An ecotoxicological approach towards the understanding of the impacts of micro- and nanoplastics in the marine environment

The omnipresence of micro- and nanoplastics (MNPs) in marine ecosystems requires an understanding of their ecotoxicological impacts. These particles can interact with microorganisms such as microalgae and bacteria, potentially disrupting food webs and biogeochemical cycles. Extracellular Polymeric Substances (EPS), secreted by microalgae in response to stress, play a pivotal role in these interactions by influencing aggregation dynamics. This PhD investigates the ecological impact of MNPs on microalgae and microbial consortia, and how these effects cascade to higher trophic levels and influence the biological pump. This PhD focused on the environmental relevance of the experimental designs, comparing virgin to weathered MNPs, and including kaolin as a natural particle control to disentangle plastic effects from particle effects. Chapter 1 examined the effects of virgin and weathered polyethylene terephthalate (PET) particles on the growth and EPS production of the microalgae Rhodomonas salina. Both particle types caused significant reductions in algal cell density. Weathered PET triggered a marked increase in EPS production, indicating a coordinated stress response. The effects of PET were more pronounced than those of kaolin. Chapter 2 extended the analysis to an algae–bacteria consortium, assessing growth dynamics, EPS composition, and aggregation behaviour. Weathered PET and kaolin induced stronger growth inhibition than virgin PET, alongside elevated EPS production. Particle exposure also enhanced aggregate formation. The findings suggest that particle properties, rather than material identity, primarily drive biological responses. Chapter 3 explored how algae EPS influences the stability of PET particles. Virgin PET exhibited greater EPS-induced stabilisation than weathered PET, highlighting the role of the particle surface characteristics. These results support a feedback mechanism in which MNP exposure triggers algae EPS production, which in turn enhances MNP stability and bioavailability. Chapter 4 investigated the indirect effects of MNPs on higher trophic levels by feeding copepods (Acartia tonsa) with MNP-exposed microalgae. Copepod physiology and behaviour, including grazing rates, fecal pellet production, and body size were significantly affected. These experimental results were integrated into an expanded NPZD model, revealing that MNP exposure alters detritus production and biomass export. Weathered PET had stronger effects than virgin PET or kaolin, underscoring the importance of accounting for weathering and natural particles in experimental designs. ix This PhD demonstrates that micro- and nanoplastics, particularly weathered particles, exert substantial direct and indirect effects on marine microbial communities and ecosystem processes. EPS was studied as a central mediator of stress responses and aggregation, with consequences extending to the biological pump. By combining experimental and modeling approaches, this work offers an integrated understanding of the ecological impacts of MNP pollution. This research contributes to a deeper understanding of the subtle yet far-reaching impacts of MNPs in marine systems.