Uptake and Biological Effects of Polystyrene Microplastics and Adsorbed Cadmium and Benzo(a)Pyrene on the Mussel Mytilus galloprovincialis

Supervisors: Amaia Orbea, Beñat Zaldibar (UPV/EHU)
The following thesis project is aiming to understand the combined impacts of microplastics (particles between a few μm and 50 μm size) and model contaminants on the health of the marine mussel Mytilus galloprovincialis. The first objective was to assess whether mussels accumulate particles of different sizes in different tissues. This was done by exposing mussels to two sizes of particle sizes (4.5 and 45 μm) in three different concentrations (10, 100 and 1000 particles/mL) for one day. The findings were conclusive, showing that smaller particles were more easily internalised by the mussels, as well as taking longer to depurate. Larger particles, on the other hand, were almost exclusively found in the stomach lumen and were released soon after uptake. These results lead to using 4.5 μm particles to research the second objective of this project, aiming to assess the health impacts of plastic on mussels, alone and in combination with a model metal and polycyclic aromatic hydrocarbon (PAH). In order to do so, mussels were exposed to pure, as well as to contaminant pre-exposed plastic particles, for three days, and then examined for different chemicals concentrations, as well as biochemical, cellular and tissue- levels. The results of this research showed that the gills of the mussels were more sensitive to exposure than the digestive gland, with increased acyl-CoA oxidase (AOx) and superdioxide dismustase (SOD) activities after exposure to PAH-pre-exposed plastic, and lowered SOD activity after exposure to cadmium pre-exposed plastic exposure. The results of the lysosomal membrane stability test were found to not be following any expected trends, with increased times of labilisation in all treatment groups. The atrophy of the digestive gland was strongly influenced by both contaminants and the pure plastics. The results of the digestive gland autometallography showed that the exposure to pure metal as well as metal pre-exposed particles led to a significant metal accumulation. Again, plastic particles were found present in the majority of samples when assessed under polarised light. The results of this research project showed that there are a multitude of responses to the plastic exposure. Although the exact absorption rates of the contaminants to the plastic particles were not measured it was found that the organisms reacted more strongly to the pure contaminant than the contaminant adsorbed to plastic.