Study Reveals Mirror-Image Pesticide Forms Differ in Accumulation and Harm Across Fish Generations

A new study published in Environmental Chemistry and Ecotoxicology has revealed significant differences in how mirror-image forms of a persistent pesticide metabolite behave when transferring from mother fish to their offspring, with important implications for environmental risk assessment. The research focused on o,p'-DDD, a metabolite of the pesticide DDT, examining how its two enantiomeric forms—molecules that are mirror images of each other—differ in their accumulation and effects across generations of zebrafish.

Lead author Lili Niu explained the motivation behind the study: 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 research 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.

The findings showed striking differences between the two forms. The S-enantiomer accumulated 134-176% more in adult fish and over 100% more in their larvae compared to the R-enantiomer. This preferential accumulation led to more severe outcomes in the next generation, including increased mortality, malformations, and reduced hatching success in the S-DDD exposed groups. Offspring consistently carried even higher chemical levels than their parents, demonstrating that maternal transfer was highly efficient for the S-form.

To understand the mechanism behind these differences, the team used 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. The small structural difference between the mirror-image molecules resulted in large differences in accumulation, hormone system effects, and offspring development.

The study, available at https://doi.org/10.1016/j.enceco.2025.10.021, emphasizes that understanding enantiomer-specific effects is crucial for improving ecological risk predictions for long-lasting pollutants. 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 suggests that evaluating only racemic mixtures—equal mixtures of both forms—may underestimate real-world hazards to wildlife.

These findings have significant implications for environmental regulation and pesticide assessment. If we ignore these differences, we risk underestimating long-term harm to wildlife, Niu added. The work demonstrates that even very low exposure in parents can create meaningful risks for the next generation, highlighting the need for more precise environmental standards that account for stereospecific effects of chemical pollutants.