Light Conditions Determine Optimal CO2 Concentrations for Nannochloropsis oceanica Growth with Carbon Fixation

To improve carbon fixation using microalgal biomass cultivated with industrial flue gas, the combined effects of various CO2 concentrations and light intensities on microalgal growth and carbon fixation were comprehensively studied. The optimal CO2 concentration for microalgal growth depended on the light intensity. For example, the optimal concentration with which the microalgal biomass achieved the highest yield was 6% under 20 μmol·m–2·s–1 light (1.24 g/L), which changed to 30% under 60 μmol·m–2·s–1 (2.12 g/L) and reached 50% under 80 μmol·m–2·s–1 (2.45 g/L). The changes in light intensity dictate whether the microalgal system was in a state of insufficient carbon, adequate carbon, or excessive carbon. The ideal combination of CO2 concentration and light intensity led to developed light capture antenna proteins (10 types of light harvesting proteins were significantly upregulated in optimal groups) and other component proteins in the photosynthetic system. This enhanced the photochemical electron yield (a maximum increase of 36.9% in Fv/Fm) and active carbon metabolism, thus promoting carbon fixation (a maximum increase of 16.3% in the proportion of carbon element in biomass) by the microalgal biomass. However, inappropriate combinations of CO2 concentration and light intensity led to the inhibition of carbon accumulation and biomass growth. For example, when the CO2 supply was weaker than the light supply, there were more protein translation errors and a greater abundance of lysosomes and phagosomes, causing increased apoptosis. Alternatively, when the CO2 supply was stronger than the optimal CO2 concentration under the current light intensity, excess CO2 pressure strengthened the phosphoric acid-related regulation process. Therefore, photosynthetic activity was dynamically regulated to promote the light quantum consumption rate to match the abundant CO2, while ion pumps were strengthened to dissipate excess CO2 to maintain a balanced intracellular environment. This study convinced the importance of the correct match of CO2 concentration and light intensity, which provided a method to optimize the effect of carbon fixation on microalgal growth in different industrial sites.