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

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 NH 4 + -N, NO 3 − -N, and NO 2 − -N uptake kinetics in nitrogen-starved (7-day). U. prolifera across eight concentration gradients (NH 4 + -N: 0.5–35 µmol·L −1 ; NO 3 − -N: 5–130 µmol·L −1 ; NO 2 − -N: 2.5–60 µmol·L −1 ). Result Michaelis-Menten modeling revealed: NO 3 − -N achieved the highest maximum uptake rate (V max = 161.29 μmol·g −1 ·h −1 ), but the greatest half-saturation constant (K m = 29.40 µmol·L −1 ); NH 4 + -N showed the strongest affinity (K m = 4.60 μmol·L −1 ); NO 2 − -N absorption plummeted at >50 µmol·L −1 (41% removal after 6h). When the initial concentrations were below 20 µmol·L −1 for NH 4 + -N, or below 90 μmol·L −1 NO 3 − -N, or below 20 μmol·L −1 for NO 2 − -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: NH 4 + -N >20 µmol·L −1 reduced removal by 40–60%, NO 3 − -N >110 μmol·L −1 induced suppression, and NO 2 − -N >30 µmol·L −1 triggered acute inhibition. Conclusion We propose optimized RAS protocols: maintain NH 4 + -N ≤20 μmol·L −1 and NO 2 − -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.