANOVA: F(2, 15) = 2.01, p = 0.168, η2 = 0.212). In contrast, lipid content decreased significantly at 6.0 vol.% CO2, from 7.30% to 5.93% of dry biomass, whereas carbohydrate content increased from 10.19% to 15.68%. C-phycocyanin accumulation was highest at 3.0 vol.% CO2 (7.73%) but decreased to 2.34% at 6.0 vol.% CO2. Total phenolic content decreased from 5.89 mg GAE g−1 at 1.5 vol.% CO2 to approximately 5.0 mg GAE g−1 at elevated CO2 concentrations. Elemental analysis revealed a significant decrease in nitrogen content and an increase in the molar C/N ratio at 6.0 vol.% CO2, while carbon and hydrogen contents showed no statistically significant changes (C: p = 0.080; H: p = 0.532). Overall, elevated CO2 was associated with changes in the biochemical composition of L. platensis, particularly increased carbohydrate content and decreased lipid content at 6.0 vol.% CO2. The results demonstrate that CO2 concentration can be used as a cultivation parameter to tailor the biochemical properties of cyanobacterial biomass, supporting the development of integrated CO2 mitigation and biomass valorization strategies.