Investigating the Molecular Response of Skeletonema marinoi to Polyethylene Nano/Microplastics: Insights into Stress Genes, Inflammation, and Extracellular Polymeric Substance Production

Background: Nano- and microplastics (N/MPs) are widespread pollutants in aquatic ecosystems and may interfere with the physiology of diatoms. However, the underlying molecular mechanisms remain poorly understood. Methods: Cultures of the diatom species Skeletonema marinoi, a key contributor to marine primary production and biogeochemical cycles, were exposed for 10 days to polyethylene (PE) N/MPs (25 and 75 µg/mL). Growth, morphology, and cell viability were monitored, while gene expression of stress, inflammation, programmed cell death (PCD), and extracellular polymeric substance (EPS) biosynthesis markers was assessed by RT-qPCR. Results: Growth was not significantly affected. Gene expression showed biphasic regulation of antioxidant enzymes, induction of HSP70/90, transient modulation of prostaglandin-related genes, and dose- and time-dependent activation of PCD markers. Selected CAZymes possibly involved in EPS synthesis were downregulated at early stages, suggesting impaired aggregation potential. Conclusions: Exposure to PE N/MPs elicited subtle yet multifaceted effects in S. marinoi, including oxidative stress, inflammatory-like responses, modulation of EPS pathways, and PCD activation. Despite apparent growth resilience, the molecular alterations observed may impact bloom dynamics and carbon export.