Adverse outcome pathways as a tool for optimization of the biomarker-based assessment of pollutant toxicity: A case study of cadmium in the blue mussels Mytilus edulis

Technogenic metals like cadmium (Cd) are discharged into the environment posing significant ecological risks to wildlife and contributing to a decline in biodiversity. Recent developments in toxicity testing and environmental risk assessment have shifted the focus towards prioritizing biological effects. To ensure effective environmental risk assessment, it is essential to develop sensitive sub-organismal biomarkers correlated with ecologically relevant organismal-level responses. Our study proposes a novel approach to enhance the evaluation of biomarker-based toxicity based on adverse outcome pathways (AOP) concept. We exposed the blue mussels M. edulis to Cd concentrations ranging from 200 to 600 mu g l(-1) (10-30 % of LC50) for 14 days, and assessed a comprehensive panel of 26 biomarkers related to molecular initiation events (Cd uptake and binding), key cellular events (oxidative stress, metabolic disturbances, and cytotoxicity), and adverse organismal outcomes (survival, energy metabolism, and reef-building capacity) in the putative AOP of Cd. Tissue Cd burdens increased linearly with exposure concentrations in the mussels. Although the condition index of the mussels remained unaffected, their biofiltration activity was suppressed at the highest Cd concentration. The negative effects of Cd exposure on the mussels' feeding were linked to reduced energetic scope for growth, impaired byssus production, increased hemocyte mortality, and altered functional traits of hemocytes. Additionally, Cd exposure induced oxidative stress, as evidenced by lower total antioxidant capacity and thiol pool, along with increased lipid peroxidation, leading to enhanced autophagy. Through Random Forest Analysis, we identified a minimal set of sensitive Cd effect biomarkers, including hemocyte mortality (contributing similar to 0.32 to classification accuracy), hemocyte acid phosphatase activity, nitric oxide and metallothionein levels in the digestive gland, and wholeorganism clearance rate and scope for growth (each contributing -0.02-0.08 to classification accuracy). These biomarkers collectively achieved a high accuracy of 90-100 % in distinguishing between control and Cdexposed groups. Increasing the number of biomarkers did not improve classification accuracy. These findings suggest that using a smaller number of optimized biomarkers based on Adverse Outcome Pathways (AOP) can enhance the assessment of contaminant-induced negative effects while reducing testing costs in environmental risk assessments.