Toxic effects of microplastics on extracellular polymeric substances (EPS) in estuarine microalgae under stress...

The accumulation of microplastics (MPs) in estuaries poses risks to aquatic organisms, but the role of extracellular polymeric substances (EPS) in microalgae's response remains unclear. This study examined the role of EPS in Scenedesmus obliquus (green alga) and Skeletonema costatum (diatom) exposed to polypropylene (PP) and polyethylene (PE) microplastics. S. obliquus was inhibited at all MP concentrations, with biomass reduced by 3.2 %-12.5 %, whereas S. costatum was inhibited at low MP concentrations but showed 4.4 %-9.9 % higher biomass than the control at high concentrations. Removal of EPS reduced the stress tolerance of microalgae, with notable differences in antioxidant responses between the two species, while S. costatum maintained stronger photosynthetic regulation. High PP concentraction increased G₀/G₁-phase cells (consistent with impaired DNA synthesis). EPS removal further disrupted the cell cycle: in S. obliquus, the proportions of G₀/G₁-phase and G₂/M-phase cells increased (the latter from 12.4 % to 18.8 %), whereas in S. costatum, apoptosis rate increased from 20.7 % to 35.9 %. EPS composition shifted under MP stress: S. obliquus reduced loosely bound EPS (LB-EPS) proteins, whereas S. costatum increased them. Both species elevated tightly bound EPS (TB-EPS) proteins and polysaccharides, with 3D-EEM fluorescence confirming structural changes. EPS aids microalgae in mitigating MP stress by stabilizing cell cycles, regulating antioxidants, and sustaining photosynthesis. These findings enhance understanding of MP to microalgae risks and interaction in estuarine ecosystems.