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Morphological defense is assumed to be an effective anti-grazer strategy in phytoplankton. Scenedesmus obliquus, a globally widespread freshwater chlorophyte, can form colonies in response to the info-chemicals of herbivorous zooplankton and survive in coexistence with grazers. However, the inducible defense response is often disturbed by abiotic or biotic factors, especially under the increasing global warming and environmental pollution. In this study, two nonlethal environmental factors, namely, elevated temperature and environmentally relevant Zn2+ concentrations, decreased colony formation of S. obliquus induced by Daphnia grazing infochemicals. Elevated temperature (30 degrees C) reduced the inducible colony size and shortened the maintenance time of defensive colonies. Decreased colony size was detected with increased Zn2+ concentration. Colony formation was inhibited even at low Zn2+ concentration (0.131 mu mol L-1), which neither retarded growth nor affected photosynthesis. Warming promoted the inhibition effect of Zn2+ on inducible colony formation of S. obliquus. Warming also enhanced Zn2+ toxicity, which caused the growth rate of S. obliquus to be hindered by high Zn2+ concentrations at elevated temperature. Specially, S. obliquus which formed inducible colonies under the condition of Daphnia infochemicals had higher tolerance to Zn2+ toxicity and thus likely exerted protective effects against heavy metals. The results indicated the combined effects of global warming and heavy-metal pollution result in more severe impact on the inducible defense of S. obliquus. (C) 2018 Elsevier Ltd. All rights reserved.