Exposure to Structurally Related Organophosphate Esters Elicits Distinct Metabolomic Perturbations in Daphnia magna

Organophosphate esters (OPEs) are found ubiquitously in the environment and exposure to these chemicals has induced behavioral, developmental, and morphological impairments in several model organisms. Yet there is still a lack of understanding of how sub-lethal exposure disrupts organism function at the molecular-level, especially for Daphnia magna. To address this knowledge gap, D. magna responses after exposure to three sub-lethal concentrations consisting of 23.5 mg/L, 59 mg/L, and 118 mg/L for tris(2-chloroethyl) phosphate (TCEP); 2.5 mg/L, 6.25 mg/L, and 12.5 mg/L for tris(2-chloro-1-methylethyl) phosphate (TCPP); 0.125 mg/L, 0.312 mg/L, and 0.625 mg/L for triphenyl phosphate (TPhP) were examined. These OPEs were selected as they are frequently detected in aquatic ecosystems. Targeted mass spectrometry metabolomics was used to measure the molecular-level regulation of key metabolites after 48 h of exposure. Metabolomics technologies can capture changes of metabolite levels in model organisms exposed to sub-lethal concentrations where key metabolic pathways responsible for vital biological functions can be elucidated. Metabolic perturbations in amino acid levels were observed after exposure to OPEs with higher octanol-water partition coefficients (TCPP and TPhP). In addition to the perturbed metabolites shared across all tested OPEs, each OPE resulted in unique perturbations to select metabolites. Most metabolite concentrations exhibited non-monotonic responses following exposure to TCEP and TPhP. In contrast, after TCPP exposure, both non-monotonic and monotonic responses were observed across the measured metabolites. This study distinguishes specific metabolic disturbances that are unique to the side chains attached to the phosphate center when exposed to D. magna. Hence, these metabolites are most sensitive to OPE sub-lethal exposure. Providing novel insights into the diverse modes of action from sub-lethal OPE exposure to a sentinel species allows for more effective risk assessment and water monitoring strategies to be implemented for the protection of freshwater ecosystems.