Microplastics alter the toxicity of benzo[a]pyrene in a mangrove oyster: An integrated biomarker approach

Microplastics (MPs) and Benzo[a]pyrene (BaP) are ubiquitous co-contaminants in marine environments, yet their combined ecotoxicological effects remain poorly understood. This study evaluated the isolated and interactive toxicity of alone linear low-density polyethylene (LLDPE) (0, 5, 50, 500 mg L^-1) and BaP (0, 3, 12, 21, 30 μg L^-1) in the mangrove oyster Crassostrea gasar, a key filter-feeding species highly vulnerable to particulate and hydrophobic contaminants. The concentrations of 5 mg/L MPs and all BaP concentrations tested are environmentally relevant. Adult oysters were exposed for 7 days in a full factorial design, and biomarkers (Glutathione S-transferase, Glutathione Peroxidase, Reduced Glutathione, Lipid Peroxidation, DNA damage, Neutral Red Retention Time) were assessed in gills and hemolymph. Alone MPs alone induced oxidative and cytogenotoxic effects, confirming that even uncontaminated plastic particles can disrupt cellular homeostasis. Significant interactive effects between MPs and BaP were observed, particularly influencing oxidative stress and DNA integrity. GPx, GST, and GSH responses were associated with DNA damage at higher exposure levels. BaP increased lipid peroxidation, reducing lysosomal membrane stability, and this impairment was exacerbated under combined exposure. The integrated biomarker response index identified the combination of 30 μg L^-1 BaP and 500 mg L^-1 MP as the most hazardous scenario. The environmentally relevant MP concentration (5 mg L^-1) also produced significant effects when combined with BaP. These findings demonstrate that MPs modulate BaP toxicity and highlight the importance of assessing co-contaminant interactions in filter-feeding organisms. Although the highest concentration tested (500 mg L^-1) exceeds environmental levels, effects were also observed at environmentally relevant concentrations. The inclusion of elevated concentrations was intended to identify effect thresholds and underlying mechanisms, providing robust data for environmental risk assessment.