Study Reveals Pesticide Metabolite's Mirror-Image Forms Pose Different Environmental Risks Across Generations

A new study published in Environmental Chemistry and Ecotoxicology reveals that mirror-image forms of a persistent pesticide metabolite behave differently when transferring from mother fish to their offspring, with significant implications for environmental risk assessment. The research demonstrates that the S-enantiomer of o,p'-DDD accumulates more efficiently in adult zebrafish and transfers more effectively to their offspring compared to its mirror-image R-form, leading to more severe developmental defects and endocrine disruption across generations.

Lead author Lili Niu explained the motivation behind the research: Many pesticides exist in two mirror-image forms, but environmental assessments usually treat them as if they're the same. We wanted to know whether that assumption is actually safe, especially across generations. The team fed adult zebrafish diets containing each form of o,p'-DDD for four weeks, then measured chemical accumulation in adults and their transfer to developing embryos, while tracking hatching success, deformities, survival, and thyroid hormone changes.

The findings showed that offspring consistently carried even higher chemical levels than their parents, demonstrating highly efficient maternal transfer. The S-enantiomer accumulated 134-176% more in adults and over 100% more in their larvae than the R-form. This differential accumulation led to more severe outcomes in the next generation, including increased mortality, malformations, and reduced hatching success in the S-DDD exposed groups. The complete study is available at https://doi.org/10.1016/j.enceco.2025.10.021.

To understand the mechanism behind these differences, the research team used computer-based molecular docking simulations to examine how each form interacts with key proteins involved in producing and regulating thyroid hormones. These simulations revealed that S-DDD binds more strongly to several thyroid-related proteins, providing a mechanistic explanation for its greater biological impact. Niu noted that what surprised the team most was how consistently the S-form caused stronger effects at every level tested, from chemical accumulation to developmental outcomes.

The research highlights that a small structural difference in the molecule led to very large differences in accumulation patterns, hormonal effects, and offspring development. The team emphasized that understanding enantiomer-specific effects will help improve ecological risk predictions for long-lasting pollutants and support the development of more accurate environmental standards. Niu added that if we ignore these differences, we risk underestimating long-term harm to wildlife, noting that even very low exposure in parents can create meaningful risks for the next generation.

These findings suggest that evaluating only racemic mixtures may underestimate real-world environmental hazards, particularly for persistent pollutants that can transfer across generations. The study provides critical evidence that regulatory assessments need to consider the specific behavior of individual enantiomers rather than treating mirror-image forms as identical in their environmental impacts and biological effects.