Are Alternatives to Conventional Plastics really Safer? Exploring Toxicity of Biobased, Biodegradable and Recycled Microplastics in Fish: The Role of Polymer Type, Associated Chemicals, and Exposure Conditions Across Multiple Biological Endpoints

Abstract

Microplastics are a pervasive and complex class of contaminants in aquatic environments, raising concerns about both their physical and chemical impacts on organisms. The plastic pollution crisis has been identified as a global threat, and several possible solutions have been proposed, though these must be evaluated to ensure that they are safer and more sustainable than conventional, fossil fuel-based plastics. This thesis investigates the biological effects of microplastics in fish, with a focus on polymer type, life stage, and associated chemicals, including those leaching from recycled plastics. Using a combination of chronic or acute dietary exposure experiments and early-life stage embryo assays, I explored how conventional, biobased/biodegradable, and recycled polymers affect behavior, physiology, and gene expression in freshwater fish. In juvenile European perch, chronic ingestion of poly(L-lactide) (PLA), a biobased and biodegradable polymer, significantly altered social behavior, indicating the risk of toxicity from a polymer often marketed as eco-friendly. The following connecting experiment, also in juvenile perch, investigated a comparison of biobased/ biodegradable and conventional polymer mixtures. It revealed that only the biobased plastics induced significant changes in fish growth and hepatic gene expression related to oxidative stress and inflammation, while conventional polymers had no detectable effects on any measured endpoints. These findings challenge assumptions about the relative safety of bioplastics. To address chemical exposure risks, leachates from recycled polyethylene pellets collected from recycling facilities in different countries were analyzed and found to contain a complex mixture of plastic additives and contaminants (NIAS), such as pesticides and pharmaceuticals. Zebrafish embryos exposed to these leachates exhibited upregulation of genes involved in adipogenesis and endocrine pathways, though no effects on behavior, morphology or survival occurred. Lastly, dietary exposure to a conventional plastic mixture in Nile tilapia demonstrated potential life stage- and exposure duration-dependent effects: inflammatory responses were observed in juveniles over long-term exposure but not in adults under acute exposure. Together, my studies highlight that polymer type, chemical content, and organismal life stage all influence the effects of microplastic exposure and toxicity of plastics chemicals. Importantly, biobased/biodegradable and recycled plastics, though widely promoted as sustainable alternatives, can exert biological effects equal to or greater than those of conventional plastics, underscoring the need for critical evaluation of emerging new plastic materials.