## 3 Results and Discussion
### Lysosomal Parameters  
Lysosomal parameters measured in adult mussels after 7 days of in vivo exposure to selected treatments are shown in Figure 2. A significant LMS reduction with respect to control was observed in haemocytes of mussels exposed to MP, CHR-MP and 0.1 mg/L CHR (Figure 2A). The latter treatment also induced a significant LMS decrease compared to MP, CHR-MP and 0.73 ng/L CHR. Similarly, the NL content measured in mussel digestive gland showed a significant increase compared to control following mussel exposure to MP, CHR-MP and 0.1 mg/L CHR (Figure 2B). No significant change was identified between CHR-MP and MP, both showing levels significantly lower compared to 0.1 mg/L CHR. NL content measured at 0.73 ng/L CHR was significantly lower compared to those measured in organisms exposed to MP, CHR-MP and 0.1 mg/L CHR.  
The intra-lysosomal content of LF increased significantly compared to control in digestive cells of mussel exposed to CHR-MP and 0.1 mg/L CHR (Figure 2C). Mussels exposed to MP showed increased though not significantly different LF levels compared to control and CHR-MP-treated mussels. LF levels measured at 0.73 ng/L CHR were similar to control and significantly lower compared to CHR-MP and 0.1 mg/L CHR.  
In bivalves, lysosomes play an active role in food intracellular uptake and cell turnover, and modulate the overall organisms' response to external stressors by regulating toxican inactivation and cell homeostasis [67]. Toxicant-induced lysosomal dysfunctions are thus generally recognized as early-warning signals of possible alterations to higher functions, such as growth and/or reproduction [67; 68]. LMS and NL alterations have been widely documented in mussels exposed to both PAHs [69; 70] and MP [18; 19] and describe the onset of a stress syndrome associated to an increased susceptibility toward physicochemical/microbiological agents and fatty acid metabolism disorders, respectively. LF are final products of the peroxidation of biological membrane lipid layers, which may result in cell injuries and altered organ functions in the long-term. Overall, LMS, NL and LF data from this study indicate that the mussel exposure to 0.1 mg/L CHR have higher deleterious consequences on lysosomal integrity and functionality than micro-sized particle exposure, and that, at the tested experimental conditions, the sorption of CHR did not increase the toxicity of plain MP on most lysosomal endpoints. Similar evidence was obtained by [PERSON] al. [31], who did not observe changes in LMS variations between mussels exposed for 26 days compared to plain and benzo[a]pyrene-loaded (0.5 to 4.5 um) polystyrene MP. The lack of effects observed at 0.73 ng/L CHR allows us to hypothesize that the amount of CHR sorbed on MP was not sufficient to trigger additional effects to those elicited by plain MP on lysosomal endpoints. However, though levels of all lysosomal parameters did not significantly differ between MP and CHR-MP, an increased LF content compared to control values was observed in CHR-MP-exposed mussel, possibly reflecting a cumulative action of MP and sorbed PAH on lipid peroxidation pathways.  
Figure 2: Lysosomal parameters (mean \(\pm\) SEM, \(n\) = 3) analysed in mussels after 7 days of exposure to 5 \(\times\) 10\({}^{3}\) units/L of polystyrene microplastics (MP) and chrysene-loaded microplastics (CHR-MP), 0.1 mg/L and 0.73 ng/L of freely dissolved CHR (without MP); (**A**), LMS assessed in mussel haemocytes; (**B**,**C**), lysosomal content of NL (**B**) and LF (**C**) in mussel digestive gland. Different letters denote statistically significant differences (\(p\) < 0.05, ANOVA—Bonferroni _post-hoc_ comparison) among treatments.