Global warming not only challenges the ability of organisms to adjust to higher temperatures, but also their ability to deal with pollutants. Whether organisms will suffer from increased toxicity of pollutants under warming will depend on thermal evolution. Besides effects on life history, effects on body stoichiometry are important as these may link effects at the organismal level to ecosystem functions through nutrient cycling. In the present research, we studied combined effects of warming and nano-ZnO in Daphnia magna, and aimed to test thermal evolution and its ability to offset the higher toxicity of ZnO at higher temperatures. Daphnia clones were resurrected from two periods from the same lake differing in temperature and were exposed to nano-ZnO at 20°C and 24°C for a whole generation. After their second clutch was released, the key life history traits (age at maturity, fecundity and intrinsic population growth rate), stoichiometry (carbon, nitrogen and phosphorus) and related physiological biomolecules (RNA:DNA, protein, sugar and lipid) were examined.
Results/Conclusions
Results showed nano-ZnO only decreased the intrinsic population growth rate (‘r’) at 24°C, indicating a strong ZnO × Temperature synergism in the old population. In sharp contrast, this synergistic effect was not present in the recent population, which indicates thermal evolution not only improved the ability to deal with the higher temperature, but also offset the synergism between nano-ZnO and warming. Similarly, nano-ZnO only decreased the RNA:DNA ratio at 24°C in the old population, while this synergism was not observed in the recent population, indicating that also at the physiological level thermal evolution could offset the synergism between nano-ZnO and warming. Interestingly, nano-ZnO increased the %P and decreased the N:P and C:P ratios only at 24°C in the recent population. Changes in the body stoichiometry may have important consequences that may scale up to affect nutrient cycling in ecosystems. We provide the first experimental evidence that a higher toxicity to a pollutant (here a nanomaterial) at higher temperatures may be offset by thermal evolution. In other words, we demonstrated evolution of the synergistic interaction between a pollutant and warming, which has been ignored in the ecological risk assessment of pollutants in a warming world.