Anthropogenic activities have introduced pharmaceutical pollutants into marine ecosystems, threatening marine organisms in conjunction with climate change. While the ecotoxicity of human drugs like selective serotonin reuptake inhibitors (SSRIs) is gaining recognition, their interactions with environmental factors remain underexplored. Our study investigates the physiological effects of fluoxetine, a common SSRI, on thermal plasticity in two diatom species, Phaeodactylum tricornutum and Thalassiosira weissflogii, while focusing on the transcriptomic responses in P. tricornutum to the interplay between fluoxetine and temperature. Fluoxetine significantly reduces growth rates, biomass production, and pigment contents. It also alters the thermal performance curve (TPC), reducing maximum growth rate (μ_max) and thermal safety margin (TSM), while increasing optimum temperature (T_opt), highlighting fluoxetine’s impact on thermal plasticity. Transcriptomic analysis of P. tricornutum reveals an antagonistic interaction between fluoxetine and temperature, with temperature driving substantial differentially expressed genes (DEGs) across fluoxetine concentrations. Functional enrichment analysis shows that fluoxetine affects genes involved in transport, metabolism, pigment synthesis, and redox balance, though many of these effects are diminished under combined conditions. Despite the minimal impact of low fluoxetine concentrations (1 μg L^-1) on growth, the molecular response indicates higher sensitivity at the genetic level. These findings emphasize the need to consider multiple environmental drivers when assessing the ecological risks of pollutants. By uncovering the physiological and molecular pathways affected by fluoxetine and its interaction with temperature, this research enhances our understanding of the compounded threats posed by pharmaceutical pollutants and climate change on marine primary producers, offering valuable insights for marine ecosystem conservation strategies.

Pharmarceutical pollution,Marine diatom,Thermal plasticity,Fluoxetine-temperature interaction,Transcriptomic response,Climate change