Effects of PAHs on nitrogen uptake by phytoplankton species: Implications for environmental risk assessment of micropollutants and algal bloom formation.

Polycyclic aromatic hydrocarbons (PAHs) are of great concern because they threaten primary productivity, but their specific effects on ecosystem functioning are scarce, hindering a comprehensive understanding of their ecological risks, especially in eutrophic waters. The present study was conducted by adding PAHs to four marine phytoplankton species and showed that naphthalene (Nap) and phenanthrene (Phe) induced both stimulatory and inhibitory effects (>50 %) on urea and NO uptake by phytoplankton species. In addition, the apparent stimulative effects (>50 %) for NH were also observed. Overall, 38.9 % of the samples exhibited stimulation effects after 24 h exposure, which increased to 61.1 % after 96 h exposure. This suggested the existence of a lag period, during which a tolerant cell population could adapt to PAHs. Significant positive correlations (P < 0.01) between low and high concentrations of PAH individuals demonstrated that the mode of action for both pollutants on nitrogen uptake by phytoplankton was the same. Species-specific responses were also observed, with 19.0 % of Thalassiosira sp. and 24.0 % of Tetraselmis sp. exhibited inhibition effects greater than 50 %, while 40.9 % of Karlodinium veneficum and 27.3 % of Rhodomonas salina demonstrated stimulation effects exceeding 50 %, providing a unique perspective for exploring the harmful algal bloom of the mixotrophic K. veneficum, in addition to the original consideration of nutrients. The internal mechanisms may lie in differences in energy consumption between N-forms, exposure time and chemical concentrations, as well as morphological characteristics and biochemical structures of the species, which require further investigation.