Microplastic (MP) pollution, particularly polystyrene (PS), poses a growing threat to aquatic ecosystems and primary producers such as microalgae. Three mangrove-derived diatoms, Nitzschia navis-varingica, Nitzschia palea, and Navicula bory, were used in this work to assess the effects of 2 μm PS MPs on lipid content and antioxidant activity. Clonal cultures were subjected to PS doses of 0, 5, 10, 50 mg/L. Nile Red fluorescence staining and Corrected Total Cell Fluorescence (CTCF) were used to measure lipid content, while IC50 values from DPPH experiment were used to measure antioxidant activity. In all species, exposure to PS initially decreased lipid fluorescence, especially at 50 mg/L, suggesting transient metabolic stress. Nevertheless, species-specific adaptation responses were noted, with lipid accumulation rising during cultivation and peaking on day 1 in N. bory and day 9 in both Nitzschia species. While N. navis-varingica did not exhibit any significant variations between treatments, N. palea and N. bory revealed significant effects of PS on lipid content (p<0.05). Significantly reduced IC50 values in all species (p<0.05) indicate that antioxidant activity increased with increasing PS concentration, with the highest activity seen at 50 mg/L. These findings demonstrate that PS promote oxidative stress and affect lipid metabolism in mangrove microalgae, while also eliciting adaptive antioxidant responses. The observed species-specific reactions point to possible ecological repercussions for trophic interactions and primary productivity in aquatic environments polluted by MPs.