Hormetic Effects of the Pesticide Chlorpyrifos Counteract the Adverse Effects of a Heat Spike Both within and across Generations in a Freshwater Rotifer.

While interactions between pesticides and global warming are increasingly recognized, their transgenerational interactive effects remain unexplored. Additionally, ecotoxicological studies rarely consider environmentally relevant low concentrations of pesticides that may induce hormetic effects, whose potential to mitigate adverse impacts of global warming remains unknown. We sequentially exposed the freshwater rotifer Brachionus calyciflorus to a 12 h heat spike (32 °C) followed by a 36 h exposure to a hormetic concentration of the pesticide chlorpyrifos (50 μg/L) in the parental generation and assessed the effects in the parental and the unexposed (grand-)offspring generations. The heat spike negatively impacted fitness-related traits including survival, fecundity, and intrinsic population growth rate. In contrast, chlorpyrifos exposure elicited hormetic responses, shortening the time to the first brood and increasing the intrinsic population growth rate. Interestingly, the heat spike-induced adverse effects were counteracted by the hormetic response to chlorpyrifos: the heat spike-induced reductions in survival, fecundity, lifespan, the proportion of rotifers producing offspring, and the intrinsic population growth rate were no longer significant when combined with subsequent chlorpyrifos exposure. Moreover, both the main and interactive effects of the heat spike and chlorpyrifos were transmitted to the unexposed offspring and grand-offspring generations, providing the first evidence of transgenerational interactive effects between a pesticide and a climate change-related factor. Our findings underscore the critical role of hormetic and transgenerational effects in assessing pollutant impacts in a warming world.