Understanding plastic degradation in the marine environment: a multi-tiered assessment
- Autores: Sara López Ibáñez
- Directores de la Tesis: Ricardo Beiras García-Sabell (dir. tes.)
- Lectura: En la Universidade de Vigo ( España ) en 2024
- Idioma: inglés
- Tribunal Calificador de la Tesis: Julián Blasco Moreno (presid.), Víctor Hernando Morales (secret.), Sofie Thijs (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencias Marinas, Tecnología y Gestión por la Universidad de A Coruña; la Universidad de Santiago de Compostela; la Universidad de Vigo; Universidade de Aveiro(Portugal); Universidade de Porto(Portugal); Universidade de Trás-os-Montes e Alto Douro(Portugal) y Universidade do Minho (Portugal)
- Menciones:
- Internacional
- Calificación: Sobresaliente Cum Laude
- Materias:
- Enlaces
- Tesis en acceso abierto en: Investigo
- Resumen
Plastics are present in all aspects of our lives. They are all-purpose materials that, given their excessive production and neglected management, are found in all types of ecosystems. Their ubiquity and persistence bring about ecological consequences that range from aesthetical nuisance to disruption of biological communities. Once in the environment, the behavior of plastics is difficult to predict, particularly in marine ecosystems where too variable parameters are present in the different compartments, in terms of degradation and associated toxicity. This is especially scarcely known in the case of bioplastics, the new materials either biodegradable and/or made from renewable resources intended to substitute conventional fossil-based plastics like polyethylene, the most currently used polymer.
In this thesis, several studies have been conducted assessing both biodegradation and mechanical degradation, as well as the derived toxicity along weathering of commercial plastic products of different nature in marine conditions. Therefore, a multi-tiered method has been developed, including controlled laboratory exposures, described in Chapter 2 (mineralization) and Chapter 3 Part 1 (aquarium exposures), along with mesocosm exposures outdoors, described in Chapter 3 Part 2 and Chapter 4. Furthermore, several samples have been collected during the exposures to be analyzed by metabarcoding with a view to identify the microorganisms with the highest ecological interest and the most abundant ones found on each plastic surfaces. Regarding mineralization, materials were tested in biphasic BOD bottles for oxygen consumption measurement, improving the existing standard protocols by using an adequate nutrient formulation following the Redfield ratio (106C:16N:1P), a natural inoculum representative of marine habitats, grinding particles below 250 m and using a truly biodegradable biopolymer (PHB) as positive control, which avoids calculation of ThOD. Preconditioning of the microbial inoculum can increase the sensitivity of the test, in contrast to pre-exposure. Ultimate biodegradability has been calculated fitting the data to a non-linear logistic model. In this case, only one material showed signs of significant biodegradability comparing to the control. When scaling up, plastic bags were selected as subject materials, and they were exposed to different habitat simulations, including benthic, pelagic and littoral. Polyethylene bags did not degrade in any exposure, while compostable ones lost weight and mechanical properties, being strength at break the most suitable endpoint to assess mechanical degradation. Littoral and benthic exposures showed the highest degradation rate. Ecotoxicity, analyzed by the sea urchin embryo test, was higher for compostable materials and lost after a few days. In some cases, it was regained, and the influence of UV radiation on it has been investigated, showing a UV-dose dependent toxicity, influenced at the same time by bag type. Microbial communities identified showed divergent colonization depending on sample type and exposure and interesting species with potential polymerdegrading ability were selected, such as Marinicella sp. and Mycobacterium sp. bacteria, as well as already known polymer degraders, present on the plastic surface, such as Penicillium sp., Debaryomyces sp. or Cladosporium sp. A higher number of fungal species were observed in benthic exposures, where the highest underwater degradation rate was found outdoors.
The whole methodology contributes to the elaboration of standard protocols by official bodies that need to be based on scientific knowledge. These standards help the industry when analyzing the potential impact of their products, thus facilitating the incorporation of more environmentally-friendly materials to the market, since companies are increasingly more interested in the improvement of the sustainability of their products, taking on environmental policies.
