Macroalgae Ulva prolifera O.F. Müller: An Efficient Biofilter for Nitrogen-Enriched Aquaculture Effluents

Wei Liu¹Meijing Zhang²Xianan Xiong¹Chiquan He¹Jinlin Liu^3*Peimin He²

• ¹Shanghai University, Shanghai, China
• ²Shanghai Ocean University, Shanghai, China
• ³Tongji University, Shanghai, China

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. 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). 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. We propose optimized RAS protocols: maintain NH4⁺-N ≤20 μmol•L⁻¹ and NO2⁻-N ≤30 μmol•L⁻¹, with 1.5-2× increased algalbiomass 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.