Physiological responses of the alien macroalga Rugulopteryx okamurae (Phaeophyceae, Heterokontophyta) to changes in nutrients and temperature.

The Asian brown macroalga Rugulopteryx okamurae has invaded the oligotrophic areas of Mediterranean coasts since 2015, with drastic impacts on environmental conditions and socioeconomic activities in coastal areas in Europe. Therefore, it is intriguing how this species is able to grow and expand at the observed rates. In this context, the physiological responses of R. okamurae to changing nutrient concentrations and temperature were analyzed. Two experiments were conducted, evaluating six combinations of nitrate and phosphate concentrations and their potential interaction with temperature. Nutrient uptake efficiency (NUE) and rates (NUR), photosynthetic responses, growth rates, and biomass composition were evaluated. Photosynthesis parameters, soluble proteins, and $N{O}_3^{-}$ -NUR increased with increasing N:P ratio; however, $P{O}_4^{3-}$ -NUR was very similar in all treatments. The species showed high capacity for nitrate assimilation, which was rapidly modulated by its external concentration and temperature (more than 90% of NO₃-NUE after 5 days in treatments with N:P rations of 5, 10, 16, 25, and 40 N to 1 P). Consequently, N-nutrients were removed from the water by R. okamurae and likely stored inside the cells. This process will allow the alga to maintain high growth rates if thalli are moved to oligotrophic areas, favoring its spreading to many marine environments. Additionally, fucoxanthin was the predominant carotenoid in this species, although its content was lower than in other brown macroalgae species (mean value of 0.51 ± 0.05 mg · g^-1 DW). However, since a huge amount of R. okamurae is observed recurrently on beaches, the use of this biomass might be proposed to compensate partially for its impacts.