Effect of Carbon Dioxide on the Growth and Nutrient Uptake of the Microalgae Chlorella sorokiniana from Digestate

Microalgae are photosynthetic microorganisms capable of capturing CO2 from both the atmosphere and industrial emissions while producing valuable biomass. Among the various factors influencing microalgal growth, CO2 availability plays a critical role. This study examined how different CO2 flow rates affect the growth and nutrient assimilation of Chlorella sorokiniana cultivated in diluted digestate from a biogas plant with nitrogen concentrations up to 5 g/L. Results showed that biomass productivity increased with CO2 supply up to a threshold, beyond which it declined. The highest mean productivity was observed at a CO2 flow rate of 0.025 LPM, which did not differ significantly from the 0.050 LPM treatment, indicating comparable performance. In contrast, the highest flow rate (0.100 LPM) led to reduced productivity, although still higher than the control (no CO2). A similar trend was observed in ammonium removal, whereas phosphorus uptake remained relatively unaffected by CO2 supply. Overall, elevated CO2 levels appeared to shift microalgal metabolism towards biomass with lower nitrogen content and increased lipid and carbohydrate accumulation.