Optimizing initial biomass to enhance microplastic sequestration via density-driven bio-flocculation in microalgal systems

Initial biomass is a key regulator of microalgal resilience to polystyrene microplastics (PS-MPs). This study investigated the responses of four microalgae–Chlorella sp., Scenedesmus sp., Cyclotella sp., and Spirulina sp.–exposed to 5 mg L−1 PS-MPs across biomass gradients of 500 – 1500 mg L−1. PS-MPs induced species- and density-dependent responses rather than uniform toxicity. Higher initial biomass increased final population yield and extracellular polymeric substance (EPS) production, but reduced specific growth rates due to intensified self-shading and resource competition. Chlorella sp. and Spirulina sp. exhibited stronger resilience, supported by pigment accumulation, coordinated antioxidant regulation, and EPS-mediated PS sequestration. Multivariate analysis further confirmed that biomass level, EPS secretion, photosynthetic performance, and oxidative status collectively structured microalgae–PS interactions. Notably, an initial biomass of 1000 mg L−1 provided the optimal balance between growth performance and remediation efficiency. These findings provide a quantitative framework for optimizing microalgae-based systems for microplastic contaminated wastewater treatment.