Marine phytoplankton, vital to ocean food webs and global nutrient cycles, are increasingly threatened by microplastic (MP) pollution. However, the precise ways these tiny plastic particles harm phytoplankton at the cellular level remain unclear. This thesis thus aims to reveal the nanoscale interactions between MPs and phytoplankton cells using cutting-edge microscopy, physiological measurement, and chemical characterization. Primarily, we uncovered how different phytoplankton species respond uniquely to MP exposure. Algae with smooth cell walls, like Phaeodactylum tricornutum and Chlorella vulgaris, developed deep indentations, wrinkles, and even internalized MPs, which led to diverse surface deformities. In contrast, diatoms with porous silica shells such as Amphora coffeaeformis and Thalassiosira weissflogii trapped MPs within their intricate frustule structures, thereby minimizing morphological damage. Atomic force microscopy (AFM) further confirmed that these MP-exposed algae lost nanostructural regularity and mechanical strength with species-specific disparities. We then explored how environmental stressors influence MP tolerance in phytoplankton by inducing nanoscale modifications to their cell surfaces. Under silicon (Si)-limited conditions, diatoms were found to produce mechanically weaker, more adhesive, and more porous frustules. These nutrient-stressed cells showed dramatically inhibited growth, suffered greater membrane damage, and were attached with more MPs in comparison to their Si-replete counterparts. Finally, we also investigated how emerging plastic pollutants like mask-released debris (MD) affect marine diatoms. MD exposure not only caused physicochemical shifts in diatom cell walls but also influenced their sinking behaviors. By connecting nanoscale damage to single-cell and population-level consequences, this thesis has demonstrated the multifaceted threats MPs pose to marine phytoplankton. 

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