Urea Utilization in Fed-Batch Spirulina Cultivation: A Practical Approach for Large-Scale Production

Reliance on nitrate as the primary nitrogen source for Spirulina (Arthrospira) platensis production poses cost challenges and hampers large-scale production, discouraging potential investors. While urea appears cost-effective relative to nitrate, its use is hindered by toxic ammonia concerns. To tackle the inhibitory effect of ammonia on microalgae growth, urea was added gradually to a fed-batch reactor using an exponential function to establish a direct correlation between urea consumption and cell growth. Three distinct protocols were explored to maintain a consistent nitrogen concentration within the culture: 50 mg/L for protocol 1, 37.33 mg/L for protocol 2, and 24 mg/L for protocol 3. Among these protocols, protocol 2 stood out as the most favorable, demonstrating superior cell concentrations and lipid content results. Protocol 2 achieved a peak cell concentration of 1100 mg/L with only 393.5 mg of urea consumption. In comparison, the control culture reached a cell concentration of 1215 mg/L but consumed 2500 mg of sodium nitrate. Notably, protocol 2 demonstrated a significantly higher nitrogen-cell conversion efficiency compared to the control culture. Lipid accumulation showed an upward trend as the nitrogen levels in the cell culture decreased. In contrast, the protein content of cells showed a decline, though the impact of the reduced nitrogen levels on macromolecular composition was not statistically significant. The developed model for urea addition in a fed-batch microalgal reactor can also prove beneficial in research aimed at biologically remediating wastewater with high ammonia content such as petrochemical, digestate, and poultry wastewater utilizing algae-bacteria consortia.