Urea as a key nitrogen source for the invasion of the southern coast of Portugal by the brown seaweed Rugulopteryx okamurae (Dyctiotales, Phaeophyceae)

The invasive seaweed Rugulopteryx okamurae, native to East Asia, is spreading rapidly along the western Mediterranean and southern Portugal, severely affecting coastal biodiversity, ecosystem structure, and economic sectors such as fisheries and tourism. This study examined the nutrient uptake kinetics of R. okamurae, including ammonium, nitrate, urea, amino acids, and phosphate, and their role in nitrogen and phosphorus budgets based on laboratory growth rates. R. okamurae demonstrated the highest uptake for ammonium (V_max = 57.95 μmol · g^-1 DW · h^-1), followed by urea (7.74 μmol · g^-1 DW · h^-1), nitrate (5.37 μmol · g^-1 DW · h^-1), and amino acids (3.71 μmol · g^-1 DW · h^-1). The species showed higher uptake affinity for urea (α = 1.8), which accounted for 70% of nitrogen uptake. Phosphate uptake was low, and total nitrogen uptake exceeded growth requirements. These findings suggest that R. okamurae relies on organic nitrogen (urea) and may guide toward effective management strategies to mitigate its spread in coastal ecosystems.