Abstract <p>Microplastic pollution in the marine environment poses a significant threat to biotic compartments. In this study, we evaluated the impact of polystyrene (PS) microbeads on the biomass, pigment levels, and biochemical constituents of two marine diatoms, <i>Amphora</i> sp. and <i>Navicula</i> sp. Moreover, the hydrophilicity of PS microbeads was assessed using contact angle measurements to determine the biodegradation potential of these diatoms. Marine diatoms were exposed to PS microbeads at a concentration of 200 mg/150 mL of culture medium separately for 30 days, while untreated diatoms served as respective controls. The results showed a significant decrease (<i>p</i> &lt; 0.05) in biomass, pigments, protein, and carbohydrate levels in both <i>Amphora</i> sp. and <i>Navicula</i> sp., indicating their vulnerability to PS microbeads. However, an increase in lipid content in both species suggests a defence mechanism in response to PS exposure. Furthermore, contact angle measurements revealed surface changes in PS microbeads from hydrophobic to hydrophilic, indicating the initiation of the biodegradation process by the marine diatoms.</p> Research highlights <p><UnorderedList Mark="Bullet"> <ItemContent> <p>Polystyrene (PS) microbeads significantly reduced biomass, pigments, proteins, and carbohydrates in <i>Amphora</i> sp. and <i>Navicula</i> sp.</p> </ItemContent> <ItemContent> <p>Increased lipid production in both diatoms suggests a protective response to MP exposure.</p> </ItemContent> <ItemContent> <p>Contact angle measurements revealed a shift in PS microbeads from hydrophobic to hydrophilic, indicating potential microalgal biodegradation.</p> </ItemContent> </UnorderedList></p>