Biological mediation of microplastic aggregation and sedimentation by a harmful dinoflagellate Prorocentrum minimum...

Coastal estuaries are hotspots of microplastics (MPs) and harmful algal blooms (HABs), yet the role of dinoflagellates in MP sinking remains unclear. We examined aggregate formation between Prorocentrum minimum and polyethylene (PE; 1.0 and 1.4 g cm^-3, 10-20 μm) and polypropylene (PP; 0.91 g cm^-3, 45-75 μm) using roller-shaker incubations. Growth, aggregate morphology, sinking velocity, and stability were evaluated microscopically and statistically. P. minimum growth was not inhibited by MPs; PE treatments showed significantly higher biomass than controls during exponential and stationary phases (p < 0.05). Aggregates appeared by Day 10 and progressively incorporated MPs and thecal fragments. The sinking ratio of PE1.0 particles increased to ∼22 % (R² = 0.96, p < 0.05), whereas PP showed negligible sedimentation (<1 %). Sinking velocities rose from 0.38 mm s^-1 (Day 10) to 0.76 mm s^-1 (Day 16; p < 0.05), then declined to 0.66 mm s^-1 by Day 31 despite larger sizes. This deviation from Stokes' law was linked to cellulose thecal plates reducing density and cohesion. Principal component analysis (PCA; PC1 = 53.9 % variance) associated sinking velocity with aggregate area, while PC2 (22.9 %) showed theca abundance negatively influenced velocity. Long-term incubations under cold, dark conditions (>70 days) showed no resuspension. These findings indicate thecate morphology limits MP export efficiency compared to extracellular polymeric substances (EPS)-rich raphidophytes. Nonetheless, scaling suggests Prorocentrum blooms may export on the order of 10¹⁰ MPs annually, highlighting species-specific traits as critical regulators of MP vertical flux and fate in coastal ecosystems.