Microplastic aggregation and sinking mediated by the harmful dinoflagellate Prorocentrum minimum under simulated marine conditions

Coastal estuaries are recognized as hotspots for both microplastics (MPs) and harmful algal blooms (HABs); however, the role of dinoflagellates in facilitating MP sinking remains poorly understood. In this study, we investigated aggregate formation between dinoflagellate Prorocentrum minimum and polyethylene (PE; densities of 1.0 and 1.4 g cm⁻³, particle size 10–20 µm) and polypropylene (PP; density 0.91 g cm⁻³, particle size 45–75 µm) using roller–shaker incubations. Phytoplankton growth, aggregate morphology, sinking velocity, and aggregate stability were evaluated through microscopic observations and statistical analyses. Growth of P. minimum was not inhibited by MP exposure; notably, PE treatments exhibited significantly higher biomass than the control during both the exponential and stationary phases (p < 0.05). Aggregates first appeared on Day 10 and progressively incorporated MPs and fragmented thecal plates. The sinking ratio of PE1.0 particles increased steadily, reaching approximately 22% (R² = 0.96, p < 0.05), whereas PP particles exhibited negligible sedimentation (<1%). Sinking velocities increased from 0.38 mm s⁻¹ on Day 10 to 0.76 mm s⁻¹ on Day 16 (p < 0.05), but subsequently declined to 0.66 mm s⁻¹ by Day 31 despite continued increases in aggregate size. This deviation from Stokes’ law was attributed to the accumulation of low-density cellulose thecal plates, which reduced aggregate density and structural cohesion. Principal component analysis (PCA) showed that PC1 explained 53.9% of the variance and was positively associated with aggregate area and sinking velocity, whereas PC2 accounted for 22.9% of the variance and indicated a negative influence of thecal plate abundance on sinking velocity. Long-term incubations conducted under cold and dark conditions (>70 days) revealed no evidence of aggregate resuspension. Collectively, these results demonstrate that thecate morphology constrains MP export efficiency relative to extracellular polymeric substance (EPS)-rich raphidophytes. Nevertheless, scaling our experimental results suggests that Prorocentrum blooms may export on the order of 10¹⁰ MP particles annually, underscoring the importance of species-specific traits as key regulators of MP vertical transport and ultimate fate in coastal ecosystems.