Micro- and nanoplastic pollution is an increasingly pervasive problem in freshwater ecosystems, yet uncertainties remain regarding how particle transformation, biological sex, and cross-compartment interactions influence biological responses and ecosystem functional processes. This thesis investigates the biological effects of polystyrene (PS) micro- and nanoplastics in freshwater systems using a multi-scale laboratory framework. PS was selected for its widespread use in microplastic ecotoxicology, environmental occurrence, and suitability for investigating controlled particle aging and biological responses. A multi-scale approach was adopted to examine responses across interconnected biological and sediment-associated levels.
Zebrafish (Danio rerio) exposures examined responses to pristine and UV-aged PS particles, focusing on gender-specific responses, oxidative stress, metabolism, gut microbiota, and reproduction. A freshwater-sediment microcosm incorporating submerged plants (Vallisneria spiralis), freshwater snails (Lymnaea stagnalis), and artificial sediment assessed organismal stress and ecosystem functional responses.
The results demonstrate that: (1) 1 µm pristine PS microplastics induce oxidative stress, gut microbiota disturbance and liver metabolic alteration in zebrafish, with females showing broader physiological and metabolic responses than males. Altered sex hormone levels and reduced egg production in females indicated potential reproduction-related adverse outcomes; (2) UV aging modifies properties of 500 nm PS nanoplastics and was associated with stronger stress-related responses than pristine particles, including oxidative stress, gut microbiota dysbiosis, and metabolic disturbance related to gut-liver axis, particularly in females; (3) mechanically weathered PS microplastics induced stronger stress-related responses in V. spiralis and L. stagnalis than pristine particles, while both pristine and mechanically weathered PS microplastics increased the ecosystem multifunctionality. This increase was mainly driven by increased dissolved organic carbon, microbial respiration and enzymatic activity, indicating short-term sediment-associated functional stimulation rather than improved ecosystem health or long-term resilience. Overall, this thesis highlights the importance of integrating ultraviolet aging, mechanical weathering, biological sex and cross-scale processes to improve freshwater PS micro- and nanoplastic risk assessment.
