Warming mitigates the enhancement effect of elevated air CO on anti-grazer morphological defense in Scenedesmus obliquus.

Atmospheric CO and temperature are increasing, which will have substantial impacts on interactions among organisms. While each stressor in isolation has been studied extensively, there has been less focus on their combined effects on the interspecies interaction. In order to reveal how warming and elevated CO interact on the induced defense of phytoplankton, we investigated the combined influences of elevated CO (750 ppm vs 390 ppm) and high temperature (28 °C and 31 °C vs 25 °C) on grazer Daphnia-induced morphological defense in Scenedesmus obliquus. Results showed that S. obliquus formed big-sized colonies (e.g., four- and eight-celled colonies) as response to Daphnia infochemicals, resulting in an increase in the number of cells per particle. Elevated CO further decreased the proportion of unicells from >40% in the populations growing at 390 ppm CO without Daphnia filtrate to <7% in the populations growing at 750 ppm CO with Daphnia filtrate, with the formation of more than 90% colonies, thus enhancing this morphological defense in S. obliquus. However, under elevated CO, increasing temperature up to 31 °C remarkably increased the four-celled colonies by at least 159% but decreased the eight-celled colonies by 37% compared with 25 °C. As a result, the maximum cells per particle were significantly decreased to the 390 ppm CO-grown level at high temperature. The time to reach the maximum cells per particle was also shortened by high temperature under elevated CO. These results suggest that high temperature has an overwhelming inhibitory effect on the enhanced anti-grazer defense by elevated CO, which provides significant implications for forecasting the predator-prey interaction changes in freshwater ecosystem under future climate regimes.