The importance of experimental time when assessing the effect of temperature on toxicity in poikilotherms

Temperature is an important factor affecting toxicity, determining chemical toxicokinetics and toxicodynamics in poikilothermic organisms. As also metabolic rates are affected by temperature, interactions between the emergence of toxic effects and time are very likely. The aim of this study was to investigate how temperature affected the toxicity of copper towards the nematode Caenorhabditis elegans when measured during short fixed timeframes compared to during full life-cycles. Copper toxicity was tested in two experiments at four or six temperatures in the range of 11 to 24°C, with copper concentrations spanning from one to 40 mg Cu/L agar, respectively. Reproduction and mortality were determined across the entire life-cycle, and the time to production of first egg and the population growth rate were calculated. The results showed that the 50% effect concentrations (EC50) of copper increased 1.5 to 2.5-fold with increasing temperature within the tested range, depending on endpoint. When calculating EC50 on reproduction after 24 or 96 hours, as is the typical setup for temperature/chemical interaction studies, results ranged from no temperature effect to effects much larger than for the full life cycle. Studies of temperature effects on toxicity must therefore be carefully designed in relation to the research question investigated.