Microplastic aggregation and sinking regulated by harmful alga Chattonella marina: Implications for vertical transport and resuspension

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Microplastics (MPs) are widespread pollutants in marine environments, with their vertical distribution strongly influenced by biological interactions such as aggregation with phytoplankton. This study experimentally examined the effects of the harmful algal bloom (HAB) species Chattonella marina (Raphidophyceae) on the aggregation, sinking, and resuspension of polyethylene (PE) and polypropylene (PP) MPs. Aggregate formation, MP sinking velocity, and sinking ratio were evaluated over a 92-day period, and the resuspension of settled aggregates was subsequently examined under low-temperature (12 °C) conditions. Aggregation was primarily driven by intracellular materials released during C. marina cell lysis, promoting MP binding and large aggregate formation. Low-density spherical PE particles (PE1.0) exhibited a logistic sinking pattern, reaching a maximum sinking ratio of 9 % with a half-saturation time of 13 days. In contrast, small PP fragments showed negligible sedimentation (<1 %). Aggregate size, sinking velocity, and MP count per aggregate did not differ significantly among spherical PE particles of varying densities (P > 0.05). During the decline phase, larger aggregates sank faster, reaching up to 76.9 m·day −1 . A strong positive correlation was found between C. marina chlorophyll a (Chl. a ) concentration and PE1.0 sinking ratio (R 2 = 0.92, P < 0.05), suggesting Chl. a as a proxy for aggregation-inducing substances during senescence. MP aggregates remained structurally stable for three months under dark, low-temperature conditions, with no degradation or resuspension. Aggregate number remained constant over the final 60 days (P > 0.05), indicating microbial resistance. These findings establish sedimentation thresholds and provide empirical data to improve models of MP vertical flux and fate. • MP sinking rate reached 9 % during the late growth phase of C. marina with MPs present. • Chl. a exhibited strong correlation with MP sinking, serving as an aggregation proxy. • Aggregates sank at 76.9 m day −1 and stayed stable for 3 months in cold, dark water.