Extracellular carotenoid production and fatty acids profile of Parachlorella kessleri under increased CO2 concentrations

Large-scale cultivations of photoautotrophic microorganisms represent a very promising and potentially cost-effective alternative for climate change mitigation, when associated to the co-production of high value bioproducts, such as fatty acids and carotenoids, considering the growing demand for natural products. During microalgae cultivation, CO₂ enrichment is a requirement to reach high productivities, although high CO₂ levels are normally stressful to microalgae. On the other hand, cellular stress is a well reported strategy to induce carotenoid and fatty acids production. This work evaluated extracellular carotenoid production from the mangrove-isolated microalga Parachlorella kessleri cultivated under 5, 15 and 30% CO₂ in stirred tank photobioreactors. In the 10th day of cultivation, CO₂ supply was interrupted until the end of the cultivation (14th day), causing a stressful and imperative condition for microalgae cells to release the red pigment. Growth kinetics, physiological parameters and bioproducts production were evaluated. Growth kinetics were similar under all tested conditions and differences were not statistically significant, with the highest values of μₘₐₓ, biomass concentration, lipid content and CO₂ fixation rate of 0.77 d⁻¹, 1.24 g L⁻¹, 241 mg g⁻¹ (dw) and 165 mg L⁻¹ d⁻¹, respectively. In contrast, total carotenoid concentrations varied significantly (p < 0.01), with the highest concentration of 0.030 μg mL⁻¹ under 5% CO₂. The produced red pigment presented antioxidant activity and characteristics of carotenoids confirmed by UV–vis and tandem mass spectrometry (MS/MS). The fatty acid profiles in the biomass varied in response to CO₂ levels in the cultivations. In general, higher CO₂ concentrations (15 and 30%) favored the production of saturated and mono-unsaturated fatty acids, suitable as biodiesel feedstock, while drastically decreased the production of the polyunsaturated.