Role of microplastics in the bioaccumulation and biological effects of PCB-153 on Paracentrotus lividus

Microplastics (MPs, 1 μm - 5 mm in length) are ubiquitous in oceans and constitute a major global concern. Given their capacity to adsorb various environmental contaminants (co-contaminants), MPs can act as vectors for the bioaccumulation of co-contaminants in marine organisms. Nevertheless, the significance of MPs as vectors for co-contaminant bioaccumulation compared to other natural particles remains poorly investigated, especially in sea urchins. Moreover, MP translocation from the gut into organs is generally poorly understood and entirely unstudied in sea urchins. Inconsistent findings on combined MP and co-contaminant toxicity in marine organisms, including sea urchins, also highlight the ongoing debate about the role of MPs in delivering toxic chemicals. Therefore, this PhD dissertation aimed to investigate the role of MPs in the bioaccumulation and biological effects of PCB-153, as an example of co-contaminants, in the sea urchin Paracentrotus lividus. First, an efficient method was developed for sorbing ¹⁴C-PCB-153 onto MPs and sediment particles, allowing us to investigate the role of MPs as vectors of PCB-153. Results showed that MPs effectively acted as vectors for PCB-153 bioaccumulation under various conditions and that biofilm-covered MPs acted as vectors for PCB-153 bioaccumulation to a similar extent as biofilm-covered sand particles in adult P. lividus. Moreover, MP translocation across the adult P. lividus intestinal wall was demonstrated to be size-dependent, with only MPs below 5 μm able to cross this barrier. Lastly, MP concentrations (1-300 MP mL⁻¹) did not exhibit any significant adverse effects on P. lividus larval development, and observed adverse effects in co-exposure settings were attributed solely to PCB-153 at its highest achievable concentration (1.54 μg L⁻¹). In conclusion, while MPs can act as vectors for co-contaminant bioaccumulation, their overall contribution is likely negligible compared to natural pathways due to their significantly lower abundance. Regarding biological effects, our findings align with the growing consensus that co-contaminants, rather than the polymer itself, are the primary drivers of MP adverse effects. However, the most prevalent MPs in the environment, especially the small ones (< 5 µm for P. lividus) capable of crossing the intestinal barrier, could exert greater toxicity than those commonly tested in laboratories.