Global warming and plastic pollution are two human-induced environmental stressors of rising concern due to their potential impacts on ocean health. To tackle plastic pollution, sustainable alternatives to conventional fossil fuel-based plastics are being developed, such as polylactic acid (PLA), a bio-based polymer made from natural-sourced feedstocks. However, in the marine environment, plastic additives and low molecular weight oligomers can be released, i.e. leached from the plastic to the surrounding seawater, and little is known about the toxicity of leachates from bio-based polymers on marine organisms. Plastics degrade because of ultraviolet (UV) irradiation, which is one of the most important triggers of polymer degradation, enhancing the chemical leaching process (i.e. release mixture of leached compounds). Currently marine organisms are exposed to a variety of environmental stressors besides plastic pollution, related to climate change like increased seawater temperature. Warming of the ocean can affect the growth, pigment content, photosynthesis, respiration and antioxidant systems of microalgae. The potential effects of plastic pollution combined with global warming is even less well understood. In this master thesis, we have three research objectives. The first aim is to assess the effect of plastic leachates from a bio-based polymer (i.e. Polylactic acid), in comparison to a reference conventional fossil fuel-based polymer (i.e. Polypropylene), on the population growth of a marine diatom. The second aim is to investigate whether the possible effect of plastic leachates is enhanced or diminished by the pre-exposure of the plastics to UV irradiation. The third aim is to understand the combined effects of leached compounds and increased water temperature (i.e. + 5 ºC) based on the worst case scenario from IPCC, in the context of climate change, on the growth of a marine diatom. To do so, we exposed the marine diatom Phaeodactylum tricornutum to a dilution series of plastic leachates from pristine and weathered self-reinforced PLA (SR-PLA) and self-reinforced polypropylene (SR-PP) following the ISO 10253:2016 protocol. The UV-weathered plastic was obtained by exposing SR-PLA and SR-PP strips (0.13 x 2 x 7.5 cm) to artificial UV radiation for 57 days simulating 18 months of natural solar exposure. To obtain the leachates, we incubated the plastic strips in artificial seawater for 20 days in the dark. Following the leachate tests, we determined the EC50 (i.e. half-maximal effective concentration) to algal growth of four related plastic associated chemicals of PLA and PP, i.e., di-lactide, dodecan-1-ol, 2,4-di-tert-butylphenol, and tributyl O-acetylcritrate. Our results on SR-PLA leachates showed that neither pristine leachate nor UV-weathered leachate influenced the growth of P. tricornutum, and up to 57 days UV radiation had no effect on the toxicity of SR-PLA leachates. The results on SR-PP showed significant inhibition only at 100% volume of weathered PP leachates. The determined EC50 of di-lactide, dodecan-1-ol and 2,4-di-tert-butylphenol were 341 ± 30 mg / L, 1.86 ± 0.06 mg / L and 1.50 ± 0.01 mg / L, respectively. No growth inhibition was observed with tributyl O-acetylcritrate up to 98 mg / L. Experiments were performed with di-lactide at increased seawater temperature (i.e., +5 °C, worst case scenario according to IPCC) that showed effect of temperature on the growth of the diatom but no interaction when combined with substance exposure. We anticipate that our results will contribute to the risk assessment of plastic additives from bio-based and fossil-fuel based polymers and contribute to assess the link between multiple stressors under the context of global warming for phytoplankton growth and to assess the potential ecological impacts of new bio-based polymers. |
