Molecular Toxicity Pathways and Transcriptomic Points of Departure (tPODs) of 6PPD-Quinone in Early-Life Stage Lake Trout ( Salvelinus namaycush )

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-quinone) has been linked to acute mortality in select salmonid species at concentrations found in surface waters after stormwater runoff events. However, little is known about the specific mechanism underlying the highly species-specific sensitivity to 6PPD-quinone, limiting our ability to predict effects across fishes and develop protective guidelines. This study aimed to characterize the molecular toxicity pathways of 6PPD-quinone in lake trout (Salvelinus namaycush), a species of significant ecological, cultural, and economic importance in North America that is among the most sensitive fishes to 6PPD-quinone exposure. Whole-transcriptome analysis of alevins exposed to graded 6PPD-quinone concentrations (0.22-7.6 μg/L) for 96 h revealed concentration-dependent dysregulation of pathways related to inflammation, vascular integrity, oxidative stress, protein homeostasis, and skeletal and craniofacial development. Notably, exposed alevins showed early activation of proinflammatory cytokine signaling and programmed cell death processes, including efferocytosis, necroptosis, and ferroptosis. Benchmark concentration (BMC) modeling of the transcriptomic data identified a transcriptomic BMC_10th (the concentration affecting the most sensitive 10th percentile of genes) of 0.211 μg/L, closely aligning with the 45-day apical BMC_10th and BMC_20th values (0.161 and 0.252 μg/L, respectively). These findings highlight that short-term transcriptomic responses in nonregulated embryonic life stages offer mechanistic insights and provide predictive, quantitative benchmarks that closely correspond with traditional apical toxicity thresholds.