Macroalga Ulva prolifera O.F. Müller: an efficient biofilter for nitrogen-enriched aquaculture effluents

Abstract

Introduction: Ulva prolifera, a dominant species in green tides, exhibits remarkable nitrogen absorption capacity, yet its kinetics as a biofilter in recirculating aquaculture systems (RAS) remain unquantified. Methods: This study systematically characterized NH4+-N, NO3−-N, and NO2−-N uptake kinetics in nitrogen-starved (7-day). U. prolifera across eight concentration gradients (NH4+-N: 0.5–35 µmol·L−1; NO3−-N: 5–130 µmol·L−1; NO2−-N: 2.5–60 µmol·L−1). Result: Michaelis-Menten modeling revealed: NO3−-N achieved the highest maximum uptake rate (Vmax = 161.29 μmol·g−1·h−1), but the greatest half-saturation constant (Km = 29.40 µmol·L−1); NH4+-N showed the strongest affinity (Km = 4.60 μmol·L−1); NO2−-N absorption plummeted at >50 µmol·L−1 (41% removal after 6h). When the initial concentrations were below 20 µmol·L−1 for NH4+-N, or below 90 μmol·L−1 NO3−-N, or below 20 μmol·L−1 for NO2−-N, 1 g of Ulva prolifera in 1 L of seawater completely achieving 100% removal efficiency for each within 6 hours. Critical inhibition thresholds were identified: NH4+-N >20 µmol·L−1 reduced removal by 40–60%, NO3−-N >110 μmol·L−1 induced suppression, and NO2−-N >30 µmol·L−1 triggered acute inhibition. Conclusion: We propose optimized RAS protocols: maintain NH4+-N ≤20 μmol·L−1 and NO2−-N ≤30 µmol·L−1, with 1.5–2× increased algal biomass when high total inorganic nitrogen concentration to sustain >90% nitrogen removal. This work demonstrates U. prolifera’s nitrogen assimilation strategy—"high ammonium affinity, high nitrate capacity, nitrite sensitivity"—providing a mechanistic foundation for valorizing green-tide macroalgae in sustainable aquaculture.