Heat stress modulates the size-dependent effects of aminated polystyrene particles on a marine diatom

Microalgae play a crucial role in maintaining marine ecosystem balance, yet they are generally more sensitive to global changes and pollutants. In this study, we assessed the effects of heat stress (elevated 4°C) and aminated polystyrene particles (PS-NH₂) of different sizes (50 nm, 1 μm, and 10 μm) on the growth of microalgae Skeletonema costatum (S. costatum). Results showed that elevated temperature mitigated the growth inhibition of S. costatum induced by PS-NH₂ particles across all sizes, but could not fully counteract their adverse effects. Transcriptome analysis revealed that S. costatum enhanced energy metabolism under both ambient and elevated temperatures to cope with microplastics (1 μm and 10 μm). However, the concurrent suppression of chlorophyll a synthesis and fatty acid degradation created an energy‑limiting condition that constrained growth. Notably, ribosome biogenesis was up-regulated only under exposed to larger microplastics (10 μm) at both temperature conditions, suggesting a size‑dependent capacity to sustain protein synthesis and proliferation. Under heat stress, both 1 μm and 10 μm microplastics up‑regulated the TCA cycle, thereby alleviating growth inhibition. This study provides new insights into how the biological effects of microplastics on microalgae are modulated by particle sizes and temperatures. It highlights the need for further research on the long-term effects of microplastics particles using environmentally relevant concentrations and heat stress, in order to better predict their enduring impacts on microalgae under ongoing climate change.