## 1 Introduction  
The demand for plastic products has globally risen over the past decades, resulting in the record of 368 million tonnes of plastics only produced in 2019 [1]. In addition to a prevalent use in modern society, the improper disposal of single-use items has progressively determined the widespread contamination of aquatic systems by plastic waste, which currently accounts for nearly the 80% of the oceanic litter [2, 3]. According to recent appraisals, the rate of plastic input in the marine environment is around 9.5 metric tonnes per year, with a two- to ten-fold increase projected by the next ten years [4, 5]. In this scenario, the potential environmental risks posed by microscopic plastic debris are capturing ever-growing attention [6, 7, 8]. Around five trillion < 5-mm plastic fragments, commonly referred to as microplastics (MP), are estimated to float into world's ocean [9]. MP mainly originates from the weathering-driven degradation of larger plastics, although a lower fraction is represented by micro-sized items designed for industrial/engineering processes (also known as primary microplastics) [10, 11]. Several reports have documented the ingestion and tissue translocation of MP in marine taxa spanning from planktonic bacteria and algae to cetaceans and vertebrates at the apex level of the trophic chain [10; 12; 13; 14; 15]. One of the effects most frequently associated to the MP ingestion is the physical blockage of the gastrointestinal tract, which produces outcomes of variable intensity: from impaired nutritional status to increased mortality rates [12; 16; 17]. Evidence of unspecific effects were also recorded in marine fish and invertebrates, including embryotoxicity, tissue inflammation, oxidative stress, transcriptional alterations, immunological and behavioural alterations [18; 19; 20; 21]. A recurring paradigm in MP pollution is represented by their potential to act as vehicle of chemical pollutants, including either additives, degradation products or waterborne POPs [22; 23; 24]. Hydrophobic compounds, such as polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) have been found to display an affinity for MP sorption up to two orders of magnitude higher than natural sediments [25; 26]. According to [PERSON] et al. [27], Log partition coefficients for PAHs on MP are comprised between 3 and 6 depending on chemical (molecular weight/Log K\({}_{\text{ow}}\)) and physical (plastic polymer type/structure/charge) factors. Consistency has been observed between the concentrations of sorbed (legacy and emerging) pollutants in MP collected in natural systems and those experimentally loaded in batch experiments, with levels spanning from a few ng/g to tens of mg/g of particles [23; 24; 26]. A recent study by [PERSON] and [PERSON] [28] found MP to have a biphasic behaviour with respect to organic pollutants after ingestion by aquatic organisms: they tend to absorb chemicals in a relatively contaminated gut fluid and to release the chemicals under clean gut conditions. In this respect, experimental evidence suggests that the desorption of persistent organic pollutants (POPs) after ingestion may synergistically increase the toxicity of MP to aquatic species. For instance, hepatic stress associated to glycogen depletion and cell necrosis was reported in fish (_Oryzaitas latipes_) exposed to beach-collected MP contaminated by PAHs, PCBs and PBDEs [29]; reduced phagocytic activity was recorded in the lugworm _Arenicola marina_ exposed to nonylphenol-loaded PVC MP [30], while mussels of the genus _Mytilus_ exposed to MP loaded with benzo[a]pyrene, fluoranthene and pyrene showed increased body burden of tested chemicals, histopathological alterations, immune and physiological disorders [18; 31; 32]. Based on the above considerations, this study aimed at assessing the adverse effects of polystyrene MP loaded with the PAH chrysene (CHR) on different life stages of the Mediterranean mussel _Mytilus galloprovincialis_.