Transcriptomic analysis of dinoflagellate Prorocentrum cordatum exposed to microplastics under different nitrogen sources

Anthropogenic activities have significantly altered marine nitrogen (N) regimes by releasing different forms of N (e.g. nitrate and urea, the latter from wastewater/agricultural runoff). Some species can grow equally well under different N sources, while others show maximum growth rates under one source (e.g. urea). Another pollutant, plastic microparticles (MP), has been found in every ecosystem/habitat on Earth. A crucial deleterious effect of MP is the disruption of the cellular membrane integrity-a key mechanism affecting the N uptake and assimilation in single-cell organisms such as dinoflagellates and microalgae. The combined effects of MP and different N sources remain an open question. This study exposed the dinoflagellate Prorocentrum cordatum to microplastics (MP, concentrations of 0 or 10 mg L) with urea (U) or nitrate as nitrogen sources, both at 440 µmol L during 16 d to investigate the response of P. cordatum in terms of physiology and nitrogen metabolism. P. cordatum showed a similar growth rate, pigment contents (chlorophyll a and carotenoids), lipid peroxidation (MDA), nitrate reductase, and nitrogen consumption rate (C) in both N sources. The addition of MP under both nitrogen sources decreased growth but increased pigments, SOD and Urease activity, and MDA level, significanlty. The C was reduced by ∼50 % in the Microplastics + nitrate treatment (MPN), whereas it remained unchanged in the Microplastics + Urea treatment (MPU). At the transcriptomic level, the MP addition resulted in an approximate 2.5-fold increase in the number of differentially expressed genes (1811) under MPU compared to MPN (731). The MPU significantly upregulated pathways involving nitrogen assimilation and metabolism, endocytosis and phagosome, and further electron transport in photosynthesis and energy metabolism, but MPN downregulated those pathways. This suggests higher cellular metabolism under MPU compared to MPN. Additionally, cells in MPU upregulated pathways of antioxidation, heat shock protein, ubiquitin-mediated proteolysis, and proteasome in response to MP-induced stress, whereas the variations of these pathways in MPN were limited. These results shed light on the combined effects of urea as a source of nitrogen and MP exposure, providing insight into the metabolic flexibility of P. cordatum under future environmental stress.